PM6686
Dual step-down controller with adjustable voltages, adjustable LDO
and auxiliary charge pump controller for notebook
Features
■
5.5 V to 28 V input voltage range
■
Dual fixed OUT1 = 1.5 V/5 V and OUT2 =
1.05 V / 3.3 V outputs or adjustable OUT1 = 0.7
V to 5.5 V and OUT2 = 0.7 V to 2.5 V outputs,
± 1.5% accuracy over valley regulation
■
Low-side MOSFETs' RDS(on) current sensing
and programmable current limit
■
Constant ON-time control
■
Frequency selectable
■
Soft-start internally fixed at 2 ms and soft-stop
■
Selectable pulse skipping at light loads
■
Selectable minimum frequency (33 kHz) in
pulse skip mode
■
Independent Power Good and EN signals
■
Latched OVP and UVP
■
Charge pump feedback
■
Fixed 3.3 V/5.0 V, or adjustable output 0.7 V to
4.5 V, ± 1.5% (LDO): 200 mA
■
3.3 V reference voltage ± 2.0%: 5 mA
■
2.0 V reference voltage ± 1.0%: 50 µA
Applications
■
Notebook computers
■
Main (3.3 V/5 V), chipset (1.5 V/1.05 V),
DDR1/2/3, graphic cards power supply
■
PDAs, mobile devices, tablet PC or slates
■
3-4 cells Li+ battery powered devices
VFQFPN-32 5 x 5 mm
Description
PM6686 is a dual step-down controller specifically
designed to provide extremely high efficiency
conversion, with lossless current sensing
technique. The constant on-time architecture
assures fast load transient response and the
embedded voltage feed-forward provides nearly
constant switching frequency operation. Pulse
skipping technique increases efficiency at very
light load. Moreover a minimum switching
frequency of 33 kHz is selectable to avoid audio
noise issues. The PM6686 provides a selectable
switching frequency, allowing three different
values of switching frequencies for the two
switching sections. The output voltages OUT1
and OUT2 can be programmed to regulate
1.5 V/5 V and 1.05 V/3.3 V outputs respectively or
can deliver two adjustable output voltages. An
optional external charge pump can be monitored.
This device embeds a linear regulator that can
provide 3.3 V/5 V or an adjustable voltage from
0.7 V to 4.5 V output. The linear regulator
provides up to 100 mA output current. LDO can
be bypassed with the switching regulator outputs
or with an external power supply (switchover
function).
When in switchover, the LDO output can source
up to 200 mA.
Table 1.
Device summary
Order codes
Package
PM6686
Packaging
Tray
VFQFPN-32L 5 x 5 mm
PM6686TR
July 2009
Tape and reel
Doc ID 15281 Rev 4
1/50
www.st.com
50
Contents
PM6686
Contents
1
Simplified application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
2
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3
2.1
Connections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2
Pin descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1
Maximum rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
4
Recommended operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5
Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6
Typical operating characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1
Screen shots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
7
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8
Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1
9
2/50
Switching sections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1.1
Output voltage set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.1.2
Constant on time control (COT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
8.1.3
PWM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1.4
SKIP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.1.5
Non audible SKIP (NA SKIP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.1.6
Gate drivers and logic supply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.1.7
Current sensing and current limit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
8.1.8
Soft-start and soft-end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Monitoring and protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.1
Overvoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.2
Undervoltage protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.3
PVCC monitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Doc ID 15281 Rev 4
PM6686
Contents
9.4
Linear regulator section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
9.5
Charge pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.6
Voltage references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.7
General device fault management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
9.7.1
10
Application information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1
11
Thermal protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
External components selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1.1
Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1.2
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
10.1.3
Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
10.1.4
MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Diode selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
11.1
Freewheeling diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
11.2
Charge pump diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
11.3
Other important components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
11.3.1
VIN filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
11.3.2
PVCC and VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
11.3.3
VREF2 and VREF3 capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
11.3.4
LDO output capacitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
11.3.5
Bootstrap circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
12
PCB design guidelines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
13
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
14
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Doc ID 15281 Rev 4
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List of figures
PM6686
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Figure 21.
Figure 22.
Figure 23.
Figure 24.
Figure 25.
Figure 26.
Figure 27.
Figure 28.
Figure 29.
Figure 30.
Figure 31.
Figure 32.
Figure 33.
Figure 34.
Figure 35.
Figure 36.
Figure 37.
Figure 38.
Figure 39.
Figure 40.
Figure 41.
Figure 42.
Figure 43.
Figure 44.
Figure 45.
Figure 46.
Figure 47.
Figure 48.
4/50
Simplified application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Pin connection (through top view) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Efficiency vs load OUT1 = 5 V, TON = VCC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Efficiency vs load OUT2 = 3.3 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Efficiency vs load OUT1 = 1.5 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Efficiency vs load OUT2 = 1.05 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Load regulationOUT1 = 5 V, TON = VCC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Load regulationOUT2 = 3.3 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Load regulation OUT1 = 1.5 V, TON = VCC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Load regulation OUT2 = 1.05 V, TON = VCC. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Switching frequency vs load OUT1 = 5 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Switching frequency vs load OUT2 = 3.3 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Section 1 line regulation OUT1 = 5 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Section 2 line regulation OUT2 = 3.3 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Section 1 line regulation OUT1 = 1,5 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Section 2 line regulation OUT2 = 1,05 V, TON = VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Stand-by mode input battery current vs input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Shut-down mode input battery current vs input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
PWM no load input currents vs input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
SKIP no load input currents vs input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
NA SKIP no load input currents vs input voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
VREF3 load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
VREF2 load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
LDO = 3,3 V load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
LDO = 5 V load regulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
OUT1 soft-start no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
OUT2 soft-start no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
OUT1 soft-start 8 A constant current load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
OUT2 soft-start loaded 8 A constant current load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
OUT1 soft-end, no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
OUT2 soft-end, no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
OUT1 soft-start, EN2 = VREF2 no loads applied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
OUT2 soft-start, EN1=VREF2 no loads applied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
soft-end, EN2 = VREF2 no loads applied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
soft-end, EN1=VREF2 no loads applied . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Load transient 0-5 A 2 A/µs OUT1 = 5 V PWM mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Load transient 0-5 A 2 A/µs OUT1 = 5 V SKIP mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Load transient 0-5 A 2 A/ µs OUT1 = 1,5 V PWM mode . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Load transient 0-5 A 2 A/ µs OUT1 = 1,5 V SKIP mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Load transient 0-5 A 2 A/µs OUT2 = 3.3 V PWM mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Load transient 0-5 A 2 A/µs OUT2 = 3.3 V SKIP mode . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Load transient 0-5 A 2 A/ µs OUT2 = 1,05 V PWM mode . . . . . . . . . . . . . . . . . . . . . . . . . 22
Load transient 0-5 A 2 A/ µs OUT2 = 1,05 V SKIP mode . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Functional block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Resistor divider to configure the output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Constant on time block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Inductor current and output voltage in PWM mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Inductor current and output voltage in SKIP mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Doc ID 15281 Rev 4
PM6686
Figure 49.
Figure 50.
Figure 51.
Figure 52.
Figure 53.
Figure 54.
Figure 55.
Figure 56.
Figure 57.
Figure 58.
Figure 59.
List of figures
Inductor current and output voltage in NA SKIP mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Internal supply diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Current waveforms in current limit conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Current limit circuit block diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
VOUT2 behavior if EN2 is connected to VREF2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Charge pump application circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
VIN pin filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
VCC and PVCC filters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Bootstrap circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Current paths, ground connection and driver traces layout . . . . . . . . . . . . . . . . . . . . . . . . 46
Package dimensions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Doc ID 15281 Rev 4
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Simplified application schematic
Simplified application schematic
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PM6686
Pin settings
2
Pin settings
2.1
Connections
Figure 2.
Pin connection (through top view)
0-
!-V
2.2
Pin descriptions
Table 2.
Pin descriptions
N°
Pin
1
VREF2
2
TON
Frequency selection pin. It provides a selectable switching frequency,
allowing three different values of switching frequencies for the switching
sections.
3
VCC
Controller supply voltage input. Bypass to GND with a 1 μF capacitor.
4
EN_LDO
5
VREF3
Internal 3.3 V high accuracy voltage reference. It can deliver 5 mA if
bypassed to GND with a 10 nF capacitor. If not used, it can be left floating.
VIN
Device supply voltage pin. VIN is used in the on-time generators of the two
switching controllers. VIN is also used to power the linear regulator when the
switchover function is not active. Connect VIN to the battery input and
bypass with a 1 µF capacitor.
6
Function
Internal 2 V high accuracy voltage reference. It can deliver 50 μA. Loading
VREF2 can affect FB and output accuracy. Bypass to GND with a 100 nF
capacitor.
Enable input for the linear regulator. The LDO is enabled if
EN_LDO is > 1.6 V and is disabled if EN_LDO < 1 V.
Doc ID 15281 Rev 4
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Pin settings
PM6686
Table 2.
N°
Pin
Function
LDO
Linear regulator output. It can provide up to 100 mA peak current. The LDO
regulates at 5 V If LDOREFIN is connected to GND. When the LDO is set at
5 V and LDO_SW is within 5 V switchover threshold, the internal regulator
shuts down and the LDO output pin is connected to LDO_SW through a
0.8 Ω switch. The LDO regulates at 3.3 V if LDOREFIN is connected to
VCC. When the LDO is set at 3.3 V and LDO_SW is within 3.3 V switchover
threshold, the internal regulator shuts down and the LDO output pin is
connected to LDO_SW through a 1.1 Ω switch. Bypass LDO output to GND
with a minimum of 4.7 µF ceramic capacitor.
LDOREFIN
Feedback of the adjustable linear regulator. Connect LDOREFIN to GND for
fixed 5 V operation. Connect LDOREFIN to VCC for fixed 3.3 V operation.
LDOREFIN can be used to program LDO output voltage from 0.7 V to 4.5 V:
LDO output is two times the voltage of LDOREFIN. The switchover function
is disabled in adjustable mode.
9
LDO_SW
Source of the switchover connection. LDO_SW is the switchover source
voltage for the LDO when LDOREFIN is connected to GND or VCC.
Connect LDO_SW to 5 V if LDOREFIN is tied to GND. Connect LDO_SW to
3.3 V if LDOREFIN is tied to VCC.
10
OUT1
Output voltage sense for the switching section 1.This pin must be directly
connected to the output voltage of the switching section. It provides also the
feedback for the switching section 1 when FB1 is tied to GND/VCC.
11
FB1
Feedback input for the switching section 1:
– If this pin is connected to GND, OUT1 operates at 5 V
– If this pin is connected to VCC, OUT1 operates at 1.5 V
– This pin is connected to a resistive voltage-divider from OUT1 to GND to
adjust the output voltage from 0.7 V to 5.5 V.
12
ILIM1
Positive current sense input for the switching section 1. It is possible to set a
threshold voltage that is compared with 1/10th of the GND-PHASE1 drop
during the off time.
13
PG1
Power Good output signal for the section 1. This pin is an open drain output
and It is pulled down when the output of the switching section 1 is out of
+/- 10% of its nominal value.
14
EN1
Enable input for the switching section 1.
– If EN1 < 0.8 V the switching section OUT1 is turned off and all faults are
cleared.
– If EN1 > 2.4 V the switching section OUT1 is turned on.
– If EN1 is connected to VREF2, the switching section OUT1 turns on after
the switching section OUT2 reaches regulation.
15
HGATE1
High-side gate driver output for section 1.
16
PHASE1
Switch node connection and return path for the high-side driver for the
section 1.It is also used as positive and negative current sense input.
17
BOOT1
Bootstrap capacitor connection for the switching section 1. It supplies the
high-side gate driver.
18
LGATE1
Low-side gate driver output for the section 1.
19
PVCC
7
8
8/50
Pin descriptions (continued)
5 V low-side gate drivers supply voltage input. Bypass to PGND with a 1 μF
capacitor.
Doc ID 15281 Rev 4
PM6686
Pin settings
Table 2.
N°
Pin descriptions (continued)
Pin
Function
CP_FB
The CP_FB is used to monitor the optional external 14 V charge pump.
Connect a resistive voltage-divider from 14 V charge pump output to GND. If
CP_FB drops below the threshold voltage, the device performs a no audible
skip cycle. This charges the charge pump output (driven by LGATE1). Leave
CP_FB floating if the charge pump feedback is not needed.
21
GND
Signal ground reference for internal logic circuitry and LDO. It must be
connected to the signal ground plan of the power supply and to the exposed
pad. The signal ground plan and the power ground plan must be connected
together in one point near the PGND pin.
22
PGND
Power ground. This pin must be connected to the power ground plan of the
power supply.
23
LGATE2
Low-side gate driver output for the section 2.
24
BOOT2
Bootstrap capacitor connection for the switching section 2. It supplies the
high-side gate driver.
25
PHASE2
Switch node connection and return path for the high-side driver for the
section 2. It is also used as positive and negative current sense input.
26
HGATE2
High-side gate driver output for section 2.
20
27
EN2
Enable input for the switching section 2.
– If EN2 < 0.8 V the switching section OUT2 is turned off and all faults are
cleared.
– If EN2 > 2.4 V the switching section OUT2 is turned on.
If EN2 is connected to VREF2, the switching section OUT2 turns on after
the switching section OUT1 reaches regulation.
28
PG2
Power Good output signal for the section 2. This pin is an open drain output
and It is pulled down when the output of the switching section 2 is out of
+ 14% / - 10% of its nominal value.
29
SKIP
Pulse skipping mode control input.
– If the pin is connected to VCC the PWM mode is enabled.
– If the pin is connected to GND, the pulse skip mode is enabled.
– If the pin is connected to VREF2 (or floating) the pulse skip mode is
enabled but and the switching frequency is kept higher than 33 kHz (Noaudible pulse skip mode).
30
OUT2
Output voltage sense for the switching section 2.This pin must be directly
connected to the output voltage of the switching section. It provides also the
feedback for the switching section 2 when REFIN2 is tied to VREF3/VCC.
31
ILIM2
Positive current sense input for the switching section 2. It is possible to set a
threshold voltage that is compared with 1/10th of the GND-PHASE2 drop
during the off time.
REFIN2
Feedback input for the switching section 2:
– If this pin is connected to VCC, OUT2 operates at 3.3 V
– If this pin is connected to VREF3, OUT2 operates at 1.05 V
– If this pin is connected to an external reference from 0.7 V to 2.5 V, OUT2
works in tracking with this reference. Bypass REFIN2 to GND with a 100
nF capacitor.
32
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9/50
Electrical data
PM6686
3
Electrical data
3.1
Maximum rating
Table 3.
Absolute maximum ratings
Parameter
Value
Unit
VIN to PGND
-0.3 to 38
V
PHASEx to PGND
-0.3 to 38
V
BOOTx to PHASEx
-0.3 to 6
V
PVCC to PGND
-0.3 to 6
V
HGATEx to PHASEx
-0.3 to BOOTx +0.3
V
LGATEx, CP_FB to PGND
-0.3 to PVCC +0.3
V
-0.3 to 6
V
-0.3 to VCC+0.3
V
-0.3 to 7
V
VREF2 to GND
-0.3 to VREF3+0.3
V
PGND to GND
-0.3 to +0.3
V
2
W
±1250
V
VCC, ENx, SKIP, PGx, LDO, REFIN2, OUTx, VREF3,
LDOREFIN, LDO_SW, TON to GND
FB1, ILIMx to GND
EN_LDO to GND
Power dissipation at TA = 25 °C
Maximum withstanding voltage range test condition: CDF-AECQ100-002- “human body model” acceptance criteria: “normal
performance”
3.2
Thermal data
Table 4.
Symbol
Parameter
Value
Unit
RthJA
Thermal resistance junction to ambient
35
°C/W
TJ
Junction operating temperature range
-40 to 125
°C
Storage temperature range
-50 to 150
°C
Operating ambient temperature range
-40 to 85
°C
TSTG
TA
10/50
Thermal data
Doc ID 15281 Rev 4
PM6686
4
Recommended operating conditions
Recommended operating conditions
Table 5.
Recommended operating conditions
Value
Symbol
Parameter
Unit
Min
Typ
Max
VIN
Input voltage range, LDO = 5 V in regulation
5.5
-
28
V
VCC
VCC operative voltage range
4.5
-
5.5
V
REFIN2 voltage range with OUT2 in
adjustable mode, VIN = 5.5 V to 28 V
0.7
-
2.5
V
OUT1 output voltage range
0.70
-
5.50
V
ILIM voltage range
0.2
-
2
V
LDOREFIN setting with LDO = 2 x LDOREFIN
(adjustable mode)
0.35
-
2.25
V
-
200
mA
-
100
mA
-
100
mA
REFIN2
OUT1
ILIM
LDOREFIN
VIN = 5.5 V to 28 V, LDO_SW = 5 V,
LDO DC output current LDOREFIN = GND
(switchover function
VIN = 5.5 V to 28 V, LDO_SW = 3.3 V,
enabled)
LDOREFIN = VCC
LDO DC output current
VIN = 5.5 V to 28 V, LDO_SW = 0 V,
(switchover function
LDOREFIN = GND, VCC
disabled)
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Electrical characteristics
5
PM6686
Electrical characteristics
VIN = 12 V, no load on LDO, OUT1, OUT2, VREF3, and VREF2. EN2 = EN1 = VCC,
LDO_SW = 5 V, PVCC = 5 V, EN_LDO = 5 V, TJ = 25 °C unless otherwise specified)
Table 6.
Electrical characteristics
Symbol
Parameter
Test condition
Min
Typ
Max
Unit
Switching controller output accuracy
OUT2
OUT1
FB1
REFIN2
VIN = 5.5 V to 28 V, REFIN2 = VCC,
SKIP = VCC
3.25
3.330 3.397
V
VIN = 5.5 V to 28 V,
REFIN2 = VREF3, SKIP = VCC
1.038
1.05
1.062
V
VIN = 5.5 V to 28 V, FB1 = VCC,
SKIP = VCC
1.482 1.500 1.518
V
VIN = 5.5 V to 28 V, FB1 = GND,
SKIP = VCC
4.975 5.050 5.125
V
Feedback accuracy with
OUT1 in adjustable mode
VIN = 5.5 V to 28 V, SKIP = VCC
0.693 0.700 0.707
V
Accuracy referred to REFIN2
REFIN2 = 0.7 V to 2.5 V,
SKIP = VCC
Output voltage
Output voltage
-1
1
%
Current limit and zero crossing comparator
ILIM
ILIM bias current
Positive current limit threshold
Zero crossing current
threshold
TA = +25 °C.
4.5
5
5.5
µA
Adjustable, VILIM = 0.5 V, GND-PHASE
35
50
65
mV
Adjustable, VILIM = 1 V or VCC, GNDPHASE
85
100
115
mV
Adjustable, VILIM = 2 V, GND-PHASE
180
200
220
mV
SKIP = GND, VREF2, or OPEN, GNDPHASE
-1
+11
mV
Switching frequency
OUT1 = 5.125 V
On-time pulse width
OUT2 = 3.368 V
TON = GND or
VREF2
0.908 1.068 1.228
TON = VCC
1.815 2.135 2.455
TON = GND
0.477 0.561 0.655
TON = VCC or
VREF2
0.796 0.936 1.076
Minimum Off-time
No-audible skip mode
operating frequency
350
SKIP = VREF2(or OPEN)
25
µs
472
33
kHz
Soft-start and soft-end
Soft-start ramp time
12/50
2
Doc ID 15281 Rev 4
4
ms
PM6686
Table 6.
Electrical characteristics
Electrical characteristics (continued)
Symbol
Parameter
Test condition
Min
Typ
Max
Unit
Linear regulator and reference
LDO
LDO output voltage
LDO_SW = GND, 5.5 V < VIN < 28 V,
LDOREFIN < 0.3 V, 0 < ILDO < 100 mA
4.925 5.000 5.075
V
LDO_SW = GND, 5.5 V < VIN < 28 V,
LDOREFIN = VREF3,
0 < ILDO < 100 mA
3.250 3.300 3.350
V
LDO accuracy in adjustable
mode
VIN = 5.5 V to 28 V, LDOREFIN = 0.35 V
to 2.25 V, no load
-2.5
LDO short circuit current
(linear regulator enabled)
LDO = 4.3 V, LDO_SW = GND
260
LDO_SW
LDO_SW switch on threshold
LDO = 5 V, rising edge of LDO_SW,
LDOREFIN = GND
4.64
LDO_SW
LDO_SW hysteresis
+2.5
%
320
380
mA
4.75
4.84
V
LDO = 5 V, falling edge of LDO_SW,
LDOREFIN = GND
200
400
mV
LDO_SW switch resistance
LDO = 5 V, rising edge of LDO_SW,
LDOREFIN = GND,
output current = 200 mA
0.81
1.275
Ω
VREF3 output voltage
No load
3.235 3.300 3.365
V
VREF3 current limit
VREF3 = GND
22
30
mA
VREF2 output voltage
No load
2.02
V
VREF2 load regulation
0 < Load < 50 μA
VREF2 sink current
VREF2 > 2.030 V
VIN shutdown current
EN1 and EN2 low, EN_LDO low
49
70
µA
VIN standby current
EN1 and EN2 low, EN_LDO high,
LDOREFIN = GND
132
180
µA
4,3
6,5
mW
VREF3
VREF2
1.980 2.000
6
mV
10
Switching regulators on,
Operating power consumption
FB1 = SKIP = GND, REFIN2 = VCC,
(VCC and VIN pins
LDOREFIN = GND,
consumption)
OUT1 = LDO_SW = 5.3V, OUT2 = 3.5 V
µA
Fault management
Rising edge of PVCC
4.33
V
Falling edge of PVCC
4
V
Referred to FB1 nominal regulation point
+11
%
Referred to REFIN2 nominal regulation
point. worst case:REFIN2 = 0.7 V
+14
%
PVCC UVLO threshold
Overvoltage trip threshold
PG threshold
Lower threshold
-13
-10
-7
%
PG low voltage
ISink = 4 mA
159
235
405
mV
PG leakage current
PG = 5 V
1
µA
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Electrical characteristics
Table 6.
PM6686
Electrical characteristics (continued)
Symbol
Parameter
Output undervoltage
shutdown threshold
Test condition
Referred to FB1, REFIN2 nominal
regulation point
Min
Typ
Max
Unit
67
70
73
%
0.528
V
Inputs and outputs
fixed OUT1 = 5 V
FB1
FB1 logic level
fixed OUT1 = 1.5 V
4.1
fixed OUT2 = 1.05 V, VCC = 5 V
REFIN2
REFIN2 logic level
fixed OUT 2= 3.3 V, VCC = 5 V
V
V
VREF3
VCC0.838
V
fixed LDO = 5 V
0.4
V
LDOREFIN LDOREFIN logic level
fixed LDO = 3.3 V
2.43
V
Pulse skip mode
SKIP
SKIP logic level
0.8
No audible skip mode (VREF2 or floating)
PWM mode
TON logic level
2.4
V
0.8
Middle level
High level
EN level
2.4
V
Switching regulators on level
EN_LDO
0.8
Delay start level
V
V
VREF2
Switching regulators off level
EN1,2
V
VREF2
Low level
TON
V
V
V
VREF2
2.4
V
Linear regulator off level
0.905
1.00
1.050
V
Linear regulator on level
1.500
1.6
1.650
V
+1
µA
EN_LDO level
Input leakage current
FB1 = 0.7 V
-1
REFIN2 = 2.5 V
12
Diode forward voltage
PVCC = -BOOT, Idiode = 10 mA
0.2
Diode Leakage current
BOOT= 30 V, PHASE = 28 V, PVCC = 5 V
Internal bootstrap diode
V
500
nA
High-side and low-side gate drivers
HGATEx high state (pull-up)
Isource = 100 mA
1.8
HGATEx low state (pull-down)
Isink = 100 mA
1.3
LGATEx high state (pull-up)
Isource = 100 mA
1.3
LGATEx low state (pull-down)
Isink = 100 mA
0.6
HGATE driver on-resistance
1.9
Ω
LGATE driver on-resistance
14/50
Doc ID 15281 Rev 4
0.8
PM6686
6
Typical operating characteristics
Typical operating characteristics
(TON = VCC (200 / 300 kHz), SKIP = GND (skip mode), LDOREFIN = SGND (LDO = 5 V),
LDO_SW = OUT1, PVCC connected to LDO, VIN = 12 V, EN1-EN2-EN_LDO are high, no
load unless specified). Measures performed on the demonstration kit (PM6686_SYSTEM
and PM6686_CHIPSET)
Efficiency traces: Green: VIN = 7 V, red: VIN = 12 V, blue: VIN = 19 V.
Figure 3.
Efficiency vs load
OUT1 = 5 V, TON = VCC
Figure 4.
Efficiency vs load
OUT2 = 3.3 V, TON = VCC
Figure 5.
Efficiency vs load
OUT1 = 1.5 V, TON = VCC
Figure 6.
Efficiency vs load
OUT2 = 1.05 V, TON = VCC
A-PWM
B-SKIP
A-NASKIP
Doc ID 15281 Rev 4
A-PWM
B-SKIP
A-NASKIP
15/50
Typical operating characteristics
PM6686
Figure 7.
Load regulation
OUT1 = 5 V, TON = VCC
Figure 8.
Figure 9.
Load regulation
OUT1 = 1.5 V, TON = VCC
Figure 10. Load regulation
OUT2 = 1.05 V, TON = VCC
Figure 11. Switching frequency vs load
OUT1 = 5 V, TON = VCC
16/50
Load regulation
OUT2 = 3.3 V, TON = VCC
Figure 12. Switching frequency vs load
OUT2 = 3.3 V, TON = VCC
Doc ID 15281 Rev 4
PM6686
Typical operating characteristics
Figure 13. Section 1 line regulation
OUT1 = 5 V, TON = VCC
Figure 14. Section 2 line regulation
OUT2 = 3.3 V, TON = VCC
Figure 15. Section 1 line regulation
OUT1 = 1,5 V, TON = VCC
Figure 16. Section 2 line regulation
OUT2 = 1,05 V, TON = VCC
Figure 17. Stand-by mode input battery Figure 18. Shut-down mode input
current vs input voltage
battery current vs input
voltage
EN1=EN2=GND
FN_LDO=GND
Doc ID 15281 Rev 4
17/50
Typical operating characteristics
18/50
PM6686
Figure 19. PWM no load input currents
vs input voltage
Figure 20. SKIP no load input currents
vs input voltage
Figure 21. NA SKIP no load input
currents vs input voltage
Figure 22. VREF3 load regulation
Figure 23. VREF2 load regulation
Figure 24. LDO = 3.3 V load regulation
Doc ID 15281 Rev 4
PM6686
Typical operating characteristics
Figure 25. LDO = 5 V load regulation
6.1
Screen shots
Typical operating characteristic (TON = VCC (200 / 300 kHz), SKIP = GND (skip mode),
FB1 = GND (OUT1 = 5 V), REFIN2 = VCC (OUT2 = 3.3 V), LDOREFIN = SGND
(LDO = 5 V), CP_FB = floating, LDO_SW = OUT1, PVCC connected to LDO, VIN = 12 V,
EN1-EN2-EN_LDO are high, no load unless specified)
Figure 26. OUT1 soft-start no load
Figure 27. OUT2 soft-start no load
Doc ID 15281 Rev 4
19/50
Typical operating characteristics
PM6686
Figure 28. OUT1 soft-start 8 A constant Figure 29. OUT2 soft-start loaded 8 A
current load
constant current load
Figure 30. OUT1 soft-end, no load
Figure 31. OUT2 soft-end, no load
Figure 32. OUT1 soft-start, EN2 = VREF2 Figure 33. OUT2 soft-start, EN1=VREF2
no loads applied
no loads applied
20/50
Doc ID 15281 Rev 4
PM6686
Typical operating characteristics
Figure 34. Soft-end, EN2 = VREF2 no
loads applied
Figure 35. Soft-end, EN1=VREF2 no
loads applied
Figure 36. Load transient 0-5 A 2 A/µs
OUT1 = 5 V PWM mode
Figure 37. Load transient 0-5 A 2 A/µs
OUT1 = 5 V SKIP mode
Figure 38. Load transient 0-5 A 2 A/ µs
OUT1 = 1.5 V PWM mode
Figure 39. Load transient 0-5 A 2 A/ µs
OUT1 = 1.5 V SKIP mode
Doc ID 15281 Rev 4
21/50
Typical operating characteristics
22/50
PM6686
Figure 40. Load transient 0-5 A 2 A/µs
OUT2 = 3.3 V PWM mode
Figure 41. Load transient 0-5 A 2 A/µs
OUT2 = 3.3 V SKIP mode
Figure 42. Load transient 0-5 A 2 A/ µs
OUT2 = 1.05 V PWM mode
Figure 43. Load transient 0-5 A 2 A/ µs
OUT2 = 1.05 V SKIP mode
Doc ID 15281 Rev 4
PM6686
7
Block diagram
Block diagram
Figure 44. Functional block diagram
VIN
VREF3
VREF2
PVCC
+
UVLO
VREF3
-
VREF2
VCC
LDOREFIN
ADJ.
LINEAR
REGULATOR
LDO
ILIM1
LDO EN INT
+
SWITCHOVER
THRESHOLD
-
LDO_SW
ILIM2
CP_FB
PVCC
PVCC
BOOT1
BOOT2
LEVEL
SHIFTER
HGATE1
LEVEL
SHIFTER
SMPS1
CONTROLLER
PHASE1
HSIDE2
SMPS2
CONTROLLER
PHASE2
OUT2
OUT1
PVCC
PVCC
LGATE2
LGATE1
FB1
REFIN2
PG1
PG2
EN1
EN2
STARTUP
CONTROLLER
THERMAL
CONTROLLER
LDO EN INT
EN_LDO
TON
SKIP
Doc ID 15281 Rev 4
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Device description
8
PM6686
Device description
The PM6686 is a dual step down controller dedicated to provide logic voltages for notebook
computers. It offers several operating configurations: it combines two synchronous buck
controllers, an internal linear regulator (LDO), two voltage references and a charge pump
controller.
Each buck controller is based on constant on time (COT) architecture. This type of control
offers a very fast load transient response with a minimum external components count.
The two switching sections (SMPS) generate two output voltages OUT1 and OUT2 that
regulate adjustable voltages. A fixed output voltage configuration can also be selected,
reducing further the external components count because no external resistor divider is
needed.
In fixed mode, OUT1 provides 5 V or 1.5 V; in adjustable mode OUT1 can regulate an output
voltage between 0.7 V and 5.5 V. In fixed mode, OUT2 provides 3.3 V or 1.05 V, in
adjustable mode OUT2 can regulate an output between 0.7 V to 2.5 V by tracking an
external reference.
The switching frequencies of both switching controllers can be adjusted to 200 kHz/300 kHz,
400 kHz/300 kHz or 400 kHz/500 kHz respectively. To maximize the efficiency at light loads
a pulse skipping mode can be selected. Moreover a pulse skipping mode with a minimum
switching frequency of 33 kHz (non audible skip operation mode) can be selected to avoid
audible noise issue. The linear regulator can provide a fixed (5 V or 3.3 V) or an adjustable
output voltage. In order to reduce the power consumption the internal LDO can be turned off
and the LDO output can be supplied with an external voltage applied at LDO_SW pin
(switch-over function).
The PM6686 supplies two voltage references: 3.3 V and 2 V. The charge pump controller
can be programmed to regulate a 14 V output. The switching sections and the LDO have
independent enable signals. Moreover the switching sections have a selectable power up
sequence and a turn off management.
The device is protected against overvoltage, undervoltage and over temperature. Two
independent Power Good signals monitor the output voltage range of each switching
sections.
8.1
Switching sections
8.1.1
Output voltage set up
The switching sections can be configured in several ways.
OUT1 output voltage is configured with FB1 pin. If FB1 pin is tied to GND the PM6686
regulates 5 V while if FB1 is connected to VCC the controller set OUT1 at 1.5 V. Using an
external resistor divider the output can be adjusted following this equation:
Equation 1
⎛ R1 ⎞
VOUT1 = 0,7V ⋅ ⎜
+ 1⎟
⎝ R2 ⎠
24/50
Doc ID 15281 Rev 4
PM6686
Device description
Figure 45. Resistor divider to configure the output voltage
VOUT
PHASE
L
CO UT
R1
FB1
R2
Where R1, R2 are the resistors of the FB1 pin divider, as shown in Figure 2.
OUT2 output voltage is programmed with REFIN2 pin. Fixed output voltage is selected
connecting REFIN2 to VREF3 (OUT2 = 1.05 V) or to VCC (OUT2 = 3.3 V).
When the REFIN2 voltage is between 0.7 V and 2.5 V, OUT2 output voltage tracks REFIN2
voltage. When REFIN2 is lower than 0.5 V the section is turned OFF.
Table 7.
Output
OUT1
OUT2
Switching output voltages configuration
control pin
Control pin
Operation mode
Output voltage
GND
Fixed
5V
VCC
Fixed
1.5 V
Resistor divider
Adj
⎛ R1 ⎞
VOUT1 = 0.7V ⋅ ⎜
+ 1⎟
⎝ R2 ⎠
VCC
Fixed
3.3 V
VREF3
Fixed
1.05 V
Ext source
Tracking
=REFIN2
connected to
FB1
REFIN2
Doc ID 15281 Rev 4
25/50
Device description
8.1.2
PM6686
Constant on time control (COT)
PM6686 implements a pseudo-fixed frequency algorithm using the COT architecture. The
two sections are completely independent with separated switching controllers (SMPS).
The COT architecture bases its algorithm on the output ripple derived across the output
capacitor's ESR. The controller has an internal on time (TON) generator triggered on the
output voltage valley: when VOUT reaches the regulation value a new TON starts. The TON
duration is given by the following equation:
Equation 2
TON = K ⋅
VOUT
VIN
Where TON is the on time duration, K is a constant, VOUT is the sensed output voltage and
VIN is the input voltage.
The duty cycle in a buck converter is:
Equation 3
TON
V
= D = OUT
TSW
VIN
The switching frequency in continuous current mode (CCM) is:
Equation 4
fSW
VOUT
VIN
D
1
=
=
=
V
TON
K
K ⋅ OUT
VIN
The switching frequency is theoretically constant, but in a real application it depends on
parasitic voltage drops that occur during the charging path (high-side switch resistance,
inductor resistance (DCR)) and discharging path (low-side switch resistance, DCR). As a
result the switching frequency increases as a function of the load current. The following
table shows the switching frequencies that can be selected through TON pin:
Table 8.
Frequency configurations
SMPS 1
SMPS 2
TON
26/50
Frequency
K
Frequency
K
VCC
200 kHz
5 µs
300 kHz
3.33 µs
VREF2 or open
400 kHz
2.5 µs
300 kHz
3.33 µs
GND
400 kHz
2.5 µs
500 kHz
2 µs
Doc ID 15281 Rev 4
PM6686
Device description
The COT architecture uses a minimum off-time (TOFFMIN) to allow inductor valley current
sense on the synchronous switch and to allow the charge of the bootstrap capacitor. A
minimum on-time is also introduced to assure the start-up sequence.
An adaptive anti-cross conduction algorithm avoids current paths between VIN and GND
during switching transition.
The PM6686 has three different operation modes selectable with SKIP pin: forced PWM
(PWM), pulse SKIP (SKIP) and non audible pulse SKIP (NA SKIP). The following
paragraphs explain in details the different features of these operation modes.
Table 9.
Operative mode configurations
Control pin
Control pin
Operation mode
connected to
VCC
PWM
GND
SKIP
VREF2 or floating
NA SKIP
SKIP
Figure 46. Constant on time block diagram
T off min
BOOT
ILIM
1/10
+
S
-
R
PHASE
Voltage
Reference
CLR
Level
Shifter
Q
HS Driver
HGAT E
Q
PH ASE
T on
generator
-
OU T
SET
-
0,25V
+
+
-
PWM
Comparator
+
VIN
0,5V
+
SET
S
Z .C.
Comparator
R
CLR
Q
PVCC
Q
LS Driver
LGATE
0,8V
-
Doc ID 15281 Rev 4
PGND
27/50
Device description
8.1.3
PM6686
PWM
PWM implements the continuous current mode (CCM). During TON, the high-side MOSFET
is turned on and the inductor current starts increasing. When the Ton is elapsed the highside MOSFET is turned off and after a dead time during which neither MOSFET conducts,
the low-side MOSFET turns on. The inductor current decreases until these three conditions
are verified:
●
Output voltage reaches the regulation voltage
●
Inductor current is below the current limit
●
TOFFMIN is elapsed
When these conditions are satisfied a new TON starts.
PWM operation mode has a quasi-constant switching frequency, avoiding any audible noise
issue and the continuous current mode assures better load transitions despite of a lower
efficiency at light loads.
Figure 47.
Inductor current and output voltage in PWM mode
Inductor
current
Output
voltage
Vreg
Ton
28/50
Toff
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t
PM6686
8.1.4
Device description
SKIP
To improve the efficiency at light load the PM6686 implements pulse skip operation mode.
When SKIP pin is tied to GND the inductor current is sensed and if it is equal to zero the
synchronous MOSFET is turned off. As a consequence the output capacitor is left floating
and the discharge depends only on the current sourced by the load. The new TON starts
when the output reaches the voltage regulation. As a consequence at light load conditions
the switching frequency decreases improving the total efficiency of the converter. Working in
discontinuous current mode, the switching and the conduction losses are decreased
skipping some cycles.
If the output load is high enough to make the system work in CCM, skip mode is
automatically changed in PWM mode.
Figure 48.
Inductor current and output voltage in SKIP mode
Inductor
current
Output
Vreg
TON TOFF
8.1.5
Non audible SKIP (NA SKIP)
To avoid audio noise the NA SKIP operation mode can be selected, connecting SKIP pin to
VREF2 or leaving it floating. In this condition if a new cycle doesn't start within 30 μs typ.
from the previous one the PM6686 turns on the low-side MOSFET to discharge the output
capacitor. The inductor current goes negative until the output reaches the voltage regulation
voltage allowing a new cycle to begin. If the switching frequency is above 33 kHz the device
works in SKIP mode.
This operation mode is useful to avoid audio noise but it lowers the efficiency at light loads if
it is compared to the SKIP mode.
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Device description
PM6686
Figure 49. Inductor current and output voltage in NA SKIP mode
Inductor
current
Output
Vreg
TMA X
TON TOFF
8.1.6
TIDLE
t
Gate drivers and logic supply
The integrated high-current drivers allow the use of different power MOSFET.
high-side driver is supplied with a bootstrap circuit with an integrated bootstrap diode. The
BOOT and the PHASE pins work respectively as supply and return rails for the HS driver.
The PVCC pin is the input for the supply of the low-side driver and PGND is the pin used as
return rail.
The PM6686 implements an anti-cross conduction protection which prevents high-side and
low-side MOSFET from being on at the same time.
The power dissipation of each driver can be calculated as:
Equation 5
PDISS = VPVCC ⋅ Q G ⋅ fsw
Where VPVCC is the voltage applied to PVCC pin (+5 V) and fSW is the switching frequency.
The power dissipated by the drivers can be reduced lowering the sections switching
frequencies and mounting MOSFET with smaller QG.
VCC pin is the input voltage rail to supply the internal logic circuit. This pin is connected
internally with a resistor to PVCC. As usual analog supply should be divided by the power
supply with a low pass filter to reduce the noise for the analog supply of the logic. Being the
resistor integrated it is enough to put a decoupling capacitor near VCC pin to realize the
filter with a components count reduction.
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PM6686
Device description
Figure 50. Internal supply diagram
8.1.7
Current sensing and current limit
The PM6686 implements a positive valley current limit to protect the application from an
overcurrent fault. Each section has an independent current limit setting. A new switching
cycle can't start until the inductor current is under the positive current limit threshold. Note
that the peak current flowing in the inductor can reach a value greater than the current limit
threshold by an amount equal to the inductor ripple current.
Figure 51. Current waveforms in current limit conditions
The inductor current is sensed during the off time TOFF by measuring the voltage drop
across the low-side MOSFET using the RDS(on) as a lossless sensing element (PHASE to
PGND voltage). The voltage drop is compared to the threshold set with ILIM pin. If ILIM is
connected to a voltage higher than VCC-1V the limit is 100mV. A current of 5 µA is sourced
from the pin ILIM; if a resistor is connected between ILIM and ground the current limit is
given by the voltage at the ILIM pin. The device sets the PHASE voltage threshold at 1/10 of
the ILIM voltage.
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Device description
PM6686
Figure 52. Current limit circuit block diagram
5uA
ILIM
+
HGATE
RILIM
PHASE
9R
+
-
R
LGAT E
Ton
Generator
Table 10.
Current limit configuration
Control pin
ILIM1
ILIM2
Control pin voltage
Threshold SET
VILIM = VCC-1 V
100 mV
0.2 V = VILIM = 2 V
VILIM = 5µA * RILIM
VILIM/10
A negative current control is also implemented: the low-side MOSFET is forced off when the
current exceeds the negative limit. This function prevents the excessive negative inductor
current during the PWM operating mode. The threshold is set approximately at the 120% of
the positive current limit.
8.1.8
Soft-start and soft-end
The two sections have independent enable pins, EN1 and EN2. A not programmable softstart procedure takes place when EN pin rises above 2.4 V typ.
To prevent high input inrush currents, the current limit is increased from 25% to 100% with
steps of 25%.
The procedure is not programmable and ends typically in 2.8 ms. The overvoltage protection
is always active while the undervoltage protection is enabled typically 20 ms after the
beginning of the soft-start procedure.
Driving one EN pin below 0.8 V makes the section perform a soft-end: gate driving signals
are pulled low and the output is discharged through an internal MOSFET with RDS(on) of
28 Ω typ.
A power up sequence for the switching sections can be selected connecting one EN pin to
VREF2.
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PM6686
Device description
The section with the EN pin connected to VREF2 begins the soft-start only when the other
section is in regulation (its PGOOD is high) and makes a soft-end suddenly when the other
section is turned off.
Figure 53. VOUT2 behavior if EN2 is connected to VREF2
EN1
VOUT1
PGOOD1
OUT2
To protect the EN1, EN2, EN_LDO and SKIP pin of the PM6686 an external divider or a
series resistor is required, in order to prevent a large inrush current flowing into the device in
case the voltage spike is exceeding the recommended operating conditions.
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Monitoring and protections
9
PM6686
Monitoring and protections
The PM6686 controls its switching output to prevent any damage or uncontrolled working
condition.
The device offers also PGOOD signals to monitor the state of each switching output voltage.
PGOOD is an open drain output: it is pulled low if the output voltage is below the 90% or
above the OVP threshold. of the nominal value.
9.1
Overvoltage protection
PM6686 provides a latched overvoltage protection (OVP). If the output voltage rises above
the +111% typ. for section 1 and above the +116% typ. for the section 2, a latched OVP
protection is activated. The controller tries to pull down the output voltage down to 0 V,
working in PWM. The current is limited by the negative current limit. The low-side MOSFET
is kept on when the output voltage is about 0 V. This management avoids high negative
undervoltage of the output rail that may damage the load.
The protection is latched and this fault is cleared toggling cleared by toggling EN or by
driving PVCC4.025V (PVCC Power On Reset).
9.2
Undervoltage protection
If during regulation the output voltage droops under the 70% of the nominal value, an
undervoltage latched fault is detected. The controller performs a soft-end procedure (see
“soft-start and soft-end” paragraph). The undervoltage fault is cleared by toggling EN or by
driving PVCC4.025V (PVCC power on reset).
9.3
PVCC monitor
The device monitors the driver supply voltage at PVCC pin. The switching sections can start
operating only if the voltage at PVCC pin is above 4,025 V typ. If PVCC falls below 3,979 V
typ., both the switching sections are turned off until the PVCC voltage goes over 4,025 V typ.
Table 11.
Faults management summary
Fault
Condition
Device reaction
Overvoltage
section1
VOUT>+111%
Negative current limit protection activated. Low-side MOSFET is turned on when
the output voltage is about 0 V. Latched fault, cleared by toggling EN or by driving
PVCC4.025V (PVCC POR).
Overvoltage
section 2
VOUT>+116%
Negative current limit protection activated. Low-side MOSFET is turned on when
the output voltage is about 0 V. Latched fault, cleared by toggling EN or by driving
PVCC4.025V (PVCC POR).
Undervoltage
VOUT 2.43 V
> 3.18 V
+3,3 V
Disabled
1.12 Ω
> 2.43 V
< 3.05 V
+3,3 V
Enabled
-
2x VLDOREFIN
Enabled
-
0,35 V < VLDOREFIN < 2,25 V
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Monitoring and protections
9.5
PM6686
Charge pump
The PM6686 can drive an external charge pump circuit whose typical application schematic
is shown in the next figure.
Figure 54. Charge pump application circuit
Vo ut
PHASE
L
COU T
LGAT E
D1
CP_F B
D2
C1
D3
C2
CP
D4
C3
C4
R1
R2
The charge pump works in 4 phases:
1. LGATE is low. C1 is charged through the D1a diode at OUT1 voltage minus the diode
drop.
2. LGATE is driven high and C1 transfers the charge to C2. C2 voltage is OUT1 voltage
plus the LGATE voltage minus the voltage drops on D1 and D2.
3. LGATE is turned low and C2 shares its charge with C3 thought D3.
4. LGATE becomes high and C3 can charge C4 thought diode D4.
Every diode used to transfer charge introduces a voltage drop that decreases the charge
pump output voltage.
Repeating this cycle several times makes the charge pump output voltage equals to:
Equation 6
VCP = VOUT1 + 2VLGATE1 − 4VDIODE
Where VCP is the charge pump output voltage, VOUT1 is the output voltage of the switching
section 1, VLGATE1 is the low-side MOSFET gate driving voltage and VDIODE is the forward
voltage droop of the diodes used in the application.
CP_FB pin must be connected to the output of the charge pump with a resistor divider;
when CP_FB pin droops below 2 V typ., OUT1 controller starts a NA SKIP cycle to boost the
voltage of the charge pump.
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PM6686
Monitoring and protections
The minimum voltage of the charge pump is:
Equation 7
⎛
R ⎞
VCP _ MIN = VCP _ FB ⋅ ⎜⎜1 + 1 ⎟⎟
⎝ R2 ⎠
Where VCP_FB is the minimum voltage of CP_FB pin(2V typ.).
In case the charge pump feedback is not used, leave the CP_FB pin floating or connect the
pin to VCC.
9.6
Voltage references
The PM6686 provides two voltage references.
The device regulates a 3,3 V voltage reference (VREF3) with ±2% accuracy over
temperature. VREF3 can source up to 5 mA. VREF3 voltage is always available if VIN is
applied. The device allows the enabling of the outputs if VREF3 is above 2,8V typ. and turns
off when VREF3 falls under 2,7 V typ.
VREF2 is a + 2 V reference with an accuracy of ±1% over temperature. It can source up to
50 µA typ. and sink up to 10 µA. VREF2is adopted as internal reference; this voltage can be
used as voltage threshold to set configuration pins (e.g. TON, SKIP pins). VREF2 is enabled
when one enable pin (EN1, EN2 or EN_LDO) is pulled high.
9.7
General device fault management
9.7.1
Thermal protection
If the internal temperature of the device exceeds typically +150 °C, the controller shuts down
immediately all the internal circuitry. Switching sections performs the soft-end management.
Toggling EN, EN LDO or cycling VIN resets the latched fault.
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Application information
PM6686
10
Application information
10.1
External components selection
10.1.1
Inductor
Once that switching frequency is defined, inductor selection depends on the desired
inductor ripple current and load transient performance.
Low inductance means great ripple current and could generate great output noise. On the
other hand, low inductor values involve fast load transient response.
A good compromise between the transient response time, the efficiency, the cost and the
size is to choose the inductor value in order to maintain the inductor ripple current ΔIL
between 20% and 50% of the maximum output current ILOAD (max). The maximum ΔIL
occurs at the maximum input voltage. With these considerations, the inductor value can be
calculated with the following relationship:
Equation 8
L=
VIN − VOUT VOUT
×
fsw × ΔIL
VIN
Where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage
and ΔIL is the selected inductor ripple current.
In order to prevent overtemperature working conditions, inductor must be able to provide an
RMS current greater than the maximum RMS inductor current ILRMS:
Equation 9
ILRMS = (ILOAD (max))2 +
(ΔIL (max))2
12
Where ΔIL(max) is the maximum ripple current:
Equation 10
ΔIL (max) =
VIN max − VOUT
V
× OUT
fsw × L
VIN max
If hard saturation inductors are used, the inductor saturation current should be much greater
than the maximum inductor peak current Ipeak:
Equation 11
Ipeak = ILOAD (max) +
ΔIL (max)
2
Using soft-saturation inductors it’s possible to choose inductors with saturation current limit
nearly to Ipeak.
Below there is a list of some inductor manufacturers.
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PM6686
Application information
Table 14.
10.1.2
Inductor manufacturer
Manufacturer
Series
Inductor value (µH)
RMS current (A)
Saturation current (A)
COILCRAFT
MSS-1048
3,3
7,22
7,6
COILCRAFT
MSS-1260
3,3
9,7
7
COILCRAFT
MLC-1550
2,5
16,5
11,4
Input capacitor
In a buck topology converter the current that flows into the input capacitor is a pulsed current
with zero average value. The input RMS current of the two switching sections can be roughly
estimated as follows:
Equation 12
ICinRMS = D1 × I12 × (1 − D1) + D 2 × I22 × (1 − D 2 )
Where D1, D2 are the duty cycles and I1, I2 are the maximum load currents of the two
sections.
Input capacitor should be chosen with an RMS rated current higher than the maximum RMS
current given by both sections.
Tantalum capacitors are good in term of low ESR and small size, but they occasionally can
burn out if subjected to very high current during the charge. Ceramic capacitors have
usually a higher RMS current rating with smaller size and they remain the best choice.
Below there is a list of some ceramic capacitor manufacturers.
Table 15.
10.1.3
Input capacitor manufacturer
Manufacturer
Series
Capacitor value (µF)
Rated voltage (V)
TAYIO YUDEN
UMK325BJ106 KM-T
10
50
TAYIO YUDEN
GMK325BJ106MN
10
35
Output capacitor
The selection of the output capacitor is based on the ESR value and on the voltage rating
rather than on the capacitor value Cout.
The output capacitor has to satisfy the output voltage ripple requirements. Lower inductor
value can reduce the size of the choke but increases the inductor current ripple ΔIL.
Since the voltage ripple VRIPPLEout is given by:
Equation 13
VRIPPLEout = R out × ΔIL
A low ESR capacitor is required to reduce the output voltage ripple. Switching sections can
work correctly even with 15mV output ripple.
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Application information
PM6686
Finally the output capacitor choice deeply impacts on the load transient response. Below
there is a list of some capacitor manufacturers.
Output capacitor manufacturer
Table 16.
10.1.4
Input capacitor manufacturer
Manufacturer
Series
Capacitor value (µF)
Rated voltage (V)
ESR max (mΩ)
SANYO
POSCAP TPB
150 to 330
2.5 to 6.3
35 to 65
SANYO
POSCAP TPF
150 to 470
2.5 to 6.3
7 to 15
MOSFET
Logic-level MOSFETs are recommended, since low-side and high-side gate drivers are
powered by PVCC. Their breakdown voltage (VBRDSS) must be higher than the maximum
input voltage.
In notebook applications, power management efficiency is a high level requirement. The
power dissipation on the power switches becomes an important factor in switching
selections. Losses of high-side and low-side MOSFETs depend on their working conditions.
The power dissipation of the high-side MOSFET is given by:
Equation 14
PDHighSide = Pconduction + Pswitching
Maximum conduction losses are approximately:
Equation 15
Pconduction = RDSon ×
VOUT
× ILOAD (max)2
VIN min
Where RDS(on) is the drain-source on resistance of the high-side MOSFET.
Switching losses are approximately:
Equation 16
Pswitching =
ΔIL
ΔI
) × t on × fsw VIN × (ILOAD (max) + L ) × t off × fsw
2
2
+
2
2
VIN × (ILOAD (max) −
Where ton and toff are the switching times of the turn off and turn off phases of the
MOSFETs.
As general rule, high-side MOSFETs with low gate charge are recommended, in order to
minimize driver losses.
Below there is a list of possible choices for the high-side MOSFETs.
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PM6686
Application information
Table 17.
High-side MOSFET manufacturer
Manufacturer
Type
Gate charge (nC)
RDS(on) (mΩ)
Rated reverse voltage (V)
ST
STS12NH3LL
10
8
30
ST
STS17NH3LL
18
4
30
The power dissipation of the low-side MOSFET is given by:
Equation 17
PDLowSide = Pconduction
Maximum conduction losses occur at the maximum input voltage:
Equation 18
⎛
V
Pconduction = RDSon × ⎜⎜1 − OUT
⎝ VIN max
⎞
⎟ × ILOAD (max)2
⎟
⎠
Choose a synchronous rectifier with low RDS(on). When high-side MOSFET turns on, the
fast variation of the phase node voltage can bring up even the low-side gate through its
gate-drain capacitance CRSS, causing cross-conduction problems. Choose a low-side
MOSFETs that minimizes the ratio CRSS/CGS (CGS = CISS - CRSS).
Below there is a list of some possible low-side MOSFETs.
Table 18.
Low-side MOSFET manufacturer
Manufacturer
Type
RDS(on) (mΩ)
CRSS
C GS
Rated reverse voltage
(V)
ST
STS17NF3LL
5.5
0.047
30
ST
STS25NH3LL
3.5
0.011
30
Dual N-channel MOSFETs can be used in applications with a maximum output current of
about 3 A. Below there is a list of some MOSFETs manufacturers.
Table 19.
Dual MOSFET manufacturer
Manufacturer
Type
RDS(on) (mΩ)
Gate charge
(nC)
Rated reverse voltage
(V)
ST
STS8DNH3LL
25
10
30
ST
STS4DNF60L
65
32
60
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Diode selection
PM6686
11
Diode selection
11.1
Freewheeling diode
A rectifier across the low-side MOSFET is recommended. The rectifier works as a voltage
clamp across the synchronous rectifier and reduces the negative inductor swing during the
dead time between turning the high-side MOSFET off and the synchronous rectifier on. It
can increase the efficiency of the switching section, since it reduces the low-side switch
losses. A Schottky diode is suitable for its low forward voltage drop (0.3 V). The diode
reverse voltage must be greater than the maximum input voltage. A minimum recovery
reverse charge is preferable. Below there is a list of some Schottky diode manufacturers.
Table 20.
11.2
Schottky diode manufacturer
Manufacturer
Series
Forward voltage
(V)
Rated reverse
voltage (V)
Reverse current
(μA)
ST
STPS1L30M
0.34
30
0.00039
ST
STPS1L20M
0.37
20
0.000075
Charge pump diode
The charge pump capacitors are fed by the current supplied by LGATE1 (output of the lowside driver for the section 1). Dual in package diodes, in series configuration, could be used
to reduce the area occupation.
Table 21.
42/50
Schottky diode manufacturer
Manufacturer
Series
Forward voltage
(V)
Rated reverse
voltage (V)
Max forward
current (A)
ST
BAT54S
0.24
40
0.3
ST
BAR43A
0.35
30
0.1
ST
BAS69-04
0.35
15
0.01
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PM6686
Diode selection
11.3
Other important components
11.3.1
VIN filter
A VIN pin low pass filter is suggested to reduce switching noise. The low pass filter is shown
in the next figure:
Figure 55. VIN pin filter
Input
Voltage
VIN
C
Typical component value is: C = 1 µF.
11.3.2
PVCC and VCC
PVCC and VCC are connected with an internal resistor (about 10 Ω); this allows reducing
the external components. Connect the +5 V supply rail only to PVCC.
Use a bypass capacitor on PVCC pin. A VCC low pass filter helps to reject switching
commutations noise, this filter can be implemented simply adding a bypass capacitor on
VCC pin.
Figure 56. VCC and PVCC filters
C2
C1
PVCC
VCC
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Diode selection
PM6686
Typical components values are: C1 = 1 µF and C2 = 1 µF.
11.3.3
VREF2 and VREF3 capacitors
A 10 nF to 100 nF ceramic capacitor on VREF2 pin must be added to ensure noise
rejection. If VREF3 voltage is not used the pin can be left floating, otherwise a 10 nF to 100
nF bypass ceramic capacitor should be mounted.
11.3.4
LDO output capacitors
Bypass the output of the linear regulator with 4,7 µF ceramic capacitor closer to the LDO
pin. In most applicative conditions the ceramic output capacitor can be enough to ensure
stability.
11.3.5
Bootstrap circuit
The external bootstrap circuit is represented in the next figure:
Figure 57. Bootstrap circuit
RBOOT
RBOOT
CBOOT
PHASE
L
The bootstrap circuit capacitor value CBOOT must provide the total gate charge to the highside MOSFET during turn on phase. A typical value is 100 nF.
A resistor RBOOT on the BOOT pin could be added in order to reduce noise when the
phase node rises up, working like a gate resistor for the turn on phase of the high-side
MOSFET. A typical value is RBOOT = 1 Ω.
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12
PCB design guidelines
PCB design guidelines
The layout is very important in terms of efficiency, stability and noise of the system. It is
possible to refer to the PM6686 demonstration board for a complete layout example.
For good PC board layout follows these guidelines:
●
Place on the top side all the power components (inductors, input and output capacitors,
MOSFETs and diodes). Refer them to a power ground plan, PGND. If possible, reserve
a layer to PGND plan. The PGND plan is the same for both the switching sections.
●
AC current paths layout is very critical (see Figure 58). The first priority is to minimize
their length. Trace the LS MOSFET connection to PGND plan as short as possible.
Place the synchronous diode D near the LS MOSFET. Connect the LS MOSFET drain
to the switching node with a short trace.
●
Place input capacitors near HS MOSFET drain. It is recommended to use the same
input voltage plan for both the switching sections, in order to put together all input
capacitors.
●
Place all the sensitive analog signals (feedbacks, voltage references, current sense
paths) on the bottom side of the board or in an inner layer. Isolate them from the power
top side with a signal ground layer, SGND. Connect the SGND and PGND plans only in
one point (a multiple vias connection is preferable to a 0 Ω resistor connection) near the
PGND device pin. Place the device on the top or on the bottom size and connect the
exposed pad and the SGND pins to the SGND plan (see Figure 58).
●
As general rule, make the high-side and low-side drivers traces wide and short.
●
The high-side driver is powered by the bootstrap circuit. It’s very important to place
capacitor CBOOT as near as possible to the BOOT pin (for example on the layer
opposite to the device). Route HGATE and PHASE traces as near as possible in order
to minimize the area between them.
●
The low-side gate driver is powered by PVCC pin. Placing PGND and LGATE pins near
the low-side MOSFETs reduces the length of the traces and the crosstalk noise
between the two sections.
●
The linear regulator outputs are referred to SGND as long as the reference voltages
VREF2 and VREF3. Place their output filtering capacitors as near as possible to the
device.
●
Place input filtering capacitors near PVCC, VCC and VIN pins.
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PCB design guidelines
PM6686
Figure 58. Current paths, ground connection and driver traces layout
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13
Package mechanical data
Package mechanical data
In order to meet environmental requirements, ST offers these devices in different grades of
ECOPACK® packages, depending on their level of environmental compliance. ECOPACK®
specifications, grade definitions and product status are available at: www.st.com.
ECOPACK is an ST trademark.
Table 22.
VFQFPN 5x5x1.0 mm 32 L pitch 0.50 mechanical data
Databook (mm)
Dim.
Min
Typ
Max
A
0.8
0.9
1
A1
0
0.02
0.05
A3
0.2
b
0.18
0.25
D
4.85
5
D2
0.3
5.15
See exposed pad variations
E
4.85
E2
(2)
5
5.15
See exposed pad variations
e
(2)
0.5
L
0.3
0.4
0.5
ddd
Table 23.
0.05
Exposed pad variations
(1)(2)D2
E2
Min
Typ
Max
Min
Typ
Max
2.90
3.10
3.20
2.90
3.10
3.20
1. VFQFPN stands for thermally enhanced very thin fine pitch quad flat package no lead. Very thin:
A = 1.00 mm max.
2. Dimensions D2 and E2 are not in accordance with JEDEC.
Doc ID 15281 Rev 4
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Package mechanical data
PM6686
Figure 59. Package dimensions
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PM6686
14
Revision history
Revision history
Table 24.
Document revision history
Date
Revision
Changes
09-Jan-2009
1
Initial release
26-Feb-2009
2
Updated input voltage range in coverpage
07-May-2009
3
Updated pin 29 description in Table 2 on page 7
23-Jul-2009
4
Updated Table 3 on page 10, Section 8.1.8 on page 32, Section 9.1
on page 34, Section 9.2 on page 34 and Table 11 on page 34
Doc ID 15281 Rev 4
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PM6686
Please Read Carefully:
Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the
right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any
time, without notice.
All ST products are sold pursuant to ST’s terms and conditions of sale.
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