L6911E
5-Bit programmable step down controller
with synchronous rectification
Features
■
Operating supply IC voltage from 5V to 12V
buses
■
Up to 1.3A gate current capability
■
TTL-compatible 5 bit programmable output
compliant with VRM 8.5 :
1.050V to 1.825V with 0.025V binary steps
■
Voltage mode PWM control
■
Excellent output accuracy: ±1% over line and
temperature variations
■
Very fast load transient response: from 0% to
100% Duty Cycle
■
Power good output voltage
■
Overvoltage protection and monitor
■
Overcurrent protection realized using the upper
MOSFET's Rds(ON)
■
200kHz internal oscillator
■
Oscillator externally adjustable from 50kHz to
1MHz
■
Soft start and inhibit functions
Applications
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Power supply for advanced
microprocessor core
■
Distributed power supply
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SO-20
Description
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The device is a power supply controller
specifically designed to provide a high
performance DC/DC conversion for high current
microprocessors. A precise 5 bit digital to analog
converter (DAC) allows to adjust the output
voltage from 1.050 to 1.825 with 25mV binary
steps.
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The high precision internal reference assures the
selected output voltage to be within ±1%. The
high peak current gate drive affords to have fast
switching to the external power mos providing low
switching losses.
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The device assures a fast protection against load
overcurrent and load over-voltage. An external
SCR is triggered to crowbar the input supply in
case of hard overvoltage. An internal crowbar is
also provided turning on the low side mosfet as
long as the over-voltage is detected. In case of
over-current detection, the soft start capacitor is
discharged an the system works in HICCUP
mode.
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Table 1. Device summary
April 2007
Part Number
Package
Packaging
L6911E
TSSOP8
Tube
L6911ETR
TSSOP8
Tape and reel
Rev 3
1/34
www.st.com
34
Contents
L6911E
Contents
1
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
Pin settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
2.1
Pin connection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2
Pin description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4
Electrical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1
Maximum ratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2
Thermal data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
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4.1
5
VID Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
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Device description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1
Oscillator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2
Digital to analog converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.3
Soft start and inhibit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.4
Driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.5
Monitor and protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.6
Inductor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.7
Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
5.8
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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5.9
Compensation network design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
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Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2
Inductor design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.3
Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
6.4
Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
6.5
Over-current protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
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Electrical characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
VRM demo board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1
L6911E
Contents
7
Connector pin orientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
8
PCB and components layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9
Package mechanical data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
10
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
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3/34
Block diagram
1
L6911E
Block diagram
Figure 1.
Block diagram
Vcc 5V to12V
Vin 5V to12V
VCC
PGOOD
OCSET
BOOT
SS
MONITOR and
PROTECTION
UGATE
OVP
RT
OSC
LGATE
VD0
VD1
VD2
VD3
-
D/A
+
+
-
GND
PWM
E/A
COMP
D98IN957
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4/34
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PGND
VD4
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1.050V to 1.825V
PHASE
VSEN
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VFB
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L6911E
Pin settings
2
Pin settings
2.1
Pin connection
Figure 2.
Pin connection (top view)
VSEN
1
20
RT
OCSET
2
19
OVP
SS/INH
3
18
VCC
VID0
4
17
LGATE
VID1
5
16
PGND
VID2
6
15
BOOT
VID3
7
14
UGATE
VID4
8
13
PHASE
COMP
9
12
PGOOD
10
11
GND
FB
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D98IN958
2.2
Pin description
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Table 2. Pin description
N°
Name
1
VSEN
2
OCSET
so
Description
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Connected to the output voltage is able to manage over-voltage conditions and
the PGOOD signal.
A resistor connected from this pin and the upper Mos Drain sets the current
limit protection. The internal 200µA current generator sinks a current from the
drain through the external resistor. The Over-Current threshold is due to the
following equation:
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I OCSET Þ R OCSET
I P = ----------------------------------------------R DSon
SS/INH
The soft start time is programmed connecting an external capacitor from this
pin and GND. The internal current generator forces through the capacitor
10µA.
This pin can be used to disable the device forcing a voltage lower than 0.4V
4-8
VID0 - 4
Voltage Identification Code pins. These input are internally pulled-up and TTL
compatible. They are used to program the output voltage as specified in
Table 6 on page 9 and to set the overvoltage and power good thresholds.
Connect to GND to program a ‘0’ while leave floating to program a ‘1’.
9
COMP
This pin is connected to the error amplifier output and is used to compensate
the voltage control feedback loop.
10
FB
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This pin is connected to the error amplifier inverting input and is used to
compensate the voltage control feedback loop.
5/34
Pin settings
L6911E
Table 2. Pin description (continued)
N°
Name
Description
11
GND
All the internal references are referred to this pin. Connect it to the PCB signal
ground.
12
PGOOD
This pin is an open collector output and is pulled low if the output voltage is not
within the above specified threshlds.
If not used may be left floating.
13
PHASE
This pin is connected to the source of the upper mosfet and provides the return
path for the high side driver. This pin monitors the drop across the upper mosfet
for the current limit.
14
UGATE
High side gate driver output.
15
BOOT
Bootstrap capacitor pin. Through this pin is supplied the high side driver and
the upper mosfet. Connect through a capacitor to the PHASE pin and through
a diode to Vcc (catode vs boot).
16
PGND
Power ground pin. This pin has to be connected closely to the low side mosfet
source in order to reduce the noise injection into the device
17
LGATE
This pin is the lower mosfet gate driver output
18
VCC
Device supply voltage. The operative supply voltage range is from 4.5 to 12V.
DO NOT CONNECT VIN to 12V if VCC IS 5V.
OVP
Over voltage protection. If the output voltage reach the 15% above the
programmed voltage this pin is driven high and can be used to drive an
external SCR that crowbar the supply voltage.
If not used, it may be left floating.
19
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Oscillator switching frequency pin. Connecting an external resistor from this pin
to GND, the external frequency is increased according to the equation:
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6
fS
20
RT
5 ⋅ 10 = 200kHz + ------------------R T ( kΩ )
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Connecting a resistor from this pin to Vcc (12V), the switching frequency is
reduced according to the equation:
7
4 ⋅ 10 f S = 200kHz – ------------------R T ( kΩ )
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6/34
If the pin is not connected, the switching frequency is 200KHz.
The voltage at this pin is fixed at 1.23V. Forcing a 50µA current into this pin, the
built in oscillator stops to switch.
L6911E
Electrical data
3
Electrical data
3.1
Maximum ratings
Table 3. Absolute maximum ratings
Parameter (1)
Symbol
Value
Unit
15
V
VBOOT-VPHASE Boot Voltage
15
V
VHGATE-VPHASE
15
V
-0.3 to Vcc+0.3
V
7
V
6.5
V
Vcc
Vcc to GND, PGND
OCSET, PHASE, LGATE
ROSC, SS, FB, PGOOD, VSEN
COMP, OVP
1. ESD immunity for pins 2 to 9 and 18 to 20 is guaranteed up to 1500V (Human Body Model).
3.2
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Thermal data
Table 4. Thermal data
Symbol
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Parameter
RthJA
Thermal resistance junction to ambient
Tmax
Maximum junction temperature
TSTG
Storage temperature range
TJ
Junction temperature range
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Value
Unit
110
°C/W
150
°C
-40 to 150
°C
0 to 125
°C
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Electrical characteristics
4
L6911E
Electrical characteristics
Table 5. Electrical characteristic (VCC = 12V; TA = 25°C unless otherwise specified)
Symbol
Parameter
Test condition
Min
Typ
Max
Unit
VCC supply current
Icc
Vcc supply current
UGATE and LGATE open
Turn-On VCC threshold
VOCSET = 4.5V
Turn-Off VCC threshold
VOCSET = 4.5V
5
mA
Power-ON
4.6
3.6
Rising VOCSET threshold
ISS
Soft start current
V
1.26
V
10
µA
Oscillator
∆VOSC
Free running frequency
RT = OPEN
180
Total Variation
6 KΩ < RT to GND < 200 KΩ
-15
Ramp amplitude
RT = OPEN
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Reference and DAC
DACOUT voltage accuracy
VID0, VID1,VID2, VID3, VID25mV
see Table 6 on page 9;
TA = 0 to 70°C
VID Pull-Up voltage
Error amplifier
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DC gain
GBWP
SR
Gain-bandwidth product
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Gate drivers
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Pr
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200
od
220
kHz
15
%
Vp-p
1
%
3.1
V
88
dB
15
MHz
10
V/µS
1.3
A
IUGATE
High side source current
VBOOT - VPHASE = 12V,
VUGATE - VPHASE = 6V
RUGATE
High side sink resistance
VBOOT - VPHASE = 12V,
IUGATE = 300mA
ILGATE
Low side source current
VCC = 12V, VLGATE = 6V
RLGATE
Low side sink resistance
Vcc=12V, ILGATE = 300mA
1.5
Output driver dead time
PHASE connected to GND
120
Over voltage trip (VSEN/DACOUT)
VSEN rising
117
120
%
OCSET current source
VOCSET = 4.5V
200
230
µA
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1.9
-1
COMP = 10pF
V
2
0.9
4
1.1
Ω
A
3
Ω
nS
Protections
IOCSET
8/34
170
L6911E
Electrical characteristics
Table 5. Electrical characteristic (VCC = 12V; TA = 25°C unless otherwise specified) (continued)
Symbol
IOVP
Parameter
Test condition
OVP sourcing current
Min
Typ
Max
Unit
VSEN > OVP trip, VOVP = 0V
60
mA
Upper threshold
(VSEN/DACOUT)
VSEN rising
108
110
112
%
Lower threshold
(VSEN/DACOUT)
VSEN falling
88
90
92
%
Hysteresis
(VSEN/DACOUT)
Upper and lower threshold
PGOOD voltage low
IPGOOD = -5mA
Power GOOD
VPGOOD
4.1
VID4
(25mV)
VID3
VID2
VID1
VID0
Output
Voltage (V)
VID4
(25mV)
VID3
VID2
0
0
1
0
0
1.050
0
1
1
1
0
1
0
0
1.075
1
0
0
0
1
1
1.100
0
1
0
0
1
1
1.125
1
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0
0
0
1
0
1.150
1
0
0
1
0
1.175
0
0
0
0
1
1
0
0
0
1
0
0
0
0
1
0
0
0
0
1
1
1
1
1
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0.5
V
VID1
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VID0
Output
Voltage (V)
0
1.450
1
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0
1.475
1
0
1
1
1.500
1
0
1
1
1.525
1
0
1
0
1.550
1
1
0
1
0
1.575
0
1
0
0
1
1.600
1.225
1
1
0
0
1
1.625
1.250
0
1
0
0
0
1.650
0
1.275
1
1
0
0
0
1.675
1
1.300
0
0
1
1
1
1.700
1
1
1.325
1
0
1
1
1
1.725
1
1
0
1.350
0
0
1
1
0
1.750
1
1
1
0
1.375
1
0
1
1
0
1.775
1
1
0
1
1.400
0
0
1
0
1
1.800
1
1
0
1
1.425
1
0
1
0
1
1.825
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0
%
VID Setting
Table 6. VID Setting
1
2
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(s)
1.200
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0
9/34
Device description
5
L6911E
Device description
The device is an integrated circuit realized in BCD technology. It provides complete control
logic and protections for a high performance step-down DC-DC converter optimized for
microprocessor power supply. It is designed to drive N Channel Mosfets in a synchronousrectified buck topology. The device works properly with Vcc ranging from 5V to 12V and
regulates the output voltage starting from a 1.26V power stage supply voltage (Vin). The
output voltage of the converter can be precisely regulated, programming the VID pins, from
1.050V to 1.825V with 25mV binary steps, with a maximum tolerance of ±1% over
temperature and line voltage variations. The device provides voltage-mode control with fast
transient response. It includes a 200kHz free-running oscillator that is adjustable from
50kHz to 1MHz. The error amplifier features a 15MHz gain-bandwidth product and 10V/ms
slew rate which permits high converter bandwidth for fast transient performance. The
resulting PWM duty cycle ranges from 0% to 100%. The device protects against overcurrent conditions entering in HICCUP mode. The device monitors the current by using the
rds(ON) of the upper MOSFET which eliminates the need for a current sensing resistor.
The device is available in SO20 package.
5.1
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Oscillator
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The switching frequency is internally fixed to 200kHz. The internal oscillator generates the
triangular waveform for the PWM charging and discharging with a constant current an
internal capacitor. The current delivered to the oscillator is tipically 50µA (FSW = 200KHz)
and may be varied using an external resistor (RT) connected between RT pin and GND or
VCC. Since the RT pin is maintained at fixed voltage (typ. 1.235V), the frequency is varied
proportionally to the current sinked (forced) from (into) the pin.
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In particular connecting it to GND the frequency is increased (current is sinked from the pin),
according to the following relationship:
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Equation 1
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du
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4.94 ⋅ 10
f S = 200kHz + -----------------------R T ( kΩ )
L6911E
Device description
Connecting RT to VCC = 12V or to VCC = 5V the frequency is reduced (current is forced into
the pin), according to the following relationships:
Equation 2
7
4.306 ⋅ 10 f S = 200kHz + ---------------------------R T ( kΩ )
VCC = 12V
Equation 3
7
15 ⋅ 10 f S = 200kHz + ------------------R T ( kΩ )
VCC = 5V
Switching frequency variations vs. RT are reported in Figure 3 on page 11.
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That forcing a 50µA current into this pin, the device stops switching because no current is
delivered to the oscillator.
Note:
Figure 3.
Switching frequency variations vs. RT
10000
Resistance [kOhm]
1000
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10
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RT to GND
RT to VCC=12V
RT to VCC=5V
10
100
1000
Frequency [kHz]
11/34
Device description
5.2
L6911E
Digital to analog converter
The built-in digital to analog converter allows the adjustment of the output voltage from
1.050V to 1.825V with 25mV binary steps as shown in the previous Table 6: VID Setting on
page 9. The internal reference is trimmed to ensure the precision of 1%.
The internal reference voltage for the regulation is programmed by the voltage identification
(VID) pins. These are TTL compatible inputs of an internal DAC that is realised by means of
a series of resistors rpoviding a partition of the internal voltage reference. The VID code
drives a multiplexer that selects a voltage on a precise point of the divider. The DAC output
is delivered to an amplifier obtaining the VPROG voltage reference (i.e. the set-point of the
error amplifier). Internal pull-ups are provided (realized with a 5µA current generator); in this
way, to program a logic "1" it is enough to leave the pin floating, while to program a logic "0"
it is enough to short the pin to GND.
The voltage identification (VID) pin configuration also sets the power-good thresholds
(PGOOD) and the over- voltage protection (OVP) thresholds.
5.3
Soft start and inhibit
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At start-up a ramp is generated charging the external capacitor CSS by means of a 10µA
constant current, as shown in Figure 4 on page 13
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When the voltage across the soft start capacitor (VSS) reaches 0.5V the lower power MOS is
turned on to discharge the output capacitor. As VSS reaches 1V (i.e. the oscillator triangular
wave inferior limit) also the upper MOS begins to switch and the output voltage starts to
increase.
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The VSS growing voltage initially clamps the output of the error amplifier, and consequently
VOUT linearly increases, as shown in Figure 4 on page 13. In this phase the system works
in open loop. When VSS is equal to VCOMP the clamp on the output of the error amplifier is
released. In any case another clamp on the non-inverting input of the error amplifier remains
active, allowing to VOUT to grow with a lower slope (i.e. the slope of the VSS voltage, see
Figure 4 on page 13). In this second phase the system works in closed loop with a growing
reference. As the output voltage reaches the desired value VPROG, also the clamp on the
error amplifier input is removed, and the soft start finishes. Vss increases until a maximum
value of about 4V.
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The Soft-Start will not take place, and the relative pin is internally shorted to GND, if both
VCC and OCSET pins are not above their own Turn-On thresholds; in this way the device
starts switching only if both the power supplies are present. During normal operation, if any
under-voltage is detected on one of the two supplies, the SS pin is internally shorted to GND
and so the SS capacitor is rapidly discharged.
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The device goes in INHIBIT state forcing SS pin below 0.4V. In this condition both external
MOSFETS are kept OFF.
L6911E
Device description
Figure 4.
Soft start
Vcc Turn-on threshold
Vcc
Vin
Vin Turn-on threshold
1V
Vss
to GND
0.5V
LGATE
Vout
Timing diagram
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Aquisition: CH1 = PHASE; CH2 = VOUT;
CH3 = PGOOD; CH4 = VSS
CH3 = PGOOD; CH4 = VSS
Driver section
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The driver capability on the high and low side drivers allows to use different types of power
MOS (also multiple MOS to reduce the Rds(ON)), maintaining fast switching transition.
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The low-side mos driver is supplied directly by Vcc while the high-side driver is supplied by
the BOOT pin.
Adaptative dead time control is implemented to prevent cross-conduction and allow to use
many kinds of mosfets. The upper mos turn-on is avoided if the lower gate is over about
200mV while the lower mos turn-on is avoided if the PHASE pin is over about 500mV. The
upper mos is in any case turned-on after 200nS from the low side turn-off.
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The peak current is shown for both the upper (Figure 5 on page 14) and the lowr (Figure 6
on page 14) driver at 5V and 12V. a 4nF capacitive load has been used in these
measurements.
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For the lower driver, the source peak current is 1.1A @ VCC = 12V and 500mA @ VCC = 5V,
and the sink peak current is 1.3A @ VCC = 12V and 500mA @ VCC = 5V.
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Similary, for the upper driver, the source peak current is 1.3A @ Vboot-Vphase = 12V and
600mA @ Vboot-Vphase = 5V, and the sink peak current is 1.3A @ Vboot-Vphase = 12V
and 550mA @ Vboot-Vphase = 5V.
13/34
Device description
L6911E
Figure 5.
High side driver peak current, Vboot-Vphase=12V (left) Vboot-Vphase=5V
(right) CH1 = High Side Gate CH4 = inductor current
Figure 6.
Low side driver peak current,
VCC=12V (left) VCC=5V (right)CH1 = Low side gate CH4 = inductor current
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14/34
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L6911E
5.5
Device description
Monitor and protection
The output voltage is monitored by means of pin 1 (VSEN). If it is not within ±10% (typ.) of
the programmed value, the powergood output is forced low.
The device provides overvoltage protection, when the output voltage reaches a value 17%
(typ.) greater than the nominal one. If the output voltage exceed this threshold, the OVP pin
is forced high (5V) and the lower driver is turned on as long as the over-voltage is detected.
The OVP pin is capable to deliver up to 60mA (min) in order to trigger an external SCR
connected to burn the input fuse. The low-side mosfet turn-on implement this function when
the SCR is not used and helps in keeping the ouput low.
To perform the overcurrent protection the device compares the drop across the high side
MOS, due to its
RDSON, with the voltage across the external resistor (ROCS) connected between the
OCSET pin and drain of the upper MOS. Thus the overcurrent threshold (IP) can be
calculated with the following relationship:
Equation 4
I OCS ⋅ R OCS
I P = ------------------------------R DSON
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where the typical value of IOCS is 200µA.
To calculate the ROCS value it must be considered the maximum RDSON (also the variation
with temperature) and the minimum value of IOCS. To avoid undesirable trigger of
overcurrent protection this relationship must be satisfied:
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Equation 5
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∆l
I P ≥ I OUTMAX + ----- = I PEAK
2
where ∆I is the inductance ripple current and IOUTMAX is the maximum output current.
In case of output short circuit the soft start capacitor is discharged with constant current
(10µA typ.) and when the SS pin reaches 0.5V the soft start phase is restarted. During the
soft start the over-current protection is always active and if such kind of event occours, the
device turns off both mosfets, and the SS capacitor is dicharged again after reaching the
upper threshold of about 4V. The system is now working in HICCUP mode, as shown in
Figure 7 on page 16 a. After removing the cause of the over-current, the device restart
working normally without power supplies turn off and on.
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15/34
Device description
Figure 7.
L6911E
Hiccup mode and Inductor ripple current vs. VOUT
9
L=1.5µH, Vin=12V
Inductor Ripple [A]
8
7
L=2µH,
Vin=12V
6
L=3µH,
Vin=12V
5
4
L=1.5µH,
Vin=5V
3
L=2µH,
Vin=5V
2
L=3µH, Vin=5V
1
0
0.5
1.5
2.5
3.5
Output V oltage [V ]
a)
5.6
b)
Inductor design
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The inductance value is defined by a compromise between the transient response time, the
efficiency, the cost and the size. The inductor has to be calculated to sustain the output and
the input voltage variation to maintain the ripple current ∆IL between 20% and 30% of the
maximum output current. The inductance value can be calculated with this relationship:
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Equation 6
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V IN – V OUT V OUT
L = ------------------------------ ⋅ -------------f s ⋅ ∆I L
V IN
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Where fSW is the switching frequency, VIN is the input voltage and VOUT is the output
voltage. Figure 7 b shows the ripple current vs. the output voltage for different values of the
inductor, with vin = 5V and Vin = 12V.
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Increasing the value of the inductance reduces the ripple current but, at the same time,
reduces the converter response time to a load transient. If the compensation network is well
designed, the device is able to open or close the duty cycle up to 100% or down to 0%. The
response time is now the time required by the inductor to change its current from initial to
final value. Since the inductor has not finished its charging time, the output current is
supplied by the output capacitors. Minimizing the response time can minimize the output
capacitance required.
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The response time to a load transient is different for the application or the removal of the
load: if during the application of the load the inductor is charged by a voltage equal to the
difference between the input and the output voltage, during the removal it is discharged only
by the output voltage. The following expressions give approximate response time for ∆I load
transient in case of enough fast compensation network response:
L6911E
Device description
Equation 7
L ⋅ ∆I t application = ----------------------------V IN – V OUT
Equation 8
L ⋅ ∆I
t removal = -------------V OUT
The worst condition depends on the input voltage available and the output voltage selected.
Anyway the worst case is the response time after removal of the load with the minimum
output voltage programmed and the maximum input voltage available.
5.7
Output capacitor
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Since the microprocessors require a current variation beyond 10A doing load transients,
with a slope in the range of tenth A/µsec, the output capacitor is a basic component for the
fast response of the power supply. In fact for first few microseconds they supply the current
to the load. The controller recognizes immediately the load transient and sets the duty cycle
at 100%, but the current slope is limited by the inductor value.
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The output voltage has a first drop due to the current variation inside the capacitor
(neglecting the effect of the ESL):
Equation 9
(s)
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∆VOUT = ∆IOUT · ESR
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A minimum capacitor value is required to sustain the current during the load transient
without discharge it. The voltage drop due to the output capacitor discharge is given by the
following equation:
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Equation 10
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∆I OUT L
∆V OUT = ------------------------------------------------------------------------------------------2 ⋅ C OUT ⋅ ( V INMIN ⋅ D MAX – V OUT )
Where DMAX is the maximum duty cycle value that is 100%. The lower is the ESR, the lower
is the output drop during load transient and the lower is the output voltage static ripple.
17/34
Device description
5.8
L6911E
Input capacitor
The input capacitor has to sustain the ripple current produced during the on time of the
upper MOS, so it must have a low ESR to minimize the losses.
The rms value of this ripple is:
Equation 11
I rms = I OUT D ⋅ ( 1 – D )
Where D is the duty cycle. The equation reaches its maximum value with D = 0.5.
The losses in worst case are:
Equation 12
2
P = ESR ⋅ I rms
5.9
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Compensation network design
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The control loop is a voltage mode (Figure 9 on page 19) that uses a droop function to
satisfy the requirements for a VRM module, reducing the size and the cost of the output
capacitor.
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This method "recovers" part of the drop due to the output capacitor ESR in the load
transient, introducing a dependence of the output voltage on the load current: at light load
the output voltage will be higher than the nominal level, while at high load the output voltage
will be lower than the nominal value.
Figure 8.
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Output transient response without (a) and with (b) the droop function
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18/34
L6911E
Device description
As shown in Figure 8 on page 18, the ESR drop is present in any case, but using the droop
function the total deviation of the output voltage is minimized. In practice the droop function
introduces a static error (Vdroop in Figure 8 on page 18) proportional to the output current.
Since a sense resistor is not present, the output DC current is measured by using the
intrinsic resistance of the inductance (a few mΩ). So the low-pass filtered inductor voltage
(that is the inductor current) is added to the feedback signal, implementing the droop
function in a simple way. Referring to the schematic in Figure 9, the static characteristic of
the closed loop system is:
Equation 13
R3 + R8 // R9 R L ⋅ R8 // R9
V OUT = V PROG + V PROG ⋅ ------------------------------------- – ---------------------------------- ⋅ I OUT
R8
R2
Where VPROG is the output voltage of the digital to analog converter (i.e. the set point) and
RL is the inductance resistance. The second term of the equation allows a positive offset at
zero load (∆V+); the third term introduces the droop effect (∆VDROOP). Note that the droop
effect is equal the ESR drop if:
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Equation 14
R L ⋅ R8 // R9
--------------------------------- = ESR
R8
Figure 9.
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Compensation network
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IN
V
L2
PHASE
R
L
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OUT
PW M
C18
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C 6 -1 5
C 20
R4
R9
C 25
R3
V
ESR
R8
ZI
PROG
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19/34
Device description
L6911E
Considering the previous relationships R2, R3, R8 and R9 may be determined in order to
obtain the desired droop effect as follow:
●
Choose a value for R2 in the range of hundreds of KΩ to obtain realistic values for the
other components.
●
From the above equations, it results:
Equation 15
+
∆V ⋅ R2 R L ⋅ I MAX
R8 = ----------------------- ⋅ -------------------------V PROG ∆V DROOP
Equation 16
∆V DROOP
1
R9 = R8 ⋅ -------------------------- ⋅ -----------------------------------R L ⋅ I MAX
∆V DROOP
1 + -------------------------R L ⋅ I MAX
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Where IMAX is the maximum output current.
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The component R3 must be chosen in order to obtain R3
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