A8585
Wide Input Voltage, 2 A Buck Regulator Family with Low IQ Mode
FEATURES AND BENEFITS
DESCRIPTION
• Automotive AEC-Q100 qualified
• Withstands surge input to 40 VIN for load dump
• Operates down to 3.4 VIN (typ), 3.6 VIN (max) for idle stop
• Uses pulse frequency modulation (PFM) for low IQ mode
• Function options:
– Selectable PWM / Low IQ PFM mode, or
– Selectable 10 μA Sleep mode (automatic PWM / Low IQ
PFM mode selection)
• Fixed output voltage options: 3.3 V or 5 V with ±1.0%
accuracy
• Delivers up to 2 A of output current
• Integrated 110 mΩ high-side MOSFET
• Adjustable switching frequency from 300 to 550 kHz (to
605 kHz with sync)
• EMI Reduction Features:
– Frequency dithering
– Controlled switching node
• External synchronization capability
• Active low NPOR output with 7.5 ms delay
• Pre-bias startup capable: VOUT increases monotonically,
The A8585 family is designed to provide the power supply
requirements of next generation car audio and infotainment
systems. The A8585 family provides all the control and
protection circuitry to produce a high current regulator with
±1% output voltage accuracy.
Continued on the next page…
Continued on the next page…
PACKAGE: 10-pin SOIC with exposed
thermal pad (suffix LK)
APPLICATIONS:
In PWM mode, the A8585 family employs current mode control
to provide simple compensation, excellent stability, and fast
transient response. In Low IQ mode, the A8585 family employs
pulse frequency modulation (PFM) to draw less than 33 µA
from 12 VIN while supplying 5 V/40 µA. When operational,
the A8585 family operates down to at least 3.6 VIN (VIN
falling). The selectable Sleep mode feature allows for very
low standby current.
Features of the A8585 family include a programmable PWM
switching frequency. The regulator switching frequency can
be synchronized to an external clock. The A8585 has external
compensation to optimize stability and transient response for
a wide range of external components and applications. The
A8585 has a fixed soft start time of 5 ms.
• Automotive:
▫ Instrument Clusters
▫ Audio Systems
▫ Navigation
▫ HVAC
• Home audio
• Network and telecom
• Industrial
Not to scale
VIN
1
CIN
2
[A8585-2, -3
only]
8
LO
VIN
SW
A8585-x
VIN
BOOT
GND
VOUT
VOUT
9
10
C BOOT
47 nF
D1
2 A/40 V
CO
5
3 PWM/PFM [A8585, -1]
EN/SLEEP [A8585-2, -3]
SYNC
4
7
RFSET
CP
RZ
R PU
FSET /SYNC PWM
NPOR
6
NPOR
COMP
See Table 2 for recommended component values
CZ
Typical Application Diagram
A8585-DS, Rev. 5
MCO-0000844
April 30, 2021
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
FEATURES AND BENEFITS (continued)
DESCRIPTION (continued)
will not cause a reset
• External compensation for maximum flexibility
• Stable with ceramic or electrolytic output capacitors
• Internally fixed soft start time of 5 ms
• Pulse-by-pulse current limit, hiccup mode short circuit, and
thermal protections
• Pin open/short and component fault tolerant
• -40°C to 150°C operating junction temperature range
• Thermally enhanced SOIC-10 surface mount package
Extensive protection features of the A8585 include pulse-by-pulse
current limit, hiccup mode short circuit protection, open/short
asynchronous diode protection, BOOT open/short voltage protection,
VIN undervoltage lockout, and thermal shutdown.
Selection Guide
Part Number
Packing*
A8585KLKTR-T
3000 pieces per 13-in. reel
A8585KLKTR-T-1
3000 pieces per 13-in. reel
A8585KLKTR-T-2
Contact factory for availability
A8585KLKTR-T-3
Contact factory for availability
*Contact Allegro™ for additional packing options
The A8585 is supplied in a 10-pin SOIC package (suffix LK)
with exposed power pad. It is lead (Pb) free, with 100% matte-tin
leadframe plating.
Output Voltage Option
5 V compatible
3.3 V compatible
5 V compatible
3.3 V compatible
Function Option
Selectable PWM / Low IQ PFM
Selectable Sleep (automatic
PWM / Low IQ PWM selection)
Absolute Maximum Ratings*
Characteristic
Input Voltage (VIN pins)
Switching Node Voltage (SW pin)
BOOT Pin Voltage
VOUT Pin Voltage
Symbol
Notes
VIN
VSW
VBOOT
VOUT
Rating
Unit
–0.3 to 40
V
–0.3 to VIN + 0.3
V
–1.0 to VIN + 3
V
Continuous
VSW – 0.3 to
VSW + 5.5
V
BOOT pin overvoltage fault condition
VSW – 0.3 to
VSW + 7
V
–0.3 to 5.5
V
–0.3 to 7
V
Continuous; rating is a function of temperature
t < 50 ns
Continuous
VOUT pin overvoltage fault condition
PWM/PFM Pin Voltage
VPWM/PFM
A8585, A8585-1
–0.3 to VIN + 0.3
V
EN/SLEEP Pin Voltage
VEN/SLEEP
A8585-2, A8585-3
–0.3 to VIN + 0.3
V
All other pins
Maximum Junction Temperature
Storage Temperature
–0.3 to 5.5
V
TJ(max)
150
ºC
Tstg
–55 to 150
ºC
*Operation at levels beyond the ratings listed in this table may cause permanent damage to the device. The Absolute Maximum ratings are stress
ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the Electrical Characteristics table is not
implied. Exposure to Absolute Maximum-rated conditions for extended periods may affect device reliability.
Thermal Characteristics: May require derating at maximum conditions; see Power Dissipation and Thermal Calculations section
Characteristic
Package Thermal Resistance
Symbol
RθJA
Test Conditions*
Value
Unit
35
ºC/W
On 4-layer PCB based on JEDEC standard
*Additional thermal information available on the Allegro website.
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
2
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
Table of Contents
Specifications
1
1
2
2
2
4
5
6
10
Functional Description
15
15
15
15
16
16
16
16
16
16
17
17
17
17
18
19
19
Typical Application Diagram
Ordering Selection Guide
Absolute Maximum Ratings
Thermal Characteristics
Functional Block Diagram
Pinout Diagrams and Terminal List
Electrical Characteristics
Characteristic Performance
Overview
Reference Voltage
PWM Switching Frequency
EN/SLEEP Input (for A8585-2 and A8585-3)
PWM/PFM Input (for A8585 and A8585-1)
PWM Synchronization
Transconductance Error Amplifier
Slope Compensation
Current Sense Amplifier
Power MOSFETs
BOOT Regulator
Pulse Width Modulation (PWM) Mode
Low-IQ Pulse Frequency Modulation (PFM) Mode
Soft Start (Startup) and Inrush Current Control
Pre-Biased Startup
Active Low Power-On Reset (NPOR) Output
Protection Features
Undervoltage Lockout (UVLO)
Pulse-by-Pulse Overcurrent Protection (OCP)
Overcurrent Protection (OCP) and Hiccup Mode
BOOT Capacitor Protection
Asynchronous Diode Protection
Overvoltage Protection (OVP)
Pin-to-Ground and Pin-to-Pin Short Protections
Thermal Shutdown (TSD)
19
19
20
20
20
21
21
21
21
Design and Component Selection
PWM Switching Frequency (RFSET)
Output Inductor (LO )
Output Capacitors
Low-IQ PFM Output Voltage Ripple Calculation
Input Capacitors
Asynchronous Diode (D1)
Bootstrap Capacitor
Compensation Components (RZ, CZ , and CP)
A Generalized Tuning Procedure
23
23
23
24
25
26
27
27
27
30
Power Dissipation and Thermal Calculations
31
PCB Component Placement and Routing
32
Package Drawing
34
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
3
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
VIN
VOUT
VIN
LDO
LDO OFF
DELAY
Digital
REGOK
+
224us↓
400mV
F>200K Hz
F
F/2
F/4
PWM /
PFM
BOOT
+
ISENSE
GCSA
blankOn
minOff
FB < 0.4V
+
FB < 0.2V
timer hold off
–
BOOT
FAULT
–
IRAM P
FSET/
SYNC PWM
A8585 and
A8585-1 only
BOOT
OFF
2.90V
A8585-2 and
A8585-3 only
1.22V
1.16V
2V, 4.1V
1.2V
OC
250mA
EN
B OOT
RE G
Q
BG
–
B OOT
OFF
UVLO
+
+
EN/
SLEEP
–
3.8V
3.4V
VOUT ↑>3.1V
REGOK
openBIAS
BOOT REG .
5.0V
VREG
S
Q
R
Q
TG
Q
SW
Current
Comp
VREG
BG
DELAY
2048 ↓
PFM
Controller
BOOT
> 4.1V
PFMready
DIODEOK
COMPLO
FBHI
Calibration
3.5A
CLAMP
ERROR
AMP
5ms
OCL
800mV
VOUT
COMP
PULL DOWN
VOUT Z
Check
P FMready
FB
FB>880mV
OCL
DIODEOK
BOOT FAULT
BOOT OFF
FAULT
LOGIC
800mA
CLAMP
1K
COMPLO
openBIAS
PULL DOWN
UVLO
REGOK
(See Fault
Table )
voutOpen
TSD
FB>880mV
NPOR
DELAY
120↑
7.5ms↓
FB tSS , OCP pulses
−
120
−
counts
Hiccup BOOT Shorted Count
BOOTUV
−
64
−
counts
Hiccup BOOT Open Count
BOOTOV
−
7
−
counts
PWM/PFM Pin Input Thresholds
PWM Threshold (High)
PFM Threshold (Low)
PWM/PFM Hysteresis
PWM/PFM Pin Input Resistance
PWM/PFM Turn-Off Delay
VIH
VIL
VHYS
A8585
4.5 V < VOUT < 5.5 V,
VPWM/PFM rising
−
−
2.6
V
A8585-1
3.0 V < VOUT < 3.6 V,
VPWM/PFM rising
−
−
2.0
V
A8585
4.5 V < VOUT < 5.5 V,
VPWM/PFM falling
1.2
−
−
V
A8585-1
3.0 V < VOUT < 3.6 V,
VPWM/PFM falling
0.8
−
−
V
A8585
4.5 V < VOUT < 5.5 V, VIH – VIL
−
400
−
mV
A8585-1
3.0 V < VOUT < 3.6 V, VIH – VIL
−
200
−
mV
120
200
280
kΩ
−
2048
−
counts
RIN
A8585,
A8585-1
tdLO_IQ
A8585,
A8585-1
From PWM/PFM transitioning low,
or NPOR transitioning high, to start
Low IQ mode
Continued on the next page…
1Negative
current is defined as coming out of the node or pin, positive current is defined as going into the node or pin.
limited depending on input voltage, output voltage, duty cycle, regulator load currents, PCB layout, and airflow.
3Configured as shown in Typical Application diagram.
4Ensured by design and characterization, not production tested.
5At 0ºC < T < 85ºC, ensured by design and characterization, not production tested.
J
2Thermally
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
8
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
ELECTRICAL CHARACTERISTICS1 (continued): Valid at 4.0 V ≤ VIN ≤ 35 V, −40°C ≤ TA = TJ ≤ 150ºC, unless
otherwise specified
Characteristic
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
155
170
−
ºC
−
20
−
ºC
5.37
5.5
5.75
V
−
-60
−
mV
4.45
4.62
4.75
V
−
60
−
mV
3.54
3.63
3.75
V
−
-40
−
mV
2.93
3.05
3.14
V
VOUT rising, relative to VNPORUV
−
40
−
mV
VOUT falling beyond VNPOROV
−
120
−
counts
Thermal Protection
Thermal Shutdown Threshold (Rising)
Thermal Shutdown Hysteresis
TSDth
PWM stops immediately, COMP is pulled low,
and soft start is reset
TSDHYS
Power On Reset (NPOR) Output
NPOR Overvoltage Threshold, 5.0V
NPOR Overvoltage Hysteresis, 5.0V
NPOR Undervoltage Threshold, 5.0V
NPOR Undervoltage Hysteresis, 5.0V
NPOR Overvoltage Threshold, 3.3V
NPOR Overvoltage Hysteresis, 3.3V
NPOR Undervoltage Threshold, 3.3V
NPOR Undervoltage Hysteresis, 3.3V
NPOR Overvoltage Delay
NPOR Delay to Rising Edge
VNPOROV
(5.0V)
VNPOROV
(HYS)(5.0V)
VOUT rising
VOUT falling, relative to VNPOROV
VNPORUV(5.0V) VOUT falling
VNPORUV
(HYS)(5.0V)
VOUT rising, relative to VNPORUV
VNPOROV(3.3V) VOUT rising
VNPOROV
(HYS)(3.3V)
VOUT falling, relative to VNPOROV
VNPORUV(3.3V) VOUT falling
VNPORUV
(HYS)(3.3V)
tdPOV_POR
VOUT rising only
5
7.5
10
ms
NPOR Low Output Voltage
VPOROL
INPOR = 4 mA
−
200
400
mV
NPOR Leakage
ILKGPOR
VNPOR = 5.5 V
−
−
1.2
µA
tdPOR
EN/SLEEP Pin Input Thresholds
EN/SLEEP Threshold (High)
VSLEEPVIH
A8585-2
A8585-3
VEN/SLEEP rising
−
1.3
2.1
V
EN/SLEEP Threshold (Low)
VSLEEPVIL
A8585-2
A8585-3
VEN/SLEEP falling
0.5
1.2
−
V
tdSLEEP
A8585-2
A8585-3
VEN/SLEEP transitioning low
115
224
400
µs
IBIASSLEEP
A8585-2
A8585-3
VEN/SLEEP = 5 V
−
500
−
nA
EN/SLEEP Delay
EN/SLEEP Input Bias Current
1Negative
current is defined as coming out of the node or pin, positive current is defined as going into the node or pin.
limited depending on input voltage, output voltage, duty cycle, regulator load currents, PCB layout, and airflow.
3Configured as shown in Typical Application diagram.
4Ensured by design and characterization, not production tested.
5At 0ºC < T < 85ºC, ensured by design and characterization, not production tested.
J
2Thermally
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
9
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
CHARACTERISTIC PERFORMANCE
Output Voltage versus Temperature
Switching Frequency versus Temperature
600
Oscillator Frequency, fOSC (kHz)
Output Voltage, VOUT (V)
5.5
5.0
VOUT = 5.0 V
4.5
4.0
3.5
VOUT = 3.3 V
3.0
2.5
2.0
-50
-25
0
25
50
75
100
125
150
175
550
fOSC = 550 kHz
500
450
400
fOSC = 415 kHz
350
300
fOSC = 300 kHz
250
200
-50
-25
0
Temperature (°C)
75
100
125
150
175
Pulse-by-Pulse Current Limit at tON(MIN)
versus Temperature
3.55
3.9
3.50
3.8
VIN Undervoltage Lockout Start Threshold, VUVLO(ON)
3.45
3.7
ILIM(TONMIN) (A)
Input Voltage, VIN (V)
50
Temperature (°C)
VIN UVLO Start and Stop Thresholds
versus Temperature
3.6
3.5
VIN Undervoltage Lockout Stop Threshold, VUVLO(OFF)
3.4
3.40
3.35
3.30
3.25
3.3
-50
-25
0
25
50
75
100
125
150
175
-50
-25
0
25
50
75
100
125
150
175
Temperature (°C)
Temperature (°C)
Error Amplifier Transconductance
versus Temperature
Error Amplifier Transconductance
versus Temperature
200
Transconductance, gm (µA/V)
140
Transconductance, gm (µA/V)
25
120
VIN > 2.5 V
100
80
VIN < 2.5 V
60
40
VOUT = 5.0 V
20
0
180
160
120
VIN < 1.65 V
100
80
60
-25
0
25
50
75
100
Temperature (°C)
125
150
175
VOUT = 3.3 V
40
20
0
-50
VIN > 1.65 V
140
-50
-25
0
25
50
75
100
125
150
175
Temperature (°C)
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
www.allegromicro.com
10
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
PWM/PFM High and Low Voltage Thresholds
versus Temperature
1.90
VOUT = 5 V
1.85
VIH
1.80
1.75
1.70
VIL
1.65
1.60
-50
-25
0
25
50
PWM/PFM High and Low Voltage Thresholds
versus Temperature
1.30
PWM/PFM Pin Input Voltage (V)
PWM/PFM Pin Input Voltage (V)
A8585
75
100
125
150
1.28
VOUT = 3.3 V
VIH
1.26
1.24
1.22
1.20
1.18
VIL
1.16
1.14
175
-50
-25
0
25
Temperature (°C)
NPOR Overvoltage and Undervoltage Thresholds
versus Temperature
3.6
NPOR Pin Voltage (V)
NPOR Pin Voltage (V)
100
125
150
175
3.8
5.6
VNPOROV(5.0V)
5.4
5.2
5.0
4.8
VNPORUV(5.0V)
4.6
4.4
VOUT = 5 V
4.2
-50
-25
VNPOROV(3.3V)
3.4
3.2
3.0
VNPORUV(3.3V)
2.8
2.6
2.4
VOUT = 3.3 V
2.2
0
25
50
75
100
125
150
2.0
175
-50
-25
0
25
Temperature (°C)
IOUT = 2 mA
200
150
100
50
0
-50
-25
0
25
50
75
Temperature (°C)
100
125
150
175
NPOR Delay to Rising Edge tdPOR (ms)
300
250
50
75
100
125
150
175
Temperature (°C)
NPOR Low Output Voltage versus Temperature
NPOR Pin Output Voltage (mV)
75
NPOR Overvoltage and Undervoltage Thresholds
versus Temperature
5.8
4.0
50
Temperature (°C)
NPOR Time Delay versus Temperature
8.20
8.15
8.10
8.05
8.00
7.95
7.90
7.85
7.80
7.75
-50
-25
0
25
50
75
100
125
150
175
Temperature (°C)
Allegro MicroSystems
955 Perimeter Road
Manchester, NH 03103-3353 U.S.A.
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11
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
DROPOUT OPERATION – TYPICAL AND WORST-CASE OPERATION
Typical Output Voltage versus Input Voltage at High Ambient
at 100%, 66%, and 33% Load
8
Output Voltage, VOUT (V)
7
6
5
IOUT = 0.67 A
4
IOUT = 2.00 A
IOUT = 1.33 A
3
2
1
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
Input Voltage, VIN (V)
Worst Case Output Voltage versus Input Voltage at High Ambient
at 100%, 66%, and 33% Load
8
Output Voltage, VOUT (V)
7
6
5
IOUT = 0.67 A
4
IOUT = 2.00 A
IOUT = 1.33 A
3
2
1
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
Input Voltage, VIN (V)
8.5
9.0
9.5
10.0
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12
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
UVLO or other faults
2048
STATE
OFF
SS
2048
PWM
LOW IQ PFM
PWM
2048
PWM
LOW IQ PFM
PWM
LOW IQ PFM
OFF
UVLO
PWM/PFM
(MODE)
VIN
FOSC/2
FOSC/4
Fosc
Pulse
Skipping
at Fosc
Fosc
Fosc
SW
VOUT<
VOUTHI
VOUT
VOUT<
NPORUV
VOUT<
VREGOK
L
M
IL_PK > IPEAK,LO_IQ
PWM/PFM is low and
IL_PK < IPEAK,LO_IQ
100mA
100mA
No Load
IOUT
COMP pulled up to set IPEAK,LO_IQ
COMP
tdPOR
NPOR
A BC
D
E
F
G
H
I
J K
Figure 1: PWM/PFM Mode Timing Diagram
A
The output voltage reaches regulation voltage after soft start startup. The A8585 family always starts in PWM mode, independent of
the PWM/PFM signal.
B
NPOR transitions high after NPOR delay (7.5 ms (typ)).
C
PWM/PFM signal is logic low, so the device enters Low IQ PFM mode after NPOR transitions high and 2048 clock cycles expire.
D
The output current increases and, even though PWM/PFM is low, the device transitions to PWM mode to maintain optimal regulation.
E
PWM/PFM transitions low after transitioning high, 2048 clock cycles occur, but the device stays in PWM mode because the output
current is too high.
F
The output current decreases and PWM/PFM has been low for a relatively long time, so the device enters Low IQ PFM mode.
G
PWM/PFM transitions high so the device is forced into PWM mode immediately, independent of the load current. If the output is at
no load condition, then the device starts pulse skipping with COMP hovering around the 400mV pedestal.
H
The output current increases from no load condition, and the device stops pulse skipping and starts switching again at fOSC .
I
The output current is low, PWM/PFM transitions low, and 2048 clock cycles later, the device enters Low IQ PFM mode due to low
current.
J
VIN has been removed but the circuit is still powered from the VOUT pin. In addition, all faults are ignored while VOUT > VOUTHI.
K
When VOUT drops below VOUTHI (1% above the regulation point) all faults are checked. UVLO is active, so the COMP pin is pulled low
and switching is disabled.
L
When VOUT drops below the VNPORUV threshold, NPOR transitions low.
M
When VOUT drops below VREGOV , the device is completely turned off.
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955 Perimeter Road
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13
REGOK
NPOR
PFM Ready
BIAS Connected
Figure 2: Operation with PWM/PFM = 0
COMP
VOUT
SW
VIN
TSD
DIODE/SW
FAULT
BOOT
FAULT
OC
FAULT
OC
HIC_EN
HICCUP
f SW/4
then
f SW/2
MODE OFF 5ms SS
f SW
Tcal
2048
7.5ms
5ms SS
REGOK=0
f SW/4
Vout shorted to GND
PWM
x120
f SW/4
O 20ms 5ms
C HICCUP SS
x120
f SW/4
then
f SW/2
O 20ms
5ms SS
C HICCUP
Tcal
2048
7.5ms
f SW
PWM
PFM
OVP
LS
LS
SW & SW &
HSPD HSPD
Light OVP
120
LS
SW &
HSPD
Heavy OVP
Tcal
2048
7.5ms
f SW
PWM
BOOT, DOIDE,
or SW FAULTS
PFM
Wakeup and
Check Faults
x1
x7 OV
x120 UV
20ms 5ms
HICCUP SS
x1
x7 OV
x120 UV
f SW/4
then
f SW/2
20ms
5ms SS
HICCUP
Tcal
2048
7.5ms
f SW
PWM
f SW
f SW
TSD SS TSD SS TSD 5ms SS
REGOK=0
Tcal
2048
7.5ms
f SW
f SW/4
then
f SW/2
5ms SS
REGOK=0
FSET Open
or Short
PWM
Tcal
2048
7.5ms
f SW
VOUT Pin
Open
0 to 1.5V at
32x period
VOUT Open
REGOK=0
PWM
f SW/4
then
f SW/2
5ms SS
Tcal
2048
7.5ms
f SW
PWM
A8585
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
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Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
FUNCTIONAL DESCRIPTION
The A8585 family are current mode, buck regulators that incorporate all the control and protection circuitry necessary to provide
the power supply requirements of car audio and infotainment
systems.
The A8585 family consists of four parts:
Part Number
PWM Mode
Selection*
10 µA Sleep
Mode
Output Voltage
(V)
A8585
yes
no
5.0
A8585-1
yes
no
3.3
A8585-2
(automatic)
yes
5.0
A8585-3
(automatic)
yes
3.3
*A synchronization signal on the FSET pin, or a heavy output load
current will force all variants into PWM mode.
The A8585 family has three modes of operation:
• Pulse width modulation (PWM) mode, delivering up to 2.0 A.
(All variants)
• Low-IQ pulse frequency modulation (PFM) mode, drawing only
approximately 10 µA from VIN while maintaining VOUT (at no
load). Under most conditions, Low-IQ PFM mode is typically
capable of supporting up to approximately 100 mA, depending
on applications. (All variants)
• For the A8585-2 and A8585-3 variants, a third mode of operation is entered when the EN/SLEEP pin is set to 0. The device
enters an ultra-low current, shutdown mode. VOUT = 0 V and the
total current drawn from VIN is less than 10 µA (typ).
In PFM mode, the device operates with lower switching frequency to achieve higher efficiency at light load. When the load
is heavy, the device automatically transitions into PWM mode to
support a relatively higher current. For the A8585 and A8585-1
variants, setting the PWM/PFM pin high disables PFM operation mode even at light load and forces the device to operate in a
PWM constant frequency mode.
The A8585 family is designed to support up to 2.0 A output.
However, the exact amount of current it will supply, before possible thermal shutdown, depends heavily on duty cycle, ambient
temperature, airflow, PCB layout, and PCB construction. Figure 3
shows calculated current ratings versus ambient temperature
for VIN = 12 V/VOUT = 3.3 V and for VIN = 12 V/VOUT = 5.0 V
at 300 kHz and 550 kHz. This analysis assumed a 4-layer PCB
according to the JEDEC standard (34°C/W), no nearby heat
sources, and no airflow.
Reference Voltage
The A8585 family incorporates an internal reference. The accuracy of the internal reference is ±1.0% for TJ from 0°C to 85°C,
and ±1.5% from −40°C to 150°C. The output voltage from the
device regulator is directly connected to the VOUT pin and is
divided down internally to 800 mV by an internal resistor divider
inside the regulator, as shown in the Functional Block diagram.
The A8585 and A8585-2 variants have a fixed 5.0 V output voltage; the A8585-1 and A8585-3 variants have a fixed 3.3 V output
voltage.
PWM Switching Frequency
The PWM switching frequency of the A8585 family is adjustable
from 300 to 550 kHz and has an accuracy of ±10% over the operating temperature range. The switching frequency is dithered to
help reduce EMI between -7.5% and 7.5% according to a random
sequence.
During startup, the PWM switching frequency changes from
25%, to 50%, and finally 100% of fSW as VOUT rises from 0 V
to the regulation voltage. The startup switching frequency is
described in detail in the Soft Start section of this datasheet.
2.1
2.0
1.9
1.8
Current Rating (A)
Overview
1.7
1.6
1.5
1.4
1.3
12Vin/5Vout/300kHz
1.2
12Vin/3.3Vout/300kHz
1.1
12Vin/5Vout/550kHz
1.0
12Vin/3.3Vout/550kHz
0.9
75
85
95
105
Ambient Temperature (°C)
115
125
Figure 3: Typical Current Derating
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A8585
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
If the regulator output is shorted to ground, VFB ≈ 0 V and the
PWM frequency is 25% of fSW . In this case, the low switching
frequency allows extra off-time between SW pulses. The extra
off-time allows the inductor current to remain under control
(remains well above 0) before the next SW pulse occurs. This
prevents the inductor current from ratcheting up, or rising, to a
value that could damage the device or the output inductor.
width, duty-cycle, and rise/fall time requirements shown in the
Electrical Characteristics table in this datasheet. During synchronization, frequency dithering is disabled.
The FSET pin includes protection features that disable the regulator when the FSET pin is shorted to GND, shorted High (to
NPOR), or when RFSET is mis-selected.
The 8585 synchronizes to the SYNC input when the FSET/
SYNCPWM pin is driven above the 1.2 V threshold. Synchronization must occur within 16 μs or else a fault may be declared
causing SW to halt operation. The 8585 will transition back to
using the RFSET resistor after a rising has not crossed the 1.2V
threshold for ~8 μs, resulting in the high side switch remaining
off for ~8 μs and causing some VOUT droop.
EN/SLEEP Input (for A8585-2 and A8585-3)
Transconductance Error Amplifier
The A8585-2 and A8585-3 variants have the EN/SLEEP logic
level input pin. To enable the device, the EN/SLEEP pin must be
a logic high (>2.1 V). The EN/SLEEP pin is rated to 40 V so this
EN/SLEEP pin may be connected directly to VIN if there is no
suitable logic signal available to wake up the regulator.
When EN/SLEEP transitions low, the device waits approximately
224 µs before shutting down. This delay provides plenty of filtering to prevent the device from prematurely entering Sleep mode
because of any small glitches that might couple onto the PCB
trace or EN/SLEEP pin.
PWM/PFM Input (for A8585 and A8585-1)
The PWM/PFM pin provides two major functions. This pin is a
control input that sets the operating mode. If PWM/PFM is logic
high the device operates only in PWM mode. If PWM/PFM is
logic low the device operates in Low-IQ PFM mode if two conditions are met: (1) the regulator is lightly loaded and (2) there
is no clock signal applied to the FSET/SYNCPWM input pin. If
PWM/PFM transitions from logic high to logic low, the device
checks that VSS > 2.3 V and NPOR = 1. If these two conditions
are satisfied, then the device will wait 2048 clock cycles and
then enter into Low IQ PFM mode. This delay provides sufficient
filtering to prevent the regulator from prematurely entering PFM
mode because of any small glitches that might couple onto the
PCB trace or PWM/PFM pin.
PWM Synchronization
If an external clock is applied to the FSET/SYNCPWM pin, the
device is forced into PWM mode and synchronizes its PWM
frequency to the external clock. Synchronization is independent
of Rfset it only needs to satisfy the >200 kHz requirement. When
synchronizing, the external clock pulses must satisfy the pulse
The transconductance error amplifier's primary function is to
control the regulator output voltage. The error amplifier is shown
in the Functional Block diagram. It is shown as a three-terminal
input device with two positive inputs and one negative input. An
on-chip resistor divider is included. The negative input is simply
connected to the internal resistor divider and is used to sense
the feedback voltage for regulation. The two positive inputs are
used for soft start and steady-state regulation. The error amplifier
performs an analog OR selection between the two positive inputs.
The error amplifier regulates to either the internal soft start voltage or the device internal reference, whichever is lower.
To stabilize the regulator, a series RC compensation network
(RZ and CZ) must be connected from the error amplifier output
(the COMP pin) to GND, as shown in the Typical Application
diagram. In most instances an additional, relatively low value
capacitor (CP) should be connected in parallel with the RZ-CZ
components to reduce the loop gain at very high frequencies.
However, if the CP capacitor is too large, the phase margin of the
converter can be reduced. Calculating RZ, CZ, and CP is covered
in detail in the Component Selection section of this datasheet.
If a fault occurs or the regulator is disabled, the COMP pin
is pulled to GND via approximately 1 kΩ and SW switching
is inhibited.
Slope Compensation
The A8585 family incorporates internal slope compensation (SE )
to allow PWM duty cycles above 50% for a wide range of input/
output voltages and inductor values. The slope compensation signal is added to the sum of the current sense amplifier output and
the PWM ramp offset. As shown in the Electrical Characteristics
table, the amount of slope compensation scales with the nominal
switching frequency (fSW) set by RFSET . The amount of slope
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Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
compensation scales with the SYNC input frequency centered
around 0.35 A/μs at 425 kHz in a similar way with RFSET.
of how each type of boot fault is handled by the A8585 are shown
in Table 1.
The value of the output inductor should be chosen such that SE
is between 0.5 × and 1 × the falling slope of the inductor current (SF).
Pulse Width Modulation (PWM) Mode
Current Sense Amplifier
The A8585 family incorporates a high-bandwidth current sense
amplifier to monitor the current in the high-side MOSFET. This
current signal is used by both the PWM and PFM control circuitry to regulate the MOSFET peak current. The current signal
is also used by the protection circuitry to prevent damage to
the device.
Power MOSFETs
The A8585 family includes a 40 V, 110 mΩ high-side N-channel
MOSFET capable of delivering at least 2.0 A. The device also
includes a 10 Ω, low-side MOSFET to help insure the boot
capacitor is always charged. The typical RDS(on) increase versus
temperature is shown in Figure 4.
BOOT Regulator
The A8585 family contains a regulator to charge the boot capacitor. The voltage across the BOOT capacitor typically is 5.0 V.
If the boot capacitor is missing the device detects a boot overvoltage. Similarly, if the BOOT capacitor is shorted the device
detects a boot undervoltage. Also, the boot regulator has a current
limit to protect itself during a short circuit condition. The details
1.8
1.6
Normalized RDS(on)
1.4
1.0
0.8
0.6
0.4
0.2
-40
A high-speed comparator and control logic, capable of typical
pulse widths down to ton(min), are included in the device. The
inverting input of the PWM comparator is connected to the output of the error amplifier. The non-inverting input is connected to
the sum of the current sense signal, the slope compensation, and a
DC offset voltage (VPWMOFFS , nominally 400 mV).
At the beginning of each PWM cycle, the CLK signal sets the
PWM flip-flop and the high-side MOSFET is turned on. When
the summation of the DC offset, slope compensation, and current
sense signal, rises above the error amplifier voltage the PWM flip
-flop is reset and the high-side MOSFET is turned off.
The PWM flip-flop is reset dominant so the error amplifier may
override the CLK signal in certain situations. For example, at
very light loads or extremely high input voltages the error amplifier (temporarily) reduces its output voltage below the 400 mV
DC offset and the PWM flip-flop ignores one or more of the
incoming CLK pulses. The high-side MOSFET does not
turn on and the regulator skips pulses to maintain output
voltage regulation.
In PWM mode, and when SW is switching in PFM mode, all
of the device fault detection circuits are active. See Figure 1
showing how faults are handled during PWM mode. Also, the
Protection Features section of this datasheet provides a detailed
description of each fault and Table 1 presents a summary.
Low-IQ Pulse Frequency Modulation (PFM)
Mode
1.2
0
The A8585 family uses fixed-frequency, peak current mode
control to provide excellent load and line regulation, fast transient
response, and ease of compensation.
-20
0
20
40
60
80
100 120 140 160
Temperature (°C)
Figure 4: Typical MOSFET RDS(on) versus Temperature
The PWM/PFM variants (A8585 and A8585-1) enter Low-IQ
PFM mode 2048 counts after the PWM/PFM pin goes low,
provided that no faults are present, the load is light, NPOR =
1 and no external clock is applied to the FSET/SYNCPWM pin.
Similarly, the EN/SLEEP variants (A8585-2 and A8585-3) enter
Low-IQ PFM mode under the same conditions (except there is
no PWM/PFM pin), and the EN/SLEEP pin is high. At light
loads the PFM comparator, which is connected to the VOUT
pin through the internal feedback resistor divider (which is the
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A8585
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
internal FB point), modulates the frequency of the SW node to
regulate the output voltage with very high efficiency.
The reference for the PFM comparator is calibrated approximately 1% above the PWM regulation point. When the voltage at
the internal FB point rises above the PFM comparator threshold
and peak inductor current falls below IPEAK(LO_IQ) minus slope
compensation, the device will enter PFM coast mode, tri-stating
the SW node and drawing extremely low current from VIN. When
voltage at the FB point falls below the PFM comparator threshold
the device will fully power-up after approximately a 2.5 µs delay
and the high-side MOSFET is repeatedly turned on, operating
at the PWM switching frequency until the voltage at the FB pin
rises above the PFM comparator threshold. VOUT will rise at a
rate determined by, and have a voltage ripple dependent on, the
input voltage, output voltage, inductor value, output capacitance,
and load.
Figure 5: PFM Mode at Light Load (5 mA)
VIN = 12 V, VOUT = 5.0 V
When the COMP pin falls to a voltage corresponding to the
Low-IQ Peak Current Threshold ( IPEAK(LO_IQ) ) value, an internal clamp prevents the COMP voltage from falling further. This
results in the output voltage rising slightly which causes the
PFM comparator to trip and the device to enter the PFM Coast
mode. Thus, when the load demands a peak inductor current that
corresponds to less than the Low-IQ Peak Current Threshold
(IPEAK(LO_IQ)) minus the impact of slope compensation at the
given duty cycle the device operates in PFM mode. This transition point from PWM to PFM mode is defined by the input voltage, output voltage, slope compensation, and inductor value.
Figure 5 demonstrates PFM mode operation for a light load
(5 mA). Figure 6 shows PWM and Low IQ PFM transitions. In
PFM mode the load steps from 0.05A (PFM operation) to 1.05A
(PWM operation) and then transitions back to 0.05A PFM mode.
Soft Start (Startup) and Inrush Current
Control
Inrush current is controlled by the internal embedded soft start
function. The soft start time of the A8585 family is fixed at 5 ms.
When the device is enabled and all faults are cleared, the internal
soft start voltage ramps upward from 0 V. When the soft start
voltage exceeds the equivalent VOUT voltage the error amplifier
output slews above the 400 mV pedestal initiating PWM switching.
Figure 6: Load Steps Between 0.05 A and 1.05 A
in PFM Mode
(A)
fSW/4
gm/2
(B)
fSW/2
gm/2
(C)
fSW
gm
When the device begins switching, the error amplifier regulates
the internal FB voltage to be the soft start voltage. During the
active portion of soft start, the regulator output voltage rises from
0 V to the nominal value.
Figure 7: Soft Start Operation
VOUT Ramps from Startup (VIN powered) to Nominal (5.0 V at 2.0 A,
for A8585/A8585-2, shown)
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A8585
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
In Figure 7 the startup sequence shows the soft start operation.
During startup, while VOUT voltage is below 25% (interval
A in Figure 7) the PWM switching frequency is reduced to 1/4
of fSW. This corresponds to VOUT < 1.25 V (for nominal 5.0 V
output: A8585/A8585-2) or < 0.825 V (for nominal 3.3 V output:
A8585-1/A8585-3). While VOUT is above 25% but below 50%
(interval B) the switching frequency is reduced to 1/2 of fSW, and
when VOUT is above 50% (at C) the switching frequency is fSW.
NPOR transitions high when the output voltage is within regulation. In PWM mode, NPOR is high when the output voltage is
typically within 92.5% to 110% of the target value. The NPOR
overvoltage and undervoltage comparators incorporate a small
amount of hysteresis (VNPORUV(HYS)(xV) ), 40 mV(typ) for 3.3 V
output variants (A8585-1/A8585-3) and 60 mV (typ) for 5.0 V
output variants (A8585/A8585-2), and filtering (5 µs, typical) to
help reduce chattering.
The error amplifier gain (gm) also changes. While VOUT is
below 50% (at A through B) gm is reduced to gm / 2. While VOUT
is above 50% (at C) the error amplifier gain is gm .
The NPOR output is immediately pulled low if either an overvoltage or an undervoltage condition occurs, or if the device
junction temperature exceeds the Thermal Shutdown Threshold
( TSDth ). For other faults, NPOR depends on the output voltage.
Table 1 summarizes all of the A8585 family fault modes and the
effects on NPOR. When powering down, if VIN goes low before
VOUT, NPOR will pulled low when the undervoltage condition
occurs, but will only remain low while VOUT remains above
~3.0V.
The reduced switching frequencies and error amplifier gain are
necessary to help improve output regulation and stability when
VOUT is very low. When VOUT is very low the PWM control
loop requires on-times near the minimum controllable on-time,
and extra low duty cycles that are not possible at the nominal
switching frequency. When the soft start voltage reaches approximately 800 mV, the error amplifier switches over to referencing
the device internal 800 mV reference for regulating the internal
(resistor divider) FB voltage.
If the device is disabled or a fault occurs, the internal fault latch
is set, and the soft start voltage is discharged to ground very
quickly. When the soft start voltage decays to approximately
200 mV, the device clears the internal fault latch.
The soft start voltage is discharged slowly when the device is in
hiccup mode. Therefore, the soft start not only determines the
startup time but also the time between soft start attempts in hiccup mode. Hiccup mode operation is discussed in more details in
the Protections section of this datasheet.
For the variants with selectable Sleep mode (A8585-2 and
A8585-3 ), when the EN/SLEEP pin goes low switching stops
after 224 µs (typ) (tdSLEEP ), and NPOR goes low (even if VOUT >
VNPORUV) and stays low until VOUT drops to about 25% of nominal value. When VOUT falls to about 25% of its nominal value,
the device goes into Sleep mode consuming IIN(SLEEP) on VIN,
and shortly after that, NPOR will be released (no longer pulled
low). Given this operation, the time that NPOR remains low is
dependent on the current consumption on VOUT and the output
capacitance. It is important to note that the high-side switch leakage can overwhelm the load current on VOUT, especially since
the IC current on VOUT is very low in this mode. Therefore
for reliable operation in this situation, the user should select an
Pre-Biased Startup
If the output of the regulator is pre-biased to some voltage, the
A8585 family modifies the normal startup routine, in order to
prevent discharging the output capacitors. As described previously, the error amplifier usually becomes active when the soft
start voltage starts to ramp. If the output is pre-biased, the internal
FB voltage is at some non-zero voltage. The COMP pin remains
low and SW is tri-stated until the soft start voltage rises above
the VFB. Figure 8 shows startup when the output voltage is prebiased to 2.0 V.
fSW/2
fSW
Active Low Power-On Reset (NPOR) Output
The A8585 family has an inverted Power-On Reset output
(NPOR) with a fixed delay ( tdPOR ) before the rising edge. The
NPOR output is an open drain output so an external pull-up resistor must be used, as shown in the Typical Application diagram.
Figure 8: Soft Start Prebiased Startup
VOUT Prebiased at 2 V, Ramps to Nominal (5.0 V at 2.0 A, for A8585/
A8585-2, shown)
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Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
NPOR resistor (if connected to VOUT) with a low enough value
to overwhelm any high-side switch leakage.
Protection Features
The A8585 was designed to satisfy the most demanding automotive and non-automotive applications. In this section, a description of each protection feature is provided, and table 1 summarizes the protection operation. All faults are available in both
PWM and PFM mode. In PFM mode all faults are monitored just
before and while switching is active but are ignored while VOUT
remains above the PFM comparator threshold and below the
NPOR OV threshold.
UNDERVOLTAGE LOCKOUT (UVLO)
An undervoltage lockout (UVLO) comparator monitors the voltage at the VIN pin and keeps the regulator disabled if the voltage
is below the stop threshold (VUVLO(OFF) ). The UVLO comparator
incorporates some hysteresis (VUVLO(HYS) ) to help reduce on-off
cycling of the regulator due to resistive or inductive drops in the
VIN path during heavy loading or during startup.
PULSE-BY-PULSE OVERCURRENT PROTECTION
(OCP)
The A8585 family monitors the current of the high-side MOSFET and if the current exceeds the pulse-by-pulse overcurrent
threshold (ILIM) then the high-side MOSFET is turned off.
Normal PWM operation resumes on the next clock pulse from the
oscillator. The device includes leading edge blanking (as defined
by Ton(min) spec) to prevent false triggering of the pulse-by-pulse
current limit when the high-side MOSFET is turned on initially.
Because the slope compensation ramp is added to the inductor
current, the A8585 family delivers more current at lower duty
cycles and less current at higher duty cycles. Figure 9 illustrates
the relationship between the current limit and duty cycle. As
shown, the current limit at min and max duty cycle remains fixed,
but the relationship vs. duty cycle is skewed with frequency due
to the fixed minoff time. Given the relationship it is best to use
the ILIM(tonmin) and ILIM(toffmin) to calculate the current limit at a
given duty cycle.
OVERCURRENT PROTECTION (OCP) AND HICCUP
MODE
An OCP counter and hiccup mode circuit protect the buck regulator when the output of the regulator is shorted to ground or when
the load is too high. When the soft start ramp is active (t tSS, and
• VCOMP clamped at its maximum voltage (OCL = 1)
As long as these two conditions are met the OCP counter remains
enabled and counts pulses from the overcurrent comparator. If the
COMP voltage decreases (OCL = 0) the OCP counter is cleared.
If the OCP counter reaches OCPLIM counts (120), a hiccup latch
is set and the COMP pin is quickly pulled down by a relatively
low resistance (1 kΩ), and switching is halted for 20 ms to
provide time for the IC to cool down. After the hiccup off-time
expires (20 ms), the soft start ramp starts, marking the beginning
of a new, normal soft start cycle as described earlier.
When the soft start voltage crosses the effective output voltage,
the error amplifier forces the voltage at the COMP pin to quickly
slew upward and PWM switching resumes. If the short circuit
at the regulator output remains, another hiccup cycle occurs.
Hiccups repeat until the short circuit is removed or the converter
is disabled. If the short circuit has been removed, the device soft
starts normally and the output voltage automatically recovers to
the target level, as shown in Figure 10.
4.2
4.0
3.8
3.6
3.4
ILIM (A)
A8585
3.2
3.0
2.8
2.6
Min. 415 kHz
2.4
During synchronization, the slope compensation scales in a similar fashion as RFSET, with slight less accuracy.
2.2
The exact current the buck regulator can support is heavily
dependent on duty cycle (VIN , VOUT , diode forward voltage Vf ),
ambient temperature, thermal resistance of the PCB, airflow,
component selection, and nearby heat sources.
1.8
Typ. 415 kHz
Max. 415 kHz
Min. 2 MHz
Typ. 2 MHz
2.0
Max. 2 MHz
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100
Duty Cycle, D (%)
Figure 9: Pulse-by-Pulse Current Limit vs Duty Cycle
at 300 kHz (dashed lines) and 550 kHz (solid lines)
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A8585
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
BOOT CAPACITOR PROTECTION
The A8585 family monitors the voltage across the BOOT capacitor to detect if the capacitor is missing or short circuited. If the
BOOT capacitor is missing, the regulator enters hiccup mode
after 7 PWM cycles. If the BOOT capacitor is shorted, the regulator enters hiccup mode after 120 to 64 PWM cycles.
For a BOOT fault, hiccup mode operates virtually the same as
described previously for an output short circuit fault (OCP), having a hiccup off time of 20ms followed by a soft start retry with
repeated attempts until the fault clears. However, OCP is the only
fault that is ignored during the soft start ramp time ( tSS ). BOOT
faults are a non-latched condition, so the device automatically
recovers when the fault is corrected.
ASYNCHRONOUS DIODE PROTECTION
If the asynchronous diode is missing or damaged (open) the SW
pin is subject to unusually high negative voltages. This negative
voltage may cause the device to malfunction and could lead to
damage. The A8585 family includes protection circuitry to detect
when the asynchronous diode is missing. If the SW pin is below
−1.25 V typically, for more than 50 ns typically, the device enters
hiccup mode after detecting 1 missing diode fault.
Also, if the asynchronous diode is shorted, the device experiences
extremely high currents through the high-side MOSFET. If this
occurs, the device enters hiccup mode after detecting 1 shorted
diode fault.
OVERVOLTAGE PROTECTION (OVP)
The A8585 family provides an always-on over voltage protection that monitors VOUT, to protect against VOUT rising at light
loads due to high side switch leakage. In this case, the high-side
switch is forced off and the low-side switch continues to operate
and can correct the OVP condition provided only a few milliamperes of pull-down current are required. When the condition
causing the overvoltage is corrected, the regulator automatically
recovers.
During an output overvoltage condition, the device tries for up
to 120 counts ( tdPOV_POR ) to clear the overvoltage condition. If
the overvoltage condition is successfully cleared within this time
period, NPOR does not go low and the device continues to operate in PFM mode. If the overvoltage fault is not cleared during
this time, then an NPOR = 0 is declared and the device works
indefinitely to reduce the output overvoltage fault. If it is suc-
cessful in clearing this fault, then there will be an approximately
7.5 ms delay ( tdPOR ) before NPOR pin returns high. Note that
the size of the regulator output capacitor may have an effect on
whether the overvoltage condition is cleared within the tdPOV_POR
time period.
PIN-TO-GROUND AND PIN-TO-PIN SHORT
PROTECTIONS
The A8585 family is designed to satisfy the most demanding
automotive applications. For example, the device is carefully
designed fundamentally to withstand a short circuit to ground at
each pin without suffering damage.
In addition, care was taken when defining the device pin-out to
optimize protection against adjacent pin-to-pin short circuits. For
example, logic pins and high voltage pins are separated as much
as possible. Inevitably, some low voltage pins had to be located
adjacent to high voltage pins, but in these instances the low
voltage pins are designed to withstand increased voltages, with
clamps and/or series input resistance, to prevent damage to the
device.
THERMAL SHUTDOWN (TSD)
The A8585 family monitors internal junction temperature and
stops switching and pulls NPOR low if it becomes too hot. Also,
to prepare for a restart, the internal soft start voltage (VSS) and
the voltage at the COMP pin are pulled low until VSS < VSSRST .
TSD is a non-latched fault, so the device automatically recovers
if the junction temperature decreases by approximately 20°C.
Figure 10: Hiccup Mode and Recovery
(to 5.0 V at 0.5 A, for A8585/A8585-2, shown)
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Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
Table 1: Summary of A8585 Family Fault Modes and Operation
During Fault Counting, Before Hiccup Mode
Fault Mode
Internal
Soft Start
VCOMP
High-Side
Switch
Low-Side
Switch
Boot
Charging
Latched
Fault
Reset
Condition
Output
shorted to
ground
Hiccup, after
120 OCL
faults
Clamped for
ILIM , then
pulled low for
Hiccup
fOSC / 4 due to
VOUT < 25%,
responds to
VCOMP
Can be
activated if
BOOT voltage
is too low
Not affected
Depends on
VOUT
No
Automatic,
remove the
short
Output
overcurrent,
VOUT > 50%
Hiccup, after
120 OCL
faults
Clamped for
ILIM , then
pulled low for
Hiccup
fOSC,
responds to
VCOMP
Can be
activated if
BOOT voltage
is too low
Not affected
Depends on
VOUT
No
Automatic,
decrease load
current
VOUT pin
open
Pulled low
after 32 cycles
Pulled low
after 32 cycles
Forced off by
COMP low
Can be
activated if
BOOT voltage
is to low
Not affected
Stays low
No
Automatic,
VOUT pin
reconnected
Boot capacitor
missing
Hiccup, after 7
Boot OV faults
Not affected
but pulled low
for Hiccup
Forced off
when Boot OV
fault occurs
Forced off
when Boot OV
fault occurs
Off after Boot
fault occurs
Depends on
VOUT
No
Automatic,
replace
capacitor
Boot capacitor
shorted
Hiccup, after
64 Boot UV
faults
Not affected
but pulled low
for Hiccup
Forced off
when Boot UV
fault occurs
Forced off
only during
Hiccup
Off only during
Hiccup
Depends on
VOUT
No
Automatic,
unshort
capacitor
Asynchronous
diode missing
Hiccup after 1
fault
Not affected
but pulled low
for Hiccup
Forced off
after 1 fault
Can be
activated if
BOOT voltage
is too low
Not affected
Depends on
VOUT
No
Automatic,
install diode
Asynchronous
diode (or SW)
hard short to
ground
Hiccup after 1
fault
Clamped for
ILIM , then
pulled low for
Hiccup
Forced off
after 1 fault
Can be
activated if
BOOT voltage
is too low
Not affected
Depends on
VOUT
No
Automatic,
remove short
Asynchronous
diode (or SW)
soft short to
ground
Hiccup, after
120 OCL
faults
Clamped for
ILIM , then
pulled low for
Hiccup
Active,
responds to
VCOMP
Can be
activated if
BOOT voltage
is too low
Not affected
Depends on
VOUT
No
Automatic,
remove short
Not affected
Transitions
low via loop
response
Forced off
Active during
tOFF(MIN)
Off when VOUT
is too high
Pulled low
when VOUT is
too high for
120 counts
No
Automatic,
VOUT returns
to normal
range
Output under
voltage
Not affected
Transitions
high via loop
response
Active,
responds to
VCOMP
Can be
activated if
BOOT voltage
is too low
Not affected
Pulled low
when VOUT is
too low
No
Automatic,
VOUT returns
to normal
range
FSET shorted
to GND or
above 1.0 V
Pulled Low
Pulled Low
Forced Off
Forced Off
Forced Off
Depends on
VOUT
No
Auto
Thermal
shutdown
(TSD)
Pulled low
until
VSS> RZ , and CZ >> CP . In most cases, RO > 2 MΩ,
1 kΩ < RZ < 100 kΩ, 220 pF < CZ < 47 nF, and CP < 50 pF, so
the following equations are very accurate.
The low frequency gain of the control section (GC(0Hz) ) is calculated as:
GC(0 Hz) = gm x R0 = AVOL
(25)
where
VOUT is the output voltage,
VFB is the reference voltage (0.8 V),
gm is the error amplifier transconductance (750 µA/V ), and
RO is the error amplifier output impedance (AVOL/gm ).
The transfer function of the Type-II compensated error amplifier
has a (very) low frequency pole (fP2 ) dominated by the output
error amplifier output impedance (RO) and the CZ compensation
capacitor:
1
fP2 =
(26)
2� × RO × CZ
The transfer function of the Type-II compensated error amplifier
also has frequency zero (fZ2) dominated by the RZ resistor and
the CZ capacitor:
Finally, consider the combined Bode plot of both the control-tooutput and the compensated error amplifier (Figure 16). Careful
examination of this plot shows that the magnitude and phase
of the entire system (red curve) are simply the sum of the error
amplifier response (blue curve) and the control to output response
(green curve). As shown in figure 16, the bandwidth of this system (fc ) is 60 kHz, the phase margin is 69 degrees, and the gain
margin is 14 dB.
Complete designs for several common output voltages at
550 kHz, 425 kHz and 300 kHz are provided in table 2.
A Generalized Tuning Procedure
This section presents a methodology to systematically apply
design considerations provided above.
1. Choose the system bandwidth (fC ). This is the frequency at
which the magnitude of the gain crosses 0 dB. Recommended
values for fC, based on the PWM switching frequency, are in
the range fSW / 20 < fC < fSW / 7.5. A higher value of fC generally
provides a better transient response, while a lower value of fC
generally makes it easier to obtain higher gain and phase margins.
2. Calculate the RZ resistor value. This sets the system bandwidth
(fC):
2 × COUT
RZ = fC ×
(29)
gmPOWER × gm
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Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
80
80
60
40
f Z2 = 2.6 kHz
f P2 = 99 Hz
20
Gain (dB)
GC0Hz = 49 dB
f P3 ≈ 100 kHz
0
40
0
-20
-40
-40
-60
-60
180
180
0
-180
10
30
100
300
103
3×103
10×103
30×103
100×103 300×103
GM ≈ 14 dB
PM ≈ 45 deg
0
-180
10
106
f c = 50 kHz
20
-20
Phase (°)
Phase (°)
Gain (dB)
60
30
100
300
103
Frequency (Hz)
3×103
10×103
30×103
100×103 300×103
106
Frequency (Hz)
Figure 15: Type-II Compensated Error Amplifier Bode
Plot
Figure 16: Bode Plot of the Complete System (red
curve)
Table 2: Recommended Component Values
RZ + CZ // CP
VOUT
(V)
3.3
(A8585-1, -3)
5.0
(A8585, -2)
3.3
(A8585-1, -3)
5.0
(A8585, -2)
3.3
(A8585-1, -3)
5.0
(A8585, -2)
fSW
(kHz)
425
550
300
RFSET
(kΩ)
59.0
45.3
86.6
LO
(µH)
CO*
(µF)
RZ
(kΩ)
CZ
(pF)
CP
(pF)
Modes
10
(DR-1050-100-R)
38
34.0
560
15
PWM and PFM
10
(DR-1050-100-R)
53
47.5
680
8
PWM and PFM
8.2
(7447713082)
38
21.5
560
18
PWM and PFM
10
(DR-1050-100-R)
53
60.4
1000
8
PWM and PFM
10
(DR-1050-100-R)
38
24.9
1500
22
PWM and PFM
22
(744770122)
53
35.7
1500
15
PWM and PFM
*The user must consider negative tolerance and DC-bias effect when choosing components to obtain CO .
3. Determine the frequency of the pole (fP1). This pole is formed
by COUT and RL. Use equation 23 (repeated here):
1
fP1 =
2� × RL × COUT
4. Calculate a range of values for the CZ capacitor. Use the following:
4
1
< CZ <
2π × RZ × fC
2π × RZ ×1.5 × fP1
(30)
To maximize system stability (that is, to have the greatest gain
margin), use a higher value of CZ . To optimize transient recovery
time, although at the expense of some phase margin, use a lower
value of CZ .
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Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
5. Calculate the frequency of the ESR zero (fZ1 ) formed by the
output capacitor(s) by using equation 24 (repeated here):
fZ1 =
1
2π × ESR × COUT
If fZ1 is at least 1 decade higher than the target crossover frequency (fC ) then fZ1 can be ignored. This is usually the case for
a design using ceramic output capacitors. Use equation 28 to
calculate the value of CP by setting fP3 to either 5 × fC or fSW / 2,
whichever is higher.
Alternatively, if fZ1 is near or below the target crossover frequency (fC ), then use equation 28 to calculate the value of CP by
setting fP3 equal to fZ1. This is usually the case for a design using
high ESR electrolytic output capacitors.
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Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
POWER DISSIPATION AND THERMAL CALCULATIONS
The power dissipated in the A8585 family is the sum of the
power dissipated from the VIN supply current (PIN ), the power
dissipated due to the switching of the high-side power MOSFET
(PSW ), the power dissipated due to the rms current being conducted by the high-side MOSFET (PCOND ), and the power dissipated by the gate drivers (PDRIVER).
The power dissipated from the VIN supply current can be calculated using the following equation:
PIN = VIN × IQ + (VIN - VGS ) × QG × fSW (31)
where
VIN is the input voltage,
IQ is the input quiescent current drawn by the device (nominally
2.5 mA),
VGS is the MOSFET gate drive voltage (typically 5 V),
QG is the MOSFET gate charge (approximately 2.5 nC), and
fSW is the PWM switching frequency.
The power dissipated by the internal high-side MOSFET during
PWM switching can be calculated using the following equation:
PSW =
where
VIN × IOUT × (tr + tf ) × fSW
2
(32)
VIN is the input voltage,
IOUT is the output current,
fSW is the PWM switching frequency, and
tr and tf are the rise and fall times measured at the SW node.
The exact rise and fall times at the SW node depend on the
external components and PCB layout so each design should be
measured at full load. Approximate values for both tr and tf range
from 10 to 15 ns.
The power dissipated by the high-side MOSFET while it is conducting can be calculated using the following equation:
2
PCOND = Irms(FET)
× RDS(on)HS
=
where
VOUT +Vf
VIN +Vf
∆IL2
2
× IOUT + 12 × RDS(on)HS
IOUT is the regulator output current,
ΔIL is the peak-to-peak inductor ripple current, and
RDS(on)HS is the on-resistance of the high-side MOSFET.
(33)
The RDS(on)of the MOSFET has some initial tolerance plus an
increase from self-heating and elevated ambient temperatures.
A conservative design should accommodate an RDS(on) with
at least a 15% initial tolerance plus 0.39%/°C increase due to
temperature.
The power dissipated by the internal gate drivers can be calculated using the following equation:
PDRIVER = QG × VGS × fSW
(34)
where
QG is the gate charge to drive the MOSFET to VGS = 5 V (about
2.5 nC),
VGS is the gate drive voltage (typically 5 V), and
fSW is the PWM switching frequency.
Bias power dissipation at VOUT pin:
Pbias = VOUT*IIN = 5 V (or 3.3 V) * 2.5 mA
Finally, the total power dissipated by the device (PTOTAL) is the
sum of the previous equations:
PTOTAL = PIN + PSW + PCOND + PDRIVER + Pbias (35)
The average junction temperature can be calculated with the following equation:
TJ = PTOTAL + RθJA + TA
(36)
where
PTOTAL is the total power dissipated as described in equation 35,
RθJA is the junction-to-ambient thermal resistance (34°C/W on a
4-layer PCB), and
TA is the ambient temperature.
The maximum junction temperature is dependent on how efficiently heat can be transferred from the PCB to ambient air. It is
critical that the thermal pad on the bottom of the IC should be
connected to a at least one ground plane using multiple vias.
As with any regulator, there are limits to the amount of heat that
can be dissipated before risking thermal shutdown. There are
trade-offs among: ambient operating temperature, input voltage,
output voltage, output current, switching frequency, PCB
thermal resistance, airflow, and other nearby heat sources.
Even a small amount of airflow will reduce the junction temperature considerably.
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A8585
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
PCB COMPONENT PLACEMENT AND ROUTING
A good PCB layout is critical if the A8585 family is to provide
clean, stable output voltages. Follow these guidelines to insure
a good PCB layout. Figure 17 shows a typical buck converter
schematic with the critical power paths/loops. Figure 18 shows an
example PCB component placement and routing with the same
critical power paths/loops from the schematic.
1. By far, the highest di/dt in the asynchronous buck regulator
occurs at the instant the high-side MOSFET turns on and the
capacitance of the asynchronous Schottky diode (200 to 1000 pF )
is quickly charged to VIN. The ceramic input capacitors must
deliver this fast, short pulse of current. Therefore, the loop, from
the ceramic input capacitors through the high-side MOSFET and
into the asynchronous diode to ground, must be minimized. Ideally these components are all connected using only the top metal
(that is, do not use vias to other power/signal layers).
2. When the high-side MOSFET is on, current flows from the
input supply and capacitors, through the high-side MOSFET, into
the load via the output inductor, and back to ground. This loop
should be minimized and have relatively wide traces.
3. When the high-side MOSFET is off, free-wheeling current
flows from ground, through the asynchronous diode, into the load
via the output inductor, and back to ground. This loop should be
minimized and have relatively wide traces.
4. The voltage on the SW node transitions from 0 V to VIN very
quickly and is the root cause of many noise issues. It is best to
place the asynchronous diode and output inductor close to the
device to minimize the size of the SW polygon. Also, keep low
level analog signals (like FB and COMP) away from the SW
polygon.
5. To have the highest output voltage accuracy, the output voltage
sense trace should be connected as close as possible to the load.
6. Place the compensation components (RZ, CZ, and CP) as close
as possible to the COMP pin. Place vias to the GND plane as
close as possible to these components.
7. Place the boot strap capacitor (CBOOT) near the BOOT pin and
keep the routing from this capacitor to the SW polygon as short
as possible.
8. When connecting the input and output ceramic capacitors, use
multiple vias to GND and place the vias as close as possible to
the pads of the components.
9. To minimize PCB losses and improve system efficiency, the
input and output traces should be as wide as possible and be
duplicated on multiple layers, if possible.
10. To improve thermal performance, place multiple vias to the
GND plane around the anode of the asynchronous diode.
11. The thermal pad under the device must connect to the GND
plane using multiple vias. More vias will ensure the lowest junction temperature and highest efficiency.
12. EMI/EMC issues are always a concern. Allegro recommends
having component locations for an RC snubber from SW to
ground. The resistor should be 1206 size.
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32
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
3
VIN
CIN3
CIN2
SW
CIN1
Q1
1
VOUT
LO
Rsnub
D1
CO1
CO2
CO3
CO4
LOAD
Csnub
2
Figure 17: Typical Buck Converter with Critical Paths/Loops Shown
Loop 1 (red): At the instant Q1 turns on, Schottky diode D1, which is
very capacitive, must be very quickly shut off (only 5 to 15 ns of charging
time). This spike of charging current must come from the local input
ceramic capacitor, CIN1. This spike of current is quite large and can be an
EMI/EMC issue if the loop is not minimized. Therefore, the input capacitor
CIN1 and Schottky diode D1 must be placed be on the same (top) layer,
be located near each other, and be grounded at virtually the same point
on the PCB.
Loop 2 (magenta): When Q1 is off, free-wheeling inductor current
must flow from ground through diode D1 (SW will be at –Vf ), into the
3
output inductor, out to the load and return via ground. While Q1 is off the
voltage on the output capacitors will decrease. The output capacitors
and Schottky diode D1 should be placed on the same (top) layer, be
located near each other, and be sharing a good, low inductance ground
connection.
Loop 3 (blue): When Q1 is on, current flows from the input supply
and input capacitors through the output inductor and into the load and
the output capacitors. At this time the voltage on the output capacitors
increases.
2
1
Figure 18: Example PCB Component Placement and Routing
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33
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
PACKAGE OUTLINE DRAWING
For Reference Only – Not for Tooling Use
(Reference DWG-0000380, Rev. 1)
NOT TO SCALE
Dimensions in millimeters
Dimensions exclusive of mold flash, gate burrs, and dambar protrusions
Exact case and lead configuration at supplier discretion within limits shown
4.90
0.55
+0.08
–0.10
8º
0º
10
0.25
0.19
B
2.41 NOM
2.41
5.60
+0.585
0.685 –0.275
2
3.30 NOM
SEATING
PLANE
0.10 C
1.00 BSC
1
0.25 BSC
Branded Face
10×
0.40
0.30
1.75
3.90 ±0.10 6.00 ±0.20
A
1
1.00
10
C
1.55 ±0.10
SEATING PLANE
GAUGE PLANE
2
3.30
C
PCB Layout Reference View
10
0.10 ±0.05
XXXXXXX
A Terminal #1 mark area
B
Exposed thermal pad (bottom surface)
C
Reference land pattern layout; all pads a minimum of 0.20 mm from all
adjacent pads; adjust as necessary to meet application process
requirements and PCB layout tolerances; when mounting on a multilayer
PCB, thermal vias at the exposed thermal pad land can improve thermal
dissipation (reference EIA/JEDEC Standard JESD51-5)
D
Branding scale and appearance at supplier discretion
Date Code
Lot Number
1
2
D Standard Branding Reference View
Line 1, 2: Maximum 7 characters per line
Line 3: Maximum 5 characters
Line 1: Part Number
Line 2: Logo A, 4-digit Date Code
Line 3: Characters 5, 6, 7, 8 of
Assembly Lot Number
Figure 19: Package LK, 10-Pin SOIC with Exposed Thermal Pad
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34
Wide Input Voltage, 2 A
Buck Regulator Family with Low IQ Mode
A8585
Revision History
Number
Date
–
March 7, 2014
Initial Release
1
April 14, 2015
Ammended Slope Compensation values and updated Package Outline Drawing
2
May 19, 2015
Increased EN/SLEEP Delay Max value and corrected typo
3
January 8, 2016
4
May 4, 2020
5
April 30, 2021
Description
Corrected Selection Guide part numbers
Minor editorial updates
Updated Package Outline Drawing (page 34)
Copyright 2021, Allegro MicroSystems.
Allegro MicroSystems reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit
improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the
information being relied upon is current.
Allegro’s products are not to be used in any devices or systems, including but not limited to life support devices or systems, in which a failure of
Allegro’s product can reasonably be expected to cause bodily harm.
The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems assumes no responsibility for its use; nor
for any infringement of patents or other rights of third parties which may result from its use.
Copies of this document are considered uncontrolled documents.
For the latest version of this document, visit our website:
www.allegromicro.com
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