A8585 Datasheet

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
•Utilizes 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
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.
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.
Continued on the next page…
Continued on the next page…
PACKAGE: 10-pin SOIC with exposed
thermal pad (suffix LK)
APPLICATIONS:
•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. 3
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
FEATURES AND BENEFITS (continued)
DESCRIPTION (continued)
•Pre-bias startup capable: VOUT increases monotonically, 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*
On 4-layer PCB based on JEDEC standard
Value
Unit
35
ºC/W
*Additional thermal information available on the Allegro website.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; 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, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; 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<740mV
Functional Block Diagram
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
4
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
Pinout Diagrams
10 BOOT
VIN 1
9 SW
VIN 2
PWM/PFM 3
PAD
8 GND
10 BOOT
VIN 1
9 SW
VIN 2
EN/SLEEP 3
PAD
8 GND
FSET/SYNCPWM 4
7 COMP
FSET/SYNCPWM 4
7 COMP
VOUT 5
6 NPOR
VOUT 5
6 NPOR
A8585 and A8585-1 variants
A8585-2 and A8585-3 variants
Terminal List Table
Number
Name
1, 2
VIN
Power input for the control circuits and the drain of the internal high-side N-channel MOSFET.
Connect this pin to a power supply of 4.0 to 35 V. A high quality, high frequency ceramic capacitor
should be placed and grounded very close to this pin.
PWM/PFM
(A8585 and A8585-1) Setting this pin high forces PWM mode. Setting this pin low allows Low IQ PFM
mode after 2048 clock cycles if two conditions are met: (1) the regulator is lightly loaded and (2) there
is no clock signal being applied to the FSET/SYNCPWM input pin.
EN/SLEEP
(A8585-2 and A8585-3) This pin must be set high to enable the device. If this pin is set low, the device
will enter a very low current shut down or sleep state (VOUT = 0 V). If the application does not require a
sleep mode, then this pin can be tied directly to VIN. Do not float this pin.
3
4
Function
FSET/SYNCPWM
Frequency setting and PWM synchronization pin. A resistor, RFSET , from this pin to GND sets the
PWM switching frequency. See figure 11 and/or equation 1 to determine the value of RFSET. Applying
a clock signal to this pin forces PWM mode (that is, it overrides a logic low on the PWM/PFM pin) and
synchronizes the PWM switching frequency.
5
VOUT
Connect this pin to the output of the regulator. This pin supplies internal circuitry when its voltage level
is high enough. Also, through an on-chip voltage divider, this pin connects to the negative feedback
input of the error amplifier. Keep the VOUT pin quiet and kelvin connect.
6
NPOR
Active low, power on reset output signal. This pin is an open drain output that transitions from low to
high impedance after the output has maintained regulation for tdPOR .
7
COMP
Output of the error amplifier, and compensation node for the current mode control loop. Connect a
series RC network from this pin to GND for loop compensation. See the Design and Component
Selection section of this datasheet for further details.
8
GND
9
SW
10
BOOT
–
PAD
Ground pin.
The source for the internal high-side N-channel MOSFET. The external free-wheeling diode (D1) and
output inductor (LO) should be connected to this pin. Both D1 and LO should be placed close to this pin
and connected with relatively wide traces.
High-side gate drive boost input. This pin supplies the drive for the high-side N-channel MOSFET.
Connect a 47 nF ceramic capacitor from BOOT to SW.
Exposed pad of the package providing enhanced thermal dissipation. This pad must be connected to
the ground plane(s) of the PCB with at least 6 vias, directly in the pad.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
5
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
ELECTRICAL CHARACTERISTICS1: Valid at 4.0 V ≤ VIN ≤ 35 V, −40°C ≤ TA = TJ ≤ 150ºC, unless otherwise
specified
Characteristic
Symbol
Test Conditions
Min.
Typ.
Max.
Unit
4.0
−
35
V
Input Voltage
Input Voltage Range2
VIN
VIN Undervoltage Lockout Start
Threshold
VUVLO(ON)
VIN rising
3.6
3.8
4.0
V
VIN Undervoltage Lockout Stop
Threshold
VUVLO(OFF) VIN falling
3.2
3.4
3.6
V
VIN Undervoltage Hysteresis
VUVLO(HYS)
−
400
−
mV
Input Supply Current
Input Supply Current (Not in PFM)
IIN
ILO_IQ
(0A,5.0V)
ILO_IQ
(40µA,5.0V)
Input Supply Current (Low IQ PFM)
−
2.5
3.5
mA
VIN = 12 V, VOUT = 5.0 V, VPWM/PFM ≤
1.2 V, IOUT = No Load, TA = 25ºC
−
10
14
µA
VIN = 12 V, VOUT = 5.0 V, VPWM/PFM ≤
1.2 V, IOUT = No Load, TA = 65ºC
−
15
−
µA
VIN = 12 V, VOUT = 5.0 V, VPWM/PFM ≤
1.2 V, IOUT = 40 µA, TA = 25ºC
−
28
33
µA
VIN = 12 V, VOUT = 5.0 V, VPWM/PFM ≤
1.2 V, IOUT = 40 µA, TA = 65ºC
−
33
−
µA
VIN = 12 V, VOUT = 3.3 V, VPWM/PFM ≤
1.2 V, IOUT = No Load, TA = 25ºC
−
7
10
µA
VIN = 12 V, VOUT = 3.3 V, VPWM/PFM ≤
1.2 V, IOUT = No Load, TA = 65ºC
−
12
−
µA
VIN = 12 V, VOUT = 3.3 V, VPWM/PFM ≤
1.2 V, IOUT = 40 µA, TA = 25ºC
−
20
24
µA
VIN = 12 V, VOUT = 3.3 V, VPWM/PFM ≤
1.2 V, IOUT = 40 µA, TA = 65ºC
−
25
−
µA
VEN/SLEEP = 0 V, TJ ≤ 85°C,
VIN = 16 V
−
5
15
µA
VEN/SLEEP = 0 V, TJ ≤ 85°C,
VIN = 35 V
−
7
25
µA
0ºC < TJ < 85ºC, VOUT = 4 × VCOMP
4.950
5.0
5.050
V
–40ºC < TJ < 150ºC,
VOUT = 4 × VCOMP
4.925
5.0
5.075
V
0ºC < TJ < 85ºC, VOUT = 2 × VCOMP
3.267
3.3
3.333
V
–40ºC < TJ < 150ºC,
VOUT = 2 × VCOMP
3.250
3.3
3.350
V
VIN = 5.8 V, IOUT = 1 A, fOSC = 300 kHz
4.9
−
−
V
VIN = 6.3 V, IOUT = 2 A, fOSC = 300 kHz
4.9
−
−
V
A8585
A8585
3,4
ILO_IQ
(0A,3.3V)
ILO_IQ
(40µA,3.3V)
Input Supply Current (Sleep Mode)
IOUT = 0 mA
IIN(SLEEP)
A8585-1
A8585-1
A8585-2
A8585-3
Voltage Regulation
EVOUT
(5.0V)
Output
A8585
A8585-2
Voltage Accuracy5
EVOUT
(3.3V)
Output Dropout Voltage4
Low IQ Mode
Ripple3,4
Low IQ Peak Current Threshold
VO(PWM)
VPP(LO_IQ)
A8585-1
A8585-3
8 V < VIN < 12 V
IPEAK(LO_IQ)
−
25
65
mVPP
640
800
930
mAPEAK
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, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
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
Error Amplifier
Open Loop Voltage Gain
AVOL
Transconductance with On-Chip
Resistor Divider Included
gm(5.0V)
gm(3.3V)
VCOMP = 1.2 V
−
65
−
dB
88.0
120.0
152.0
μA/V
A8585
A8585-2
2.5 V < VOUT
0 V < VOUT < 2.5 V
46.4
64.0
81.6
μA/V
A8585-1
A8585-3
1.65 V < VOUT
133.3
181.8
230.3
μA/V
0 V < VOUT < 1.65 V
70.3
97.0
123.6
μA/V
VCOMP = 1.2 V
−
±75
−
μA
TJ = 25ºC, VBOOT – VSW = 4.5 V, IDS = 1.0 A
−
110
130
mΩ
TJ < 85ºC, VEN/SLEEP ≤ 0.8 V, VSW = 0 V,
VIN = 16 V
−
−
10
µA
TJ ≤ 150ºC,VEN/SLEEP ≤ 0.8 V, VSW = 0 V,
VIN = 16 V
−
60
150
µA
VIN = 12 V, IOUT = 1 A
−
0.72
−
V/ns
RDS(on)LS
TJ = 25ºC, VIN ≥ 6 V, IDS = 0.1 A
−
−
10
Ω
BOOT Voltage Enable Threshold
VBOOT(TH)
VBOOT rising
1.8
2.0
2.2
V
BOOT Voltage Enable Hysteresis
VBOOT(HYS)
−
400
−
mV
RFSET = 86.6 kΩ
270
300
330
KHz
RFSET = 61.9 kΩ
373
415
457
KHz
RFSET = 45.3 kΩ
Output Current
IEA
Internal MOSFET Parameters2
High-Side MOSFET4
RDS(on)HS
High-Side MOSFET Leakage5
ILKGHS
SW Node Rising/Falling Slew Rate4
Low-Side MOSFET
SR
BOOT Regulator
Oscillator and PWM Timing
PWM Switching Frequency
fOSC
495
550
605
KHz
PWM Frequency Dithering
fDITHER
No dithering with synchronization
−
±7.5
−
%
Minimum Controllable On-Time
tON(MIN)
VIN = 12 V, IOUT = 1 A
−
100
140
ns
Minimum Switch Off-Time
tOFF(MIN)
VIN = 12 V, IOUT = 1 A
−
135
160
ns
FSET/SYNCPWM Synchronization Timing
Synchronization Frequency Range
fSW_MULT
375
−
605
KHz
Synchronization Input Duty Cycle
DSYNC
−
−
80
%
Synchronization Input Pulse Width
twSYNC
200
−
−
ns
Synchronization Input Rise Time4
trSYNC
−
10
15
ns
Synchronization Input Fall
Time4
−
10
15
ns
Synchronization Rising Threshold4
VSYNC LO
VFSET/SYNCPWM Rising
−
−
1.5
V
Threshold4
VSYNC HI
VFSET/SYNCPWM Falling
0.9
−
−
V
Synchronization Falling
tfSYNC
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, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
7
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
3.0
3.5
4.0
A
2.0
2.6
3.0
A
Current Control Loop
PWM Pulse-by-Pulse Limit
COMP to SW Current Gain
ILIM(TONMIN) tON = tON(MIN)PWM
ILIM(TONMAX)
tON = (1 / fSW) – tOFF(MIN)PWM , no PWM
synchronization
gmPOWER
−
3
−
A/V
−
0.35
−
A/μs
RFSET = 86.6 kΩ
0.18
0.25
0.34
A/μs
RFSET = 61.9 kΩ
0.25
0.34
0.45
A/μs
RFSET = 45.3 kΩ
0.34
0.45
0.58
A/μs
2.5
5.0
7.5
ms
−
fOSC / 4
−
kHz
During synchronization
Slope Compensation
SE
Soft Start
Soft Start Ramp Time
tSS
fSS(5.0V)
A8585
A8585-2
Soft Start Switching Frequency
fSS(3.3V)
A8585-1
A8585-3
0 V < VOUT < 1.25 V
1.25 V < VOUT < 2.5 V
fOSC / 2
kHz
2.5 V < VOUT
−
fOSC
−
kHz
0 V < VOUT < 0.825 V
−
fOSC / 4
−
kHz
0.825 V < VOUT < 1.65 V
1.65 V < VOUT
fOSC / 2
kHz
−
fOSC
−
kHz
Hiccup Mode
Hiccup Off-Time
HICOFF
All hiccup faults such as VOUT shorted to GND
−
20
−
ms
Hiccup Overcurrent Protection (OCP)
Count
OCPLIM
t > 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, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; 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, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; 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, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; 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, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
14
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
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
15
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
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
16
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
The A8585 family utilizes fixed-frequency, peak current mode
control to provide excellent load and line regulation, fast transient
response, and ease of compensation.
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.
Normalized RDS(on)
1.4
Low-IQ Pulse Frequency Modulation (PFM)
Mode
1.2
1.0
0.8
0.6
0.4
0.2
0
-40
-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
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
17
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)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
18
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)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
19
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), the
OCP hiccup counter is disabled. Two conditions must be met for
the OCP counter to be enabled and begin counting:
• 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)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
20
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 up 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)
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
21
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<VSSRST
and TSD = 0
Pulled low
until
VSS<VSSRST
and TSD = 0
Forced Off
Disabled
Off
Pulled Low
No
Auto, part
cools down
Output
overvoltage,
(VOUT >
3.6 V/5.5 V)
NPOR
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
22
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
Design and Component Selection
PWM Switching Frequency (RFSET)
The PWM switching frequency is set by connecting a resistor
from the FSET/SYNCPWM pin to ground. Figure 11 is a graph
showing the relationship between the typical switching frequency
(y-axis) and the FSET resistor (x-axis). For a required switching
frequency ( fSW ), the FSET resistor value can be calculated as
follows:
RFSET =
27770
fSW
– 4.78
(1)
where fSW is in kHz and RFSET is in kΩ.
When the PWM switching frequency is chosen, the user should
be aware of the minimum controllable on-time ( tON( MIN) ) and
minimum off time of the A8585 family. If the system required ontime is less than tON( MIN) then switch node jitter occurs and the
output voltage has increased ripple or oscillations.
The PWM switching frequency should be calculated as follows:
fSW =
VOUT
tON(MIN) ×VIN(MAX)
(2)
where VOUT is the output voltage, tON(MIN) is the minimum
controllable on-time of the A8585 family (100 ns (typ), 140 ns
(max) ), and VIN(MAX) is the maximum required operational input
voltage (not the peak surge voltage).
2000
Swtiching Frequency, fSW (kHz)
1800
1600
If the device synchronization function is employed, then the base
switching frequency should be chosen such that jitter does not
result at the maximum synchronized switching frequency according to equation 2.
Output Inductor (LO )
For a peak current mode regulator it is common knowledge
that, without adequate slope compensation, the system becomes
unstable when the duty cycle is near or above 50%. However, the
slope compensation in the A8585 family is a fixed value (SE ).
Therefore, it is important to calculate an inductor value such that
the falling slope of the inductor current ( SF ) works well with
the device slope compensation. Equations 3a and 3b can be used
to calculate a range of values for the output inductor based on
the well known approach of providing slope compensation that
matches 50% to 100% of the down slope of the inductor current.
VOUT +Vf
2 × SE
SE = 0.13 × f 2SW + 0.69 × f SW+ 0.031
More recently, Dr. Raymond Ridley presented a formula to
calculate the amount of slope compensation required to critically
damp the double poles at half the PWM switching frequency (this
approach includes the duty cycle (D), which should be calculated
at the minimum input voltage to insure optimal stability):
VOUT + Vf
VIN(MIN) + Vf
1 – 0.18 ×
SE
VOUT + Vf
(4)
To avoid dropout (saturation of the buck regulator), VIN(MIN)
must be approximately 1 to 1.5 V above VOUT when calculating
the inductor value with equation 4.
800
600
400
200
0
(3b)
where SE is in A/µs and fSW is in MHz.
LO ≥
1000
VOUT +Vf
SE
(3a)
where LO is in µH, Vf is the forward voltage of the asynchronous
diode, and the slope compensation (SE ) is a function of switching
frequency, as follows:
1400
1200
≤ LO ≤
0
10
20
30
40 50
60 70
80
90 100 110 120 130 140 150
FSET Resistor, RFSET (kΩ)
If equations 3a or 4 yield an inductor value that is not a standard
value then the next closest available value should be used. The
final inductor value should allow for 10% to 20% of initial tolerance and 20% to 30% of inductor saturation.
Figure 11: PWM Switching Frequency versus RFSET
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
23
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
The saturation current of the inductor should be higher than the
peak current capability of the device. Ideally, for output short
circuit conditions, the inductor should not saturate given the highest pulse-by-pulse current limit at minimum duty cycle (ILIM(0)),
4.0 A (max). This may be too costly. At the very least, the inductor should not saturate given the peak operating current according
to the following equation:
IPEAK = 4.1 –
SE × (VOUT + Vf )
1.15 × fSW × (VIN(MAX) + Vf )
(5)
where VIN(MAX) is the maximum continuous input voltage, such
as 18 V (not a surge voltage, like 40 V).
Starting with equation 5 and subtracting half of the inductor
ripple current provides us with an interesting equation to predict
the typical DC load capability of the regulator at a given duty
cycle (D):
SE × D VOUT × (1 – D )
(6)
–
IOUT(DC) ≤ 4.1 –
fSW
2 × fSW × LO
After an inductor is chosen it should be tested during output short
circuit conditions. The inductor current should be monitored
using a current probe. A good design should ensure the inductor
or the regulator are not damaged when the output is shorted to
ground at maximum input voltage and the highest expected ambient temperature.
Output Capacitors
The output capacitors filter the output voltage to provide an
acceptable level of ripple voltage, and they also store energy to
help maintain voltage regulation during a load transient. The
voltage rating of the output capacitors must support the output
voltage with sufficient design margin.
The output voltage ripple (ΔVOUT ) is a function of the output
capacitor parameters: CO , ESRCO , and ESLCO:
∆VOUT =
∆IL × ESRCO
+
VIN – VOUT
LO
∆IL
+
8 fSWC O
× ESLCO
(7)
The type of output capacitors determines which terms of equa-
tion 7 are dominant. For ceramic output capacitors the ESRCO
and ESLCO are virtually zero, so the output voltage ripple will be
dominated by the third term of equation 7:
∆VOUT ≤
∆IL
8 fSWC O
(8)
To reduce the voltage ripple of a design using ceramic output
capacitors simply: increase the total capacitance, reduce the
inductor current ripple (that is, increase the inductor value), or
increase the switching frequency.
For electrolytic output capacitors the value of capacitance will be
relatively high, so the third term in equation 7 will be very small
and the output voltage ripple will be determined primarily by the
first two terms of equation 7:
∆VOUT = ∆IL × ESRCO +
VIN – VOUT
× ESLCO
(9)
To reduce the voltage ripple of a design using electrolytic output
capacitors simply: decrease the equivalent ESRCO and ESLCO
by using a high(er) quality capacitor, or add more capacitors in
parallel, or reduce the inductor current ripple (that is, increase the
inductor value).
LO
The ESR of some electrolytic capacitors can be quite high so
Allegro recommends choosing a quality capacitor for which the
ESR or the total impedance is clearly documented in the capacitor datasheet. Also, the ESR of electrolytic capacitors usually
increases significantly at cold ambients, as much as 10 X, which
increases the output voltage ripple and, in most cases, reduces the
stability of the system.
The transient response of the regulator depends on the quantity
and type of output capacitors. In general, minimizing the ESR of
the output capacitance will result in a better transient response.
The ESR can be minimized by simply adding more capacitors
in parallel or by using higher quality capacitors. At the instant
of a fast load transient (di/dt), the output voltage changes by the
amount:
∆VOUT = ∆IL × ESRCO +
di
× ESLCO
(10)
After the load transient occurs, the output voltage will deviate
from its nominal value for a short time. The length of this time
depends on the system bandwidth, the output inductor value, and
output capacitance. Eventually, the error amplifier brings the
output voltage back to its nominal value.
dt
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
24
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
The speed at which the error amplifier brings the output voltage
back to the setpoint depends mainly on the closed-loop bandwidth of the system. A higher bandwidth usually results in a
shorter time to return to the nominal voltage. However, a higher
bandwidth system may be more difficult to obtain acceptable gain
and phase margins. Selection of the compensation components
(RZ , CZ , and CP ) are discussed in more detail in the Compensation Components section of this datasheet.
Low-IQ PFM Output Voltage Ripple Calculation
After choosing an output inductor and output capacitor(s) it is
important to calculate the output voltage ripple ( (∆VOUT(PFM) )
during Low-IQ PFM mode. With ceramic output capacitors the
output voltage ripple in PWM mode is usually negligible, but that
is not the case during Low-IQ PFM mode.
The PFM mode comparator requires about 20 mV or greater of
voltage ripple on the VOUT pin, and generates groups of pulses to
meet this requirement. However, if a single pulse results in a voltage ripple greater than 20 mV, then the voltage ripple would be
dictated by that single pulse. To calculate the voltage ripple from
that single pulse, first the peak inductor current must be calculated with slope compensation accounted for. The IPEAK(LO_IQ)
specification does not include slope compensation, therefore the
peak inductor current operating point is calculated as follows:
IPEAK(LO_IQ)
(11)
SE × LO
1+
VIN – VOUT
Then, calculate the MOSFET on-time and off-time (figure 12).
The on-time is defined as the time it takes for the inductor current
to reach IPEAK_L :
IPEAK_L × LO
(12)
tON =
VIN – VOUT – IPEAK_L × (RDS(on)HS + LO(DCR) )
where RDS(on) is the on-resistance of the internal high-side
MOSFET (110 mΩ (typ)) and LO(DCR) is the DC resistance of the
output inductor, LO .
IPEAK_L =
During this rising time interval, the length of time for the inductor current to rise from 0 A to IOUT is:
t1 =
IOUT × LO
(13)
The off-time is defined as the time it takes for the inductor current to decay from IPEAK_L to 0 A:
VIN – VOUT – IPEAK_L × (RDS(on)HS + LO(DCR) )
IPEAK_L × LO
(14)
During this falling time interval, the length of time for the inductor current to fall from IOUT to 0 A is:
tOFF =
VOUT + Vf
IOUT × LO
(15)
Given the peak inductor current (IPEAK_L ) and the rise and fall
t2 =
VOUT + Vf
VOUT
Vpp(LO_IQ)
IL
I PEAK(LO_IQ)
I OUT
t
t1
t ON
t2
t OFF
Figure 12: Illustration of Calculating the Output Ripple
Voltage in PFM Mode
times (tON and tOFF) for the inductor current, the output voltage
ripple can be calculated for a signal pulse as follows:
IPEAK_L – IOUT
× (tON + tOFF – t1 – t2 )
VPP(LO_IQ) =
(16)
2 × COUT
If VPP(LO_IQ) is greater than the ~20 mV ripple that the PFM
comparator requires, then the output capacitance or inductor
can be adjusted to reduce the PFM mode voltage ripple. In PFM
mode decreasing the inductor value reduces the PFM ripple, but
may negatively impact the PWM voltage ripple, maximum load
current in PWM mode, or change the mode of operation from
CCM to DCM.
If VPP(LO_IQ) is less than the ~20 mV requirement, the A8585
operates with multiple pulses at the PWM frequency to meet the
ripple requirement. The fixed frequency operation may result in
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
25
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
DCM or CCM operation during the multiple pulses.
Three factors should be considered when choosing the input
capacitors. First, the capacitors must be chosen to support the
maximum expected input surge voltage with adequate design
margin. Second, the capacitor rms current rating must be higher
than the expected rms input current to the regulator. Third, the
capacitors must have enough capacitance and a low enough ESR
to limit the input voltage dV/dt to something much less than the
hysteresis of the UVLO circuitry (nominally 400 mV for the
A8585 family) at maximum loading and minimum input voltage.
The input capacitors must deliver an rms current (IRMS) according to the following formula:
IRMS = IOUT D × ( 1 – D)
where the duty cycle (D) is defined as:
(17)
D ≈ (VOUT + Vf ) / (VIN + Vf )
(18)
and Vf is the forward voltage of the asynchronous diode, D1.
Figure 13 shows the normalized input capacitor rms current
versus duty cycle. To use this graph, simply find the operational
duty cycle (D) on the x-axis and determine the input/output current multiplier on the y-axis. For example, at a 20% duty cycle,
the input/output current multiplier is 0.40. Therefore, if the
regulator is delivering 2.0 A of steady-state load current, the input
capacitor(s) must support 0.40 × 2.0 A or 0.8 Arms.
The input capacitor(s) must limit the voltage deviations at the
VIN pin to something significantly less than the device UVLO
hysteresis during maximum load and minimum input voltage.
The following equation allows us to calculate the minimum input
capacitance:
IOUT × D × (1– D)
CIN ≥
(19)
0.85 × fSW × ∆VIN(MIN)
where ΔVIN(MIN) is chosen to be much less than the hysteresis
of the VIN UVLO comparator (ΔVIN(MIN) ≤ 150 mV is recommended), and fSW is the nominal PWM frequency.
IRMS / IOUT
Input Capacitors
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
0
10
20
30
40
50
60
Duty Cycle, D (%)
70
80
90
100
Figure 13: Normalized Input Capacitor Ripple versus
Duty Cycle
value of 0.25 at 50% duty cycle. So, for example, a very conservative design based on IOUT = 2.0 A, fSW = 85% of 425 kHz,
D × (1–D) = 0.25, and ΔVIN =150 mV yields:
CIN ≥
2.0 (A) × 0.25
= 9.2 µF
361 (kHz) × 150 (mV)
A good design should consider the DC-bias effect on a ceramic
capacitor: as the applied voltage approaches the rated value, the
capacitance value decreases. This effect is very pronounced with
the Y5V and Z5U temperature characteristic devices (as much as
90% reduction), so these types should be avoided. The X5R and
X7R type capacitors should be the primary choices due to their
stability versus both DC bias and temperature.
For all ceramic capacitors, the DC-bias effect is even more pronounced on smaller sizes of device case, so a good design uses
the largest affordable case size (such as 1206 or 1210). Also, it is
advisable to select input capacitors with plenty of design margin
in the voltage rating to accommodate the worst case transient
input voltage (such as a load dump as high as 40 V for automotive applications).
The D × (1–D) term in equation 17 has an absolute maximum
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
26
A8585
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
Asynchronous Diode (D1)
There are three requirements for the asynchronous diode. First,
the asynchronous diode must be able to withstand the regulator input voltage when the high-side MOSFET is on. Therefore,
choose a diode with a reverse voltage rating (VR) higher than
the maximum expected input voltage (that is, the surge voltage).
Second, the forward voltage of the diode ( Vf ) should be minimized or the regulator efficiency suffers. Also, if Vf is too high(20)
the missing-diode protection in the A8585 family could be inappropriately activated. A Schottky type diode that can maintain
a very low Vf when the regulator output is shorted to ground, at
the coldest ambient temperature, is highly recommended. Third,
the asynchronous diode must conduct the output current when
the high-side MOSFET is off. Therefore, the average forward
current rating of this diode ( If(av) ) must be high enough to deliver
the load current according to the following equation:
If(av) ≥ IOUT(MAX) (1 – DMIN )(21)
where DMIN is the minimum duty cycle, as defined in equation 19
and IOUT(MAX) is the maximum continuous output current of the
regulator.
Bootstrap Capacitor
A bootstrap capacitor must be connected between the BOOT and
SW pins to provide floating gate drive to the high-side MOSFET.
Usually, 47 nF is an adequate value. This capacitor should be a
high-quality ceramic capacitor, such as an X5R or X7R, with a
voltage rating of at least 16 V.
The A8585 family incoporates a 10 Ω low-side MOSFET to
insure that the bootstrap capacitor is always charged, even when
the converter is lightly loaded or pre-biased.
Compensation Components (RZ, CZ , and CP)
To properly compensate the system, it is important to understand
where the buck power stage, load resistance, and output capacitance form their poles and zeros in frequency. Also, it is important to understand that the (Type II) compensated error amplifier
introduces a zero and two more poles, and where these should be
placed to maximize system stability, provide a high bandwidth,
and optimize the transient response.
First, consider the power stage of the A8585 family, the output
capacitors, and the load resistance. This circuitry is commonly
referred as the control-to-output transfer function. The low frequency gain of this circuitry depends on the COMP to SW current
gain ( gmPOWER ), and the value of the load resistor (RL ). The DC
gain (GCO(0HZ) ) of the control-to-output is:
GCO(0Hz) = gmPOWER × RL(22)
The control-to-output transfer function has a pole (fP1), formed
by the output capacitance (COUT) and load resistance (RL),
located at:
fP1 =
1
2� × RL × COUT
(23)
The control-to-output transfer function also has a zero (fZ1)
formed by the output capacitance (COUT) and its associated ESR:
fZ1 =
1
2� × ESR × COUT
(24)
For a design with very low-ESR type output capacitors (such
as ceramic or OSCON output capacitors), the ESR zero, fZ1, is
usually at a very high frequency so it can be ignored. On the
other hand, if the ESR zero falls below or near the 0 dB crossover frequency of the system (as happens with electrolytic output
capacitors), then it should be cancelled by the pole formed by the
CP capacitor and the RZ resistor (discussed and identified later
as fP3 ).
A Bode plot of the control-to-output transfer function for the
configuration shown in the Typical Application diagram, with
VOUT = 5.0 V, IOUT = 2.0 A, and RL = 2.5 Ω, is shown in figure
14. The pole at fP1 can easily be seen at 1.2 kHz while the ESR
zero, fZ1 , occurs at a very high frequency, 600 kHz (this is typical
for a design using ceramic output capacitors). Note, there is more
than 90° of total phase shift because of the double-pole at half the
switching frequency.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
27
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
80
Gain (dB)
40
20
GCO(0Hz) = 17 dB
f P1 = 1.2 kHz
0
-20
-40
-60
Phase (°)
180
0
-180
10
1
(27)
2� × RZ × CZ
Lastly, the transfer function of the Type-II compensated error
amplifier has a (very) high frequency pole (fP3) dominated by the
RZ resistor and the CP capacitor:
1
fP3 =
(28)
2� × RZ × CP
A Bode plot of the error amplifier and its compensation network
is shown in figure 15, where fP2, fP3, and fZ2 are indicated on the
Gain plot. Notice that the zero (fZ2 at 2.6 kHz) has been placed
so that it is just above the pole at fP1 previously shown at 1.2 kHz
in the control-to-output Bode plot (Figure 14). Placing fZ2 just
above fP1 results in excellent phase margin, but relatively slow
transient recovery time, as we will see later.
fZ2 =
60
Double Pole at
275 kHz
30
100
300
103
3×103
10×103
30×103
100×103 300×103
106
Frequency (Hz)
Figure 14: Control-to-Output Bode Plot
Next, consider the error amplifier ( gm ), and the compensation
network RZ-CZ-CP . It greatly simplifies the transfer function derivation if RO >> 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
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
28
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 <
2� × RZ × fC
2� × 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 .
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
29
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
2� × 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.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
30
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
=
VOUT +Vf
VIN +Vf
∆IL2
2
× IOUT + 12 × RDS(on)HS
where
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.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
31
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.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
33
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
PACKAGE OUTLINE DRAWING
For Reference Only – Not for Tooling Use
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
+0.08
–0.10
3.30 ±25
8º MAX
0º MIN
10
0.25 MAX
0.19 MIN
B
2.41 ±0.25
3.91
+0.08
–0.10
6.00 ±0.20
A
0.685 ±0.20
1
2
Branded Face
0.25 BSC
SEATING PLANE
10X
C
1.55 ±0.10
0.10
C
GAUGE PLANE
SEATING
PLANE
0.40 MAX
0.30 MIN
1.00 BSC
0.10 ±0.05
A
Terminal #1 mark area
B
Exposed thermal pad (bottom surface)
Figure 19: Package LK, 10-Pin SOIC with Exposed Thermal Pad
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
34
Wide Input Voltage, 2 A Buck
Regulator Family with Low IQ Mode
A8585
Revision History
Revision
Current
Revision 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
Description of Revision
Corrected Selection Guide part numbers
Copyright ©2016, Allegro MicroSystems, LLC
Allegro MicroSystems, LLC 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, LLC assumes no responsibility for its
use; nor for any infringement of patents or other rights of third parties which may result from its use.
For the latest version of this document, visit our website:
www.allegromicro.com
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
35
Similar pages