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ISL6431
TM
Data Sheet
June 2001
File Number
9018
Advanced Pulse-Width Modulation (PWM)
Controller for Home Gateways
The ISL6431 is a high efficiency, fixed frequency,
synchronous buck PWM controller. It is designed for use in
applications that convert 5V to lower distributed voltages
required for set-top box, cable modem, DSL modem and
residential home gateway core processor, memory and
peripheral power supplies.
This device makes simple work out of implementing a
complete control and protection scheme for a DC-DC
stepdown converter. Designed to drive N-channel MOSFETs
in a synchronous buck topology, the ISL6431 integrates the
control, output adjustment, monitoring and protection
functions into a single 8-pin package.
The ISL6431 provides simple, single feedback loop, voltage-
mode control with fast transient response. The output
voltage can be precisely regulated to as low as 0.8V, with a
maximum tolerance of ±1.5% over temperature and line
voltage variations. A fixed frequency oscillator reduces
design complexity, while balancing typical application cost
and efficiency.
The error amplifier features a 15MHz gain-bandwidth
product and 6V/µs slew rate which enables high converter
bandwidth for fast transient performance. The resulting
PWM duty cycles range from 0% to 100%.
Protection from overcurrent conditions is provided by
monitoring the r
DS(ON)
of the upper MOSFET to inhibit PWM
operation appropriately. This approach simplifies the
implementation and improves efficiency by eliminating the
need for a current sense resistor.
Features
• Operates from +5V Input
• 0.8V to V
IN
Output Range
- 0.8V Internal Reference
- ±1.5% Over Line Voltage and Temperature
• Drives N-Channel MOSFETs
• Simple Single-Loop Control Design
- Voltage-Mode PWM Control
• Fast Transient Response
• Lossless, Programmable Overcurrent Protection
- Uses Upper MOSFET’s r
DS(ON)
• Small Converter Size
- 300kHz Fixed Frequency Oscillator
- Internal Soft Start
- 8 Lead SOIC Package
• High Conversion Efficiency
• Synchronous/Standard Buck Configuration
Applications
• Cable Modems, Set Top Boxes, and DSL Modems
• DSP and Core Communications Processor Supplies
• Power Supplies for Microprocessors and Embedded
Controllers
• Memory Supplies
• Personal Computer Peripherals
• Industrial Power Supplies
• 5V-Input DC-DC Regulators
Ordering Information
PART NUMBER
ISL6431CB
ISL6431IB
ISL6431EVAL1
TEMP. RANGE
(
o
C)
0 to 70
-40 to 85
PACKAGE
8 Ld SOIC
8 Ld SOIC
PKG.
NO.
M8.15
M8.15
• Low-Voltage Distributed Power Supplies
Pinout
BOOT 1
UGATE 2
GND 3
LGATE 4
8 PHASE
7 COMP/OCSET
6 FB
5 VCC
Evaluation Board
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 321-724-7143
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Intersil and Design is a trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2001, All Rights Reserved
ISL6431
Block Diagram
VCC
SAMPLE
AND
HOLD
+
OC
COMPARATOR
-
POR AND
SOFTSTART
BOOT
UGATE
PHASE
+
0.8V
ERROR
AMP
+
PWM
COMPARATOR
+
INHIBIT
GATE
CONTROL
LOGIC
PWM
-
FB
COMP/OCSET
20µA
-
-
VCC
LGATE
OSCILLATOR
FIXED 300kHz
GND
Typical Application
V
CC
C
3
C
4
C
5
VCC
D
BOOT
1
BOOT
R
1
5
ISL6431
COMP/OCSET
7
UGATE
2
8
PHASE
C
6
L
OUT
+V
O
= 0.8 to V
IN
R
2
C
2
C
1
FB
6
3
GND
4
LGATE
C
7
R
3
R
4
2
ISL6431
Absolute Maximum Ratings
Supply Voltage, V
CC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +6.0V
Absolute Boot Voltage, V
BOOT
. . . . . . . . . . . . . . . . . . . . . . . +15.0V
Upper Driver Supply Voltage, V
BOOT
- V
PHASE
. . . . . . . . . . . +6.0V
Input, Output or I/O Voltage . . . . . . . . . . . GND -0.3V to VCC +0.3V
ESD Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Class 2
Thermal Information
Thermal Resistance (Typical, Note 1)
θ
JA
(
o
C/W)
SOIC Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
100
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . 150
o
C
Maximum Storage Temperature Range . . . . . . . . . . -65
o
C to 150
o
C
Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . 300
o
C
(SOIC - Lead Tips Only)
Operating Conditions
Supply Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . +5V ±10%
Ambient Temperature Range . . . . . . . . . . . . . . . . . . . -40
o
C to 85
o
C
Junction Temperature Range. . . . . . . . . . . . . . . . . . -40
o
C to 125
o
C
CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the
device at these or any other conditions above those indicated in the operational sections of this specification is not implied.
NOTE:
1.
θ
JA
is measured with the component mounted on a low effective thermal conductivity test board in free air. See Tech Brief TB379 for details.
Electrical Specifications
PARAMETER
VCC SUPPLY CURRENT
Nominal Supply
Recommended Operating Conditions, Unless Otherwise Noted V
CC
= 5.0V±5% and T
A
= 25
o
C
SYMBOL
TEST CONDITIONS
MIN
TYP
MAX
UNITS
I
VCC
ISL6431CB; UGATE and LGATE Open
ISL6431IB; UGATE and LGATE Open
-
2.5
3.2
3.2
-
3.8
mA
mA
POWER-ON RESET
Rising VCC POR Threshold
POR
ISL6431CB
ISL6431IB
VCC POR Threshold
Hysteresis
OSCILLATOR
Frequency
f
OSC
∆V
OSC
ISL6431CB; VCC = 5V
ISL6431IB; VCC = 5V
Ramp Amplitude
ISL6431CB
ISL6431IB
REFERENCE
Reference Voltage Tolerance
ISL6431CB
ISL6431IB
Nominal Reference Voltage
V
REF
ISL6431CB
ISL6431IB
ERROR AMPLIFIER
DC Gain
ISL6431CB
ISL6431IB
Gain-Bandwidth Product
GBWP
ISL6431CB
ISL6431IB
Slew Rate
SR
ISL6431CB; COMP = 10pF
ISL6431IB; COMP = 10pF
GATE DRIVERS
Upper Gate Source Current
I
UGATE-SRC
I
UGATE-SNK
I
LGATE-SRC
ISL6431CB; V
BOOT
- V
PHASE
= 5V, V
UGATE
= 4V
ISL6431IB; V
BOOT
- V
PHASE
= 5V, V
UGATE
= 4V
Upper Gate Sink Current
ISL6431CB
ISL6431IB
Lower Gate Source Current
ISL6431CB; V
CC
= 5V, V
LGATE
= 4V
ISL6431IB; V
CC
= 5V, V
LGATE
= 4V
-
-
-
-
-
-
1.0
1.0
1.0
1.0
1.0
1.0
-
-
-
-
-
-
A
A
A
A
A
A
-
-
14
14
-
4.5
82
82
-
-
8.0
8.0
-
-
-
-
-
9.2
dB
dB
MHz
MHz
V/µs
V/µs
-
-
-
-
-
-
0.800
0.800
1.5
1.5
-
-
%
%
V
V
-
230
-
-
300
300
1.5
1.5
-
340
-
-
kHz
kHz
V
P-P
V
P-P
ISL6431CB
ISL6431IB
-
4.17
-
0.01
4.30
4.30
0.20
0.20
-
4.50
-
0.85
V
V
V
V
3
ISL6431
Electrical Specifications
PARAMETER
Lower Gate Sink Current
Recommended Operating Conditions, Unless Otherwise Noted V
CC
= 5.0V±5% and T
A
= 25
o
C
(Continued)
SYMBOL
I
LGATE-SNK
ISL6431CB
ISL6431IB
PROTECTION / DISABLE
OCSET Current Source
I
OCSET
V
DISABLE
ISL6431CB
ISL6431IB
Disable Threshold
ISL6431CB
ISL6431IB
17
14
-
-
20
20
-
-
22
24
0.8
0.8
µA
µA
V
V
TEST CONDITIONS
MIN
-
-
TYP
2.0
2.0
MAX
-
-
UNITS
A
A
Functional Pin Descriptions
VCC (Pin 5)
This is the main bias supply for the ISL6431, as well as the
lower MOSFET’s gate. Connect a well-decoupled 5V supply
to this pin.
(I
OCSET
), and the upper MOSFET on-resistance (r
DS(ON)
)
set the converter overcurrent (OC) trip point according to the
following equation:
I
OCSET
xR
OC SET
I
PEAK
= -------------------------------------------------
r
DS
(
ON
)
FB (Pin 6)
This pin is the inverting input of the internal error amplifier.
Use this pin, in combination with the COMP/OCSET pin, to
compensate the voltage-control feedback loop of the
converter.
Internal circuitry of the ISL6431 will not recognize a voltage
drop across R
OCSET
larger than 0.5V. Any voltage drop
across R
OCSET
that is greater than 0.5V will set the
overcurrent trip point to:
0.5V
-
I
PEAK
= ---------------------
r
DS
(
ON
)
GND (Pin 3)
This pin represents the signal and power ground for the IC.
Tie this pin to the ground island/plane through the lowest
impedance connection available.
An overcurrent trip cycles the soft-start function.
Pulling OCSET to a level below 0.8V will disable the
controller. Disabling the ISL6431 causes the oscillator to
stop, the LGATE and UGATE outputs to be held low, and the
softstart circuitry to re-arm.
During soft-start, and all the time during normal converter
operation, this pin represents the output of the error
amplifier. Use this pin, in combination with the FB pin, to
compensate the voltage-control feedback loop of the
converter.
PHASE (Pin 8)
Connect this pin to the upper MOSFET source. This pin is
used to monitor the voltage drop across the upper MOSFET
for overcurrent protection. This pin is also monitored by the
continuously adaptive shoot-through protection circuitry to
determine when the upper MOSFET has turned off.
UGATE (Pin 2)
Connect this pin to the upper MOSFET’s gate. This pin
provides the PWM-controlled gate drive for the upper
MOSFET. This pin is also monitored by the adaptive shoot-
through protection circuitry to determine when the upper
MOSFET has turned off. Do not insert any circuitry between
this pin and the gate of the upper MOSFET, as it may
interfere with the internal adaptive shoot-through protection
circuitry and render it ineffective.
LGATE (Pin 4)
Connect this pin to the lower MOSFET’s gate. This pin
provides the PWM-controlled gate drive for the lower
MOSFET. This pin is also monitored by the adaptive shoot-
through protection circuitry to determine when the lower
MOSFET has turned off. Do not insert any circuitry between
this pin and the gate of the lower MOSFET, as it may
interfere with the internal adaptive shoot-through protection
circuitry and render it ineffective.
BOOT (Pin 1)
This pin provides ground referenced bias voltage to the
upper MOSFET driver. A bootstrap circuit is used to create a
voltage suitable to drive a logic-level N-channel MOSFET.
Functional Descriptions
Initialization
The ISL6431 automatically initializes upon receipt of power.
The Power-On Reset (POR) function continually monitors the
bias voltage at the VCC pin. The POR function initiates the
Overcurrent Protection (OCP) sampling and hold operation
after the supply voltage exceeds its POR threshold. Upon
COMP/OCSET (Pin 7)
This is a multiplexed pin. During a short period of time
following power-on reset (POR), this pin is used to determine
the overcurrent threshold of the converter. Connect a
resistor (R
OCSET
) from this pin to the drain of the upper
MOSFET (V
CC
). R
OCSET
, an internal 20µA current source
4
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