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TM
Data Sheet
ISL6431
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 rDS(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.
Ordering Information
TEMP. RANGE
PART NUMBER
(oC)
PACKAGE
ISL6431CB
0 to 70 8 Ld SOIC
ISL6431IB
-40 to 85 8 Ld SOIC
ISL6431EVAL1
Evaluation Board
PKG.
NO.
M8.15
M8.15
Features
• Operates from +5V Input
• 0.8V to VIN 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 rDS(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
• Low-Voltage Distributed Power Supplies
Pinout
BOOT 1
UGATE 2
GND 3
LGATE 4
8 PHASE
7 COMP/OCSET
6 FB
5 VCC
1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 321-724-7143 | Intersil and Design is a trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2001, All Rights Reserved

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Block Diagram
ISL6431
VCC
SAMPLE
AND
HOLD
+
-
OC
COMPARATOR
POR AND
SOFTSTART
+
0.8V
-
FB
COMP/OCSET
20µA
ERROR
AMP
+
-
PWM
COMPARATOR
+
-
INHIBIT
GATE
CONTROL
PWM LOGIC VCC
OSCILLATOR
FIXED 300kHz
GND
BOOT
UGATE
PHASE
LGATE
Typical Application
VCC
C3 C4
VCC
R1
5
BOOT
1
ISL6431
COMP/OCSET
7
UGATE
2
PHASE
R2 8
C2
C1
63
LGATE
4
FB GND
DBOOT
C5
C6
LOUT
C7
R3
R4
+VO = 0.8 to VIN
2

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ISL6431
Absolute Maximum Ratings
Supply Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +6.0V
Absolute Boot Voltage, VBOOT . . . . . . . . . . . . . . . . . . . . . . . +15.0V
Upper Driver Supply Voltage, VBOOT - VPHASE . . . . . . . . . . . +6.0V
Input, Output or I/O Voltage . . . . . . . . . . . GND -0.3V to VCC +0.3V
ESD Classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Class 2
Operating Conditions
Supply Voltage, VCC . . . . . . . . . . . . . . . . . . . . . . . . . . . . +5V ±10%
Ambient Temperature Range . . . . . . . . . . . . . . . . . . . -40oC to 85oC
Junction Temperature Range. . . . . . . . . . . . . . . . . . -40oC to 125oC
Thermal Information
Thermal Resistance (Typical, Note 1)
θJA (oC/W)
SOIC Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
100
Maximum Junction Temperature . . . . . . . . . . . . . . . . . . . . . . 150oC
Maximum Storage Temperature Range . . . . . . . . . . -65oC to 150oC
Maximum Lead Temperature (Soldering 10s) . . . . . . . . . . . . 300oC
(SOIC - Lead Tips Only)
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. θJA 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 Recommended Operating Conditions, Unless Otherwise Noted VCC = 5.0V±5% and TA = 25oC
PARAMETER
SYMBOL
TEST CONDITIONS
MIN TYP MAX
VCC SUPPLY CURRENT
Nominal Supply
IVCC
ISL6431CB; UGATE and LGATE Open
ISL6431IB; UGATE and LGATE Open
- 3.2 -
2.5 3.2 3.8
POWER-ON RESET
Rising VCC POR Threshold
POR
ISL6431CB
- 4.30 -
ISL6431IB
4.17 4.30 4.50
VCC POR Threshold
Hysteresis
ISL6431CB
ISL6431IB
- 0.20 -
0.01 0.20 0.85
OSCILLATOR
Frequency
fOSC
ISL6431CB; VCC = 5V
ISL6431IB; VCC = 5V
- 300 -
230 300 340
Ramp Amplitude
VOSC
ISL6431CB
ISL6431IB
- 1.5 -
- 1.5 -
REFERENCE
Reference Voltage Tolerance
ISL6431CB
- - 1.5
ISL6431IB
- - 1.5
Nominal Reference Voltage
VREF
ISL6431CB
ISL6431IB
- 0.800 -
- 0.800 -
ERROR AMPLIFIER
DC Gain
ISL6431CB
- 82 -
ISL6431IB
- 82 -
Gain-Bandwidth Product
GBWP ISL6431CB
14 -
-
ISL6431IB
14 -
-
Slew Rate
SR ISL6431CB; COMP = 10pF
- 8.0 -
ISL6431IB; COMP = 10pF
4.5 8.0 9.2
GATE DRIVERS
Upper Gate Source Current
IUGATE-SRC ISL6431CB; VBOOT - VPHASE = 5V, VUGATE = 4V
-
1.0
-
ISL6431IB; VBOOT - VPHASE = 5V, VUGATE = 4V
-
1.0
-
Upper Gate Sink Current
IUGATE-SNK ISL6431CB
- 1.0 -
ISL6431IB
- 1.0 -
Lower Gate Source Current
ILGATE-SRC ISL6431CB; VCC = 5V, VLGATE = 4V
ISL6431IB; VCC = 5V, VLGATE = 4V
- 1.0 -
- 1.0 -
UNITS
mA
mA
V
V
V
V
kHz
kHz
VP-P
VP-P
%
%
V
V
dB
dB
MHz
MHz
V/µs
V/µs
A
A
A
A
A
A
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ISL6431
Electrical Specifications Recommended Operating Conditions, Unless Otherwise Noted VCC = 5.0V±5% and TA = 25oC (Continued)
PARAMETER
SYMBOL
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Lower Gate Sink Current
ILGATE-SNK ISL6431CB
ISL6431IB
- 2.0 -
- 2.0 -
A
A
PROTECTION / DISABLE
OCSET Current Source
IOCSET
ISL6431CB
ISL6431IB
17 20 22
14 20 24
µA
µA
Disable Threshold
VDISABLE
ISL6431CB
ISL6431IB
- - 0.8 V
- - 0.8 V
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.
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.
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.
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.
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.
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 (ROCSET) from this pin to the drain of the upper
MOSFET (VCC). ROCSET, an internal 20µA current source
(IOCSET), and the upper MOSFET on-resistance (rDS(ON))
set the converter overcurrent (OC) trip point according to the
following equation:
IPEAK
=
I--O-----C----S----E----T----x---R-----O----C----S----E----T--
rDS(ON)
Internal circuitry of the ISL6431 will not recognize a voltage
drop across ROCSET larger than 0.5V. Any voltage drop
across ROCSET that is greater than 0.5V will set the
overcurrent trip point to:
IPEAK
=
------0---.--5----V-------
rDS(ON)
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.
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.
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
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ISL6431
completion of the OCP sampling and hold operation, the POR
function initiates the Soft Start operation.
Over Current Protection
The overcurrent function protects the converter from a shorted
output by using the upper MOSFET’s on-resistance, rDS(ON),
to monitor the current. This method enhances the converter’s
efficiency and reduces cost by eliminating a current sensing
resistor.
14A
12A
10A
8A
6A
4A
2A
0A
TIME (50µs/DIV.)
FIGURE 1. OVERCURRENT OPERATION
The overcurrent function cycles the soft-start function in a
hiccup mode to provide fault protection. A resistor
(ROCSET) programs the overcurrent trip level (see Typical
Application diagram).
Immediately following POR, the ISL6431 initiates the
Overcurrent Protection sampling and hold operation. First,
the internal error amplifier is disabled. This allows an internal
20µA current sink to develop a voltage across ROCSET. The
ISL6431 then samples this voltage at the COMP pin. This
sampled voltage, which is referenced to the VCC pin, is held
internally as the Overcurrent Set Point.
When the voltage across the upper MOSFET, which is also
referenced to the VCC pin, exceeds the Overcurrent Set
Point, the overcurrent function initiates a soft-start sequence.
Figure 1 shows this operation with an overload condition. This
current is repeated with a 21ms period. Note that the inductor
current increases to over 14A during the Soft Start interval and
causes an overcurrent trip. The converter dissipates very little
power with this method. The measured input power for the
conditions of Figure 1 is only 0.25W.
The overcurrent function will trip at a peak inductor current
(IPEAK) determined by:
IPEAK
=
I--O-----C----S----E----T-----x-----R-----O-----C----S----E----T-
rDS(ON)
where IOCSET is the internal OCSET current source (20µA
typical). The OC trip point varies mainly due to the
MOSFET’s rDS(ON) variations. To avoid overcurrent tripping
in the normal operating load range, find the ROCSET resistor
from the equation above with:
1. The maximum rDS(ON) at the highest junction temperature.
2. The minimum IOCSET from the specification table.
3. Determine IPEAK for
IPEAK
>
IO U T (MA X )
+
(-------I---)
2
,
where I is the output inductor ripple current.
For an equation for the ripple current see the section under
component guidelines titled ‘Output Inductor Selection’.
Soft Start
The POR function initiates the soft start sequence after the
overcurrent set point has been sampled. Soft start clamps the
error amplifier output (COMP pin) and reference input (non-
inverting terminal of the error amp) to the internally generated
Soft Start voltage. Figure 2 shows a typical soft start interval.
Initially the clamp on the error amplifier (COMP/OCSET pin)
controls the converter’s output voltage. The oscillator’s
triangular waveform is compared to the ramping error amplifier
voltage. This generates PHASE pulses of increasing width that
charge the output capacitor(s). With sufficient output voltage,
the clamp on the reference input controls the output voltage.
When the internally generated Soft Start voltage exceeds the
feedback (FB pin) voltage, the output voltage is in regulation.
This method provides a rapid and controlled output voltage rise.
VOUT
500mV/DIV.
0V
TIME (1ms/DIV.)
FIGURE 2. SOFT START INTERVAL
Application Guidelines
Layout Considerations
As in any high frequency switching converter, layout is very
important. Switching current from one power device to another
can generate voltage transients across the impedances of the
interconnecting bond wires and circuit traces. These
interconnecting impedances should be minimized by using
wide, short printed circuit traces. The critical components
should be located as close together as possible, using ground
plane construction or single point grounding.
5