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®
Data Sheet
October 2002
ISL6431A
FN9088
Advanced Pulse Width Modulation (PWM)
Controller for Broadband Applications
The ISL6431A 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 DSL modems, cable and DSL routers and
broadband gateway core processor, memory and peripheral
power supplies.
The ISL6431A 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 ISL6431A integrates
the control, output adjustment, monitoring and protection
functions into a single 8-Lead package.
The ISL6431A 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. The device is capable of sinking, as
well as sourcing current. A fixed frequency oscillator reduces
design complexity, while balancing typical application cost
and efficiency.
The error amplifier features a 15MHz gain-bandwidth
product and 8V/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
ISL6431ACB
ISL6431EVAL1
0 to 70 8 Ld SOIC
Evaluation Board
PKG.
NO.
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
- High-Bandwidth Error Amplifier
- Full 0% to 100% Duty Cycle
• Lossless, Programmable Overcurrent Protection
- Uses Upper MOSFET’s rDS(on)
• Converter Can Source and Sink Current
• Small Converter Size
- 300kHz Fixed Frequency Oscillator
- Internal Soft Start
- 8-Lead SOIC Package
Applications
• Cable/DSL Routers and DSL Modems
• DSP and Core Communications Processor Supplies
• Broadband Gateways
• Memory Supplies
• 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 registered trademark of Intersil Americas Inc.
Copyright © Intersil Americas Inc. 2002. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.

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Block Diagram
ISL6431A
SAMPLE
AND
HOLD
+
-
OC
COMPARATOR
VCC
POR AND
SOFTSTART
+
0.8V
-
FB
COMP/OCSET
20µA
Typical Application
ERROR
AMP
+
-
PWM
COMPARATOR
+
-
GATE
CONTROL
PWM LOGIC VCC
OSCILLATOR
FIXED 300kHz
GND
VCC
CDCPL
CBULK
ROCSET
RF
CF
VCC
5 BOOT
1
COMP/OCSET
ISL6431A
72
8
UGATE
PHASE
CI
63
4 LGATE
FB GND
DBOOT
CHF
CBOOT
LOUT
COUT
+VO
ROFFSET
RS
BOOT
UGATE
PHASE
LGATE
2

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ISL6431A
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
PARAMETER
Recommended Operating Conditions, Unless Otherwise Noted. VCC = 5.0V ±5% and TA = 25oC
SYMBOL
TEST CONDITIONS
MIN TYP MAX
VCC SUPPLY CURRENT
Nominal Supply
POWER-ON RESET
IVCC
2.6 3.2 3.8
Rising VCC POR Threshold
POR
4.19 4.30 4.50
VCC POR Threshold Hysteresis
0.01 0.20 0.85
OSCILLATOR
Frequency
Ramp Amplitude
REFERENCE
fOSC
VOSC
VCC = 5V
250 300 340
- 1.5 -
Reference Voltage Tolerance
Nominal Reference Voltage
VREF
- - 1.5
- 0.800 -
ERROR AMPLIFIER
DC Gain
- 82 -
Gain-Bandwidth Product
GBWP
14 -
-
Slew Rate
SR COMP = 10pF
4.65 8.0
9.2
GATE DRIVERS
Upper Gate Source Current
Upper Gate Sink Current
Lower Gate Source Current
Lower Gate Sink Current
PROTECTION / DISABLE
IUGATE-
SRC
VBOOT - VPHASE = 5V, VUGATE = 4V
IUGATE-SNK
ILGATE-SRC VVCC = 5V, VLGATE = 4V
ILGATE-SNK
- -1 -
-1-
- -1 -
-2-
OCSET Current Source
Disable Threshold
IOCSET
VDISABLE
17 20 22
- - 0.8
UNITS
mA
V
V
kHz
VP-P
%
V
dB
MHz
V/µs
A
A
A
A
µA
V
3

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ISL6431A
Functional Pin Description
VCC (Pin 5)
This pin provides the bias supply for the ISL6431A, 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’s source. This pin is
used to monitor the voltage drop across the upper MOSFET
for overcurrent protection.
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.
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 ISL6431A 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.
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
COMP/OCSET pin, to compensate the voltage-control
feedback loop of the converter.
Pulling COMP/OCSET to a level below 0.8V disables the
controller. Disabling the ISL6431A causes the oscillator to
stop, the LGATE and UGATE outputs to be held low, and the
softstart circuitry to re-arm.
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.
Functional Description
Initialization
The ISL6431A 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
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.
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 ISL6431A 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
ISL6431A 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 the inductor current after a fault is introduced
while running at 15A. The continuous fault causes the
ISL6431A to go into a hiccup mode with a typical period of
25ms. The inductor current increases to 18A 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 1.5W.
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OUTPUT INDUCTOR
CURRENT
5A/DIV.
ISL6431A
method provides a rapid and controlled output voltage rise. The
entire startup sequence typically take about 11ms.
COMP/OCSET
1V/DIV.
VOUT
500mV/DIV.
TIME (5ms/DIV.)
FIGURE 1. OVERCURRENT OPERATION
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 > IOUT(MAX) + (----2---I--)- ,
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 start up interval
where the COMP/OCSET pin has been released from a
grounded (system shutdown) state. Initially, the COMP/OCSET
is used to sample the overcurrent setpoint by disabling the error
amplifier and drawing 20µA through ROCSET. Once the
overcurrent level has been sampled, the soft start function is
initiated. The clamp on the error amplifier (COMP/OCSET pin)
initially controls the converter’s output voltage during soft start.
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). When the
internally generated Soft Start voltage exceeds the feedback
(FB pin) voltage, the output voltage is in regulation. This
TIME (2ms/DIV.)
FIGURE 2. SOFT START INTERVAL
Current Sinking
The ISL6431A incorporates a MOSFET shoot-through
protection method which allows a converter to sink current
as well as source current. Care should be exercised when
designing a converter with the ISL6431A when it is known
that the converter may sink current.
When the converter is sinking current, it is behaving as a
boost converter that is regulating it’s input voltage. This
means that the converter is boosting current into the VCC
rail, which supplies the bias voltage to the ISL6431A. If there
is nowhere for this current to go, such as to other distributed
loads on the VCC rail, through a voltage limiting protection
device, or other methods, the capacitance on the VCC bus
will absorb the current. This situation will allow voltage level
of the VCC rail to increase. If the voltage level of the rail is
boosted to a level that exceeds the maximum voltage rating
of the ISL6431A, then the IC will experience an irreversible
failure and the converter will no longer be operational.
Ensuring that there is a path for the current to follow other
than the capacitance on the rail will prevent this failure
mode.
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