ISL6520A
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.
Functional Pin Description
VCC
This pin provides the bias supply for the ISL6520A, as well
as the lower MOSFET’s gate. Connect a well-decoupled 5V
supply to this pin.
Pulling COMP/OCSET to a level below 0.8V disables the
controller. Disabling the ISL6520A causes the oscillator to
stop, the LGATE and UGATE outputs to be held low, and the
softstart circuitry to re-arm.
FB
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.
LGATE
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.
GND
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
Functional Description
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.
Initialization
The ISL6520A 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.
UGATE
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.
Overcurrent Protection
BOOT
The overcurrent function protects the converter from a shorted
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.
output by using the upper MOSFET’s on-resistance, r ,
DS(ON)
to monitor the current. This method enhances the converter’s
efficiency and reduces cost by eliminating a current sensing
resistor.
COMP/OCSET
The overcurrent function cycles the soft-start function in a
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
hiccup mode to provide fault protection. A resistor (R
)
OCSET
programs the overcurrent trip level (see “Typical Application”
on page 3).
resistor (R
) from this pin to the drain of the upper
OCSET
MOSFET (V ). R
, an internal 20µA current source
OCSET
CC
Immediately following POR, the ISL6520A initiates the
Overcurrent Protection sampling and hold operation. First,
the internal error amplifier is disabled. This allows an internal
(I
OCSET
), and the upper MOSFET on-resistance (r
)
DS(ON)
set the converter overcurrent (OC) trip point according to
Equation 1:
20mA current sink to develop a voltage across R
. The
OCSET
ISL6520A 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.
I
xR
OCSET
OCSET
I
= -------------------------------------------------
PEAK
r
DSON
(EQ. 1)
Internal circuitry of the ISL6520A will not recognize a voltage
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
ISL6520A 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.
drop across R
across R
larger than 0.5V. Any voltage drop
OCSET
that is greater than 0.5V will set the
OCSET
overcurrent trip point to:
0.5V
I
= ----------------------
PEAK
r
DSON
(EQ. 2)
An overcurrent trip cycles the soft-start function.
FN9016 Rev 6.00
Dec 10, 2009
Page 5 of 12
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