6
FN9030.8
March 10, 2006
OCSET
Connect a resistor (R
OCSET
) from this pin to the drain of the
upper MOSFET. R
OCSET
, an internal 200µA current source
(I
OCS
), and the upper MOSFET on-resistance (r
DS(ON)
) set
the converter overcurrent (OC) trip point according to the
following equation:
An overcurrent trip cycles the soft-start function.
SS
Connect a capacitor from this pin to ground. This capacitor,
along with an internal 10µA current source, sets the soft-start
interval of the converter.
COMP and FB
COMP and FB are the available external pins of the error
amplifier. The FB pin is the inverting input of the error
amplifier and the COMP pin is the error amplifier output.
These pins are used to compensate the voltage-control
feedback loop of the converter.
EN
This pin is the open-collector enable pin. Pull this pin below
1V to disable the converter. In shutdown, the soft-start pin is
discharged and the UGATE and LGATE pins are held low.
GND
Signal ground for the IC. All voltage levels are measured
with respect to this pin.
PHASE
Connect the PHASE pin to the upper MOSFET source. This
pin is used to monitor the voltage drop across the MOSFET
for overcurrent protection. This pin also provides the return
path for the upper gate drive.
UGATE
Connect UGATE to the upper MOSFET gate. This pin
provides the 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
This pin provides bias voltage to the upper MOSFET driver.
A bootstrap circuit may be used to create a BOOT voltage
suitable to drive a standard N-Channel MOSFET.
PGND
This is the power ground connection. Tie the lower MOSFET
source to this pin.
LGATE
Connect LGATE to the lower MOSFET gate. This pin provides
the 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.
PVCC
Provide a bias supply for the lower gate drive to this pin.
VCC
Provide a 12V bias supply for the chip to this pin.
Functional Description
Initialization
The ISL6522 automatically initializes upon receipt of power.
Special sequencing of the input supplies is not necessary.
The Power-On Reset (POR) function continually monitors
the input supply voltages and the enable (EN) pin. The POR
monitors the bias voltage at the VCC pin and the input
voltage (V
IN
) on the OCSET pin. The level on OCSET is
equal to V
IN
Less a fixed voltage drop (see overcurrent
protection). With the EN pin held to V
CC
, the POR function
initiates soft-start operation after both input supply voltages
exceed their POR thresholds. For operation with a single
+12V power source, V
IN
and V
CC
are equivalent and the
+12V power source must exceed the rising V
CC
threshold
before POR initiates operation.
The POR function inhibits operation with the chip disabled
(EN pin low). With both input supplies above their POR
thresholds, transitioning the EN pin high initiates a soft-start
interval.
Soft-Start
The POR function initiates the soft-start sequence. An internal
10µA current source charges an external capacitor (C
SS
) on
the SS pin to 4V. Soft-start clamps the error amplifier output
(COMP pin) to the SS pin voltage. Figure 3 shows the soft-
start interval. At t
1
in Figure 3, the SS and COMP voltages
reach the valley of the oscillator’s triangle wave. 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). This
interval of increasing pulse width continues to t2, at which
point the output is in regulation and the clamp on the COMP
pin is released. This method provides a rapid and controlled
output voltage rise.
I
PEAK
I
OCS
R
OCSET
•
r
DS ON()
--------------------------------------------=
ISL6522
