monitored by the adaptive shoot through protection circuitry to
determine when the lower MOSFET has turned off.
Provide a bias supply for the lower gate drive to this pin.
Provide a 12V bias supply for the chip to this pin.
The ISL6522A 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 (VIN) on the OCSET pin. The level on OCSET
is equal to VIN less a fixed voltage drop (see overcurrent
protection). With the EN pin held to VCC, the POR function
initiates soft-start operation after both input supply voltages
exceed their POR thresholds. For operation with a single
+12V power source, VIN and VCC are equivalent and the
+12V power source must exceed the rising VCC 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
The POR function initiates the soft-start sequence. An internal
10µA current source charges an external capacitor (CSS) 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 t1 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.
CLAMP ON VCOMP
RELEASED AT STEADY STATE
t0 t1 t2
t2 – t1
CSS = Soft Start Capacitor
ISS = Soft Start Current = 10µA
VOSC(MIN) = Bottom of Oscillator = 1.35V
VIN = Input Voltage
∆VOSC = Peak to Peak Oscillator Voltage = 1.9V
VOUTSteadyState = Steady State Output Voltage
FIGURE 3. SOFT-START INTERVAL
FIGURE 4. OVERCURRENT OPERATION
The overcurrent function protects the converter from a
shorted output by using the upper MOSFETs 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. An internal 200µA
(typical) current sink develops a voltage across ROCSET that