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
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
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:
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
IO U T (MA X )
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’.
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.
FIGURE 2. SOFT START INTERVAL
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.