ISL6431A Datasheet, PDF (5/10 Pages) Intersil Corporation – Advanced Pulse Width Modulation (PWM) Controller for Broadband Applications

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.
V_OUT
500mV/DIV.
TIME (5ms/DIV.)
FIGURE 1. OVERCURRENT OPERATION
The overcurrent function will trip at a peak inductor current
(I_PEAK) determined by:
I_PEAK
=
I--_{O-----C----S----E----T-----x-----R-----O----C-----S---E----T--}
r_DS(ON)
where I_OCSET is the internal OCSET current source (20µA
typical). The OC trip point varies mainly due to the
MOSFET’s r_DS(ON) variations. To avoid overcurrent tripping
in the normal operating load range, find the R_OCSET resistor
from the equation above with:
1. The maximum r_DS(ON) at the highest junction
temperature.
2. The minimum I_OCSET from the specification table.
3. Determine I_PEAK for I_PEAK > I_OUT(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 R_OCSET. 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 V_CC
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 V_CC rail, through a voltage limiting protection
device, or other methods, the capacitance on the V_CC bus
will absorb the current. This situation will allow voltage level
of the V_CC 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.
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