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ISL8201M
Following POR (and 6.8ms delay), the ISL8201M initiates
the overcurrent protection sample and hold operation. The
low side gate driver is disabled to allow an internal 21.5μA
VOUT
500mV/ diV
FIGURE 18. PRE-BIASED START-UP - OVERCHARGED
If the V IN for the synchronous buck converter is from a
different supply that comes up after P VCC , the soft-start will
go through its cycle, but with no output voltage ramp. When
V IN turns on, the output will follow the ramp of the V IN from
zero up to the final expected voltage (at close to 100% duty
cycle, with COMP/EN pin >4V). If V IN is too fast, there may
be excessive inrush current charging the output capacitors
(only the beginning of the ramp, from zero to V OUT matters
here). If this is not acceptable, then consider changing the
sequencing of the power supplies, sharing the same supply,
current source to develop a voltage across R SET . The
ISL8201M samples this voltage (which is referenced to the
PGND pin) at the ISET pin, and holds it in a counter and
DAC combination. This sampled voltage is held internally as
the overcurrent set point, for as long as power is applied, or
until a new sample is taken after coming out of a shut-down.
The actual monitoring of the low side MOSFET
ON-resistance starts 200ns (nominal) after the edge of the
internal PWM logic signal (that creates the rising external
low side gate signal). This is done to allow the gate transition
noise and ringing on the PHASE pin to settle out before
monitoring. The monitoring ends when the internal PWM
edge (and thus low side gate signal) goes low. The OCP can
be detected anywhere within the above window.
If the converter is running at high duty cycles around 75% for
600kHz operation, then the low side gate pulse width may
not be wide enough for the OCP to properly sample the
r DS(ON) . For those cases, if the low side gate signal is too
narrow (or not there at all) for 3 consecutive pulses, then the
third pulse will be stretched and/or inserted to the 425ns
minimum width. This allows for OCP monitoring every third
pulse under this condition. This can introduce a small
pulse-width error on the output voltage, which will be
corrected on the next pulse; and the output ripple voltage will
have an unusual 3-clock pattern, which may look like jitter.
The overcurrent function will trip at a peak inductor current
I PEAK = --------------------------------------------
or adding sequencing logic to the COMP/EN pin to delay the
soft-start until the V IN supply is ready (see “Input Voltage
Considerations” on page 11).
(I PEAK ) determined by Equation 2:
2 × I SET × R SET
r DS ( ON )
(EQ. 2)
If ISL8201M is disabled after soft-start (by pulling COMP/EN
pin low), and afterwards enabled (by releasing the
COMP/EN pin), then the full initialization (including OCP
sample) will take place. However, there is no new OCP
sampling during overcurrent retries. If the output is shorted
to GND during soft-start, the OCP will handle it, as described
in the next section.
Overcurrent Protection (OCP)
The overcurrent function protects the converter from a
shorted output by using the low side MOSFET
ON-resistance, r DS(ON) , to monitor the current. A resistor
where:
I SET is the internal I SET current source (21.5μA typical).
R SET is equivalent resistance between ISET and PGND
pins.
r DS(ON) is typically 6.1m Ω @ (V PVCC = V GS = 10V, I DS = 30A)
and 9m Ω @ (V PVCC = V GS = 4.5V, I DS = 30A).
Note: ISL8201M has integrated 3.57k Ω resistance (R SET-IN ).
Therefore, the equivalent resistance of R SET can be
expressed in Equation 3:
R SET = -------------------------------------------------------
(R SET ) programs the overcurrent trip level.
This method enhances the converter's efficiency and
R SET-EX × R SET-IN
R SET-EX + R SET-IN
(EQ. 3)
reduces cost by eliminating a current sensing resistor. If
overcurrent is detected, the output immediately shuts off. It
cycles the soft-start function in a hiccup mode (2 dummy
soft-start time-outs, then up to one real one) to provide fault
protection. If the shorted condition is not removed, this cycle
will continue indefinitely.
10
The scale factor of 2 doubles the trip point of the MOSFET
voltage drop, compared to the setting on the R SET resistor.
The OC trip point varies in a system mainly due to the
MOSFET r DS(ON) variations (i.e. over process, current and
temperature). To avoid overcurrent tripping in the normal
operating load range, find the R SET resistor from Equation 4,
and with Steps 1 to 3:
FN6657.2
October 21, 2010
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