ISL8204M 查看數據表（PDF） - Renesas Electronics
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ISL8204M Datasheet PDF : 19 Pages
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ISL8204M, ISL8206M
The ramp is created digitally, so there will be 64 small discrete
steps. There is no simple way to change this ramp rate externally.
After an initialization period (t3 to t4), the error amplifier (COMP/EN
pin) is enabled and begins to regulate 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 capacitors. When the
internally generated soft-start voltage exceeds the reference voltage
(0.6V), the soft-start is complete and the output should be in
regulation at the expected voltage. This method provides a rapid and
controlled output voltage rise; there is no large in-rush current
charging the output capacitors. The entire start-up sequence from
POR typically takes up to 17ms; up to 10.2ms for the delay and OCP
sample and 6.8ms for the soft-start ramp.
Figure 20 shows the normal curve for start-up; initialization
begins at t0, and the output ramps between t1 and t2. If the
output is pre-biased to a voltage less than the expected value (as
shown Figure 21), neither internal MOSFET will turn on until the
soft-start ramp voltage exceeds the output; VOUT starts
seamlessly ramping from there.
VOUT
FIGURE 21. PRE-BIASED START-UP
VOUT
T2
VOUT
T0
T1
FIGURE 20. NORMAL START-UP
If the output is pre-biased to a voltage above the expected value
(as shown Figure 22), neither MOSFET will turn on until the end of
the soft-start, at which time it will pull the output voltage down to
the final value. Any resistive load connected to the output will help
pull down the voltage (at the RC rate of the R of the load and the C
of the output capacitance).
500mV/DIV
FIGURE 22. PRE-BIASED START-UP - OVERCHARGED
If VIN for the synchronous buck converter is from a different
supply that comes up after PVCC, the soft-start will go through its
cycle, but with no output voltage ramp. When VIN turns on, the
output will follow the ramp of the VIN from zero up to the final
expected voltage (at close to 100% duty cycle, with COMP/EN pin
>4V). If VIN is too fast, there may be excessive in-rush current
charging the output capacitors (only the beginning of the ramp,
from zero to VOUT matters here). If this is not acceptable, then
consider changing the sequencing of the power supplies, sharing
the same supply, or adding sequencing logic to the COMP/EN pin
to delay the soft-start until the VIN supply is ready (see “Input
Voltage Considerations” on page 13).
If ISL8204M, ISL8206M 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
FN6999 Rev 4.00
October 28, 2014
Page 11 of 19
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