ISL8201M Ver la hoja de datos (PDF) - Renesas Electronics
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ISL8201M Datasheet PDF : 16 Pages
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ISL8201M
If the output is pre-biased to a voltage above the expected value
(as shown Figure 18), 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).
t1
t2 t3
t4
t5
COMP/EN
IISSEETT
VOUT
3.4ms
3.4ms
FIGURE 15. ISET AND SOFT-START OPERATION
Soft-Start and Pre-Biased Outputs
The soft-start internally ramps the reference on the
non-inverting terminal of the error amp from 0V to 0.6V in a
nominal 6.8ms. The output voltage will thus follow the ramp,
from zero to final value, in the same 6.8ms (the actual ramp
seen on the VOUT will be less than the nominal time), due to
some initialization timing, between t3 and t4.
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 16 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 17), the ISL8201M will detect that condition.
Neither internal MOSFET will turn on until the soft-start ramp
voltage exceeds the output; VOUT starts seamlessly ramping
from there.
VOUT
t2
t0
t1
FIGURE 16. NORMAL START-UP
VOUT
FIGURE 17. PRE-BIASED START-UP
FN6657 Rev 3.00
October 28, 2014
Page 9 of 16
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