ISL6545, ISL6545A
ISL6545, ISL6545A
soft-start operation is initiated, and the output voltage ramps
up between t4 and t5.
Functional Description
Initialization (POR and OCP Sampling)
Figure 1 shows a simplified timing diagram. The
LGATE
STARTS
SWITCHING
Power-On-Reset (POR) function continually monitors the
bias voltage at the VCC pin. Once the rising POR threshold
is exceeded (VPOR ~4V nominal), the POR function initiates
the Overcurrent Protection (OCP) sample and hold
operation (while COMP/SD is ~1V). When the sampling is
complete, VOUT begins the soft-start ramp.
COMP/SD (0.25V/DIV)
0.4V
If the COMP/SD pin is held low during power-up, that will just
delay the initialization until it is released, and the COMP/SD
V
LGATE/OCSET (0.25V/DIV)
OUT
voltage is above the V
trip point.
DISABLE
(0.5V/DIV)
GND>
3.4ms
3.4ms
0 - 3.4ms
6.8ms
V
(2V/DIV)
CC
t0 t1
t2
t3 t4
t5
FIGURE 2. LGATE/OCSET AND SOFT-START OPERATION
Soft-Start and Pre-Biased Outputs
~4V POR
Functionally, 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
V
(1V/DIV)
OUT
COMP/SD (1V/DIV)
ramp seen on the V
due to some initialization timing, between t3 and t4).
will be less than the nominal time,
OUT
GND>
The ramp is created digitally, so there will be 64 small
discrete steps. There is no simple way to change this ramp
rate externally, and it is the same for either frequency
version of the IC (300kHz or 600kHz).
FIGURE 1. POR AND SOFT-START OPERATION
Figure 2 shows a typical power-up sequence in more detail.
The initialization starts at T0, when either V rises above
CC
, or the COMP/SD pin is released (after POR). The
After an initialization period (t3 to t4), the error amplifier
(COMP/SD 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 inrush 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.
V
POR
COMP/SD will be pulled up by an internal 20µA current
source, but the timing will not begin until the COMP/SD
exceeds the V
trip point (at t1). The external
DISABLE
capacitance of the disabling device, as well as the
compensation capacitors, will determine how quickly the
20µA current source will charge the COMP/SD pin. With
typical values, it should add a small delay compared to the
soft-start times. The COMP/SD will continue to ramp to ~1V.
From t1, there is a nominal 6.8ms delay, which allows the
V
pin to exceed 6.5V (if rising up towards 12V), so that
the internal bias regulator can turn on cleanly. At the same
time, the LGATE/OCSET pin is initialized, by disabling the
CC
LGATE driver and drawing I
(nominal 21.5µA)
OCSET
. This sets up a voltage that will represent
Figure 3 shows the normal curve in blue; 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 by the magenta curve, the ISL6545x will detect that
condition. Neither MOSFET will turn on until the soft-start
through R
OCSET
the OCSET trip point. At t2, there is a variable time period for
the OCP sample and hold operation (0ms to 3.4ms nominal;
the longer time occurs with the higher overcurrent setting).
The sample and hold uses a digital counter and DAC to save
the voltage, so the stored value does not degrade, for as
ramp voltage exceeds the output; V
starts seamlessly
OUT
ramping from there. If the output is pre-biased to a voltage
above the expected value, as in the red curve, neither
MOSFET will turn on until the end of the soft-start, at which
long as the V
is above V . See “Overcurrent Protection
CC
POR
(OCP)” on page 7 for more details on the equations and
variables. Upon the completion of sample and hold at t3, the
FN6305 Rev 7.00
October 7, 2015
Page 6 of 17
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