ISL6545ACRZ Intersil, ISL6545ACRZ Datasheet - Page 6

• Manufacturer Part Number: ISL6545ACRZ
• Description: IC PWM BUCK BST VM 10DFN
• Manufacturer: Intersil

Specifications of ISL6545ACRZ

• Pwm Type: Voltage Mode
• Number Of Outputs: 1
• Frequency - Max: 660kHz
• Duty Cycle: 100%
• Voltage - Supply: 4.5 V ~ 14.4 V
• Buck: Yes
• Boost: Yes
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: No
• Operating Temperature: 0°C ~ 70°C
• Package / Case: 10-DFN
• Frequency-max: 660kHz
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Functional Description
Initialization (POR and OCP Sampling)
Figure 1 shows a simplified timing diagram. The
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.
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
voltage is above the V
Figure 2 shows a typical power-up sequence in more detail.
The initialization starts at T0, when either V
V
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
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
the internal bias regulator can turn on cleanly. At the same
time, the LGATE/OCSET pin is initialized, by disabling the
LGATE driver and drawing I
through R
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
long as the V
(OCP)” on page 7 for more details on the equations and
variables. Upon the completion of sample and hold at t3, the
GND>
POR
CC
pin to exceed 6.5V (if rising up towards 12V), so that
, or the COMP/SD pin is released (after POR). The
FIGURE 1. POR AND SOFT-START OPERATION
OCSET
CC
DISABLE
is above V
. This sets up a voltage that will represent
~4V POR
DISABLE
trip point (at t1). The external
POR
OCSET
COMP/SD (1V/DIV)
6
. See “Overcurrent Protection
trip point.
(nominal 21.5µA)
CC
V
rises above
V
OUT
CC
(2V/DIV)
(1V/DIV)
ISL6545, ISL6545A
soft-start operation is initiated, and the output voltage ramps
up between t4 and t5.
Soft-Start and Pre-Biased Outputs
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
ramp seen on the V
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, and it is the same for either frequency
version of the IC (300kHz or 600kHz).
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.
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
ramp voltage exceeds the output; V
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
GND>
FIGURE 2. LGATE/OCSET AND SOFT-START OPERATION
0.4V
t0 t1
3.4ms
LGATE/OCSET (0.25V/DIV)
COMP/SD (0.25V/DIV)
OUT
3.4ms
will be less than the nominal time,
t2
0 - 3.4ms
OUT
t3 t4
starts seamlessly
SWITCHING
STARTS
6.8ms
LGATE
(0.5V/DIV)
V
March 3, 2011
OUT
FN6305.6
t5