ISL6545A Intersil Corporation, ISL6545A Datasheet - Page 6

• Manufacturer Part Number: ISL6545A
• Description: Single Synchronous Buck Pulas Width MoDulation
• Manufacturer: Intersil Corporation

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 V
(V
Overcurrent Protection (OCP) sample and hold operation
(while COMP/SD is ~1V). When the sampling is complete,
V
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
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 LGATE driver
and drawing I
sets up a voltage that will represent the OCSET trip point. At
T2, there is a variable time period for the OCP sample and hold
operation (0 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
Overcurrent Protection on page 7 for more details on the
equations and variables. Upon the completion of sample and
GND>
GND>
GND>
GND>
OUT
POR
POR
CC
, or the COMP/SD pin is released (after POR). The
begins the soft-start ramp.
~4V nominal), the POR function initiates the
FIGURE 1. POR AND SOFT-START OPERATION
pin. Once the rising POR threshold is exceeded
OCSET
DISABLE
~4V POR
(nominal 21.5μA) through R
DISABLE
trip point (at T1). The external
COMP/SD (1V/DIV)
6
trip point.
CC
is above V
CC
rises above
POR
V
V
OCSET
CC
OUT
. See the
(2V/DIV)
(1V/DIV)
ISL6545, ISL6545A
. This
CC
hold at T3, the 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 zero 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
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 ISL6545 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
time it will pull the output voltage down to the final value. Any
GND>
GND>
GND>
GND>
FIGURE 2. LGATE/OCSET AND SOFT-START OPERATION
0.4V
T0 T1
OUT
3.4ms
LGATE/OCSET (0.25V/DIV)
will be less than the nominal time, due to
COMP/SD (0.25V/DIV)
3.4ms
T2
0 - 3.4ms
OUT
T3 T4
starts seamlessly
SWITCHING
November 15, 2006
STARTS
6.8ms
LGATE
(0.5V/DIV)
V
OUT
FN6305.3
T5