ISL6545 INTERSIL [Intersil Corporation], ISL6545 Datasheet - Page 9

• Manufacturer Part Number: ISL6545
• Description: 5V or 12V Single Synchronous Buck Pulse-Width Modulation PWM Controller
• Manufacturer: INTERSIL [Intersil Corporation]

Application” on page 3 for more detail; R
resistor; R
The recommended value for R
accuracy) and then R
equation below. Since R
(see Feedback Compensation section), it is often easier to
change R
compensation calculations do not need to be repeated. If
V
voltages less than 0.6V are not available as shown in
Equation 2.
Input Voltage Considerations
The Typical Application diagram on page 3 shows a
standard configuration where V
12V (±20%); in each case, the gate drivers use the V
voltage for LGATE and BOOT/UGATE. In addition, V
allowed to work anywhere from 6.5V up to the 14.4V
maximum. The V
allowed for long-term reliability reasons, but transitions
through it to voltages above 6.5V are acceptable.
There is an internal 5V regulator for bias; it turns on between
5.5V and 6.5V; some of the delay after POR is there to allow
a typical power supply to ramp up past 6.5V before the
soft-start ramps begins. This prevents a disturbance on the
output, due to the internal regulator turning on or off. If the
transition is slow (not a step change), the disturbance should
be minimal. So while the recommendation is to not have the
output enabled during the transition through this region, it
may be acceptable. The user should monitor the output for
their application, to see if there is any problem.
The V
as V
sources, such as outputs of other regulators. If V
up first, and the V
initialization is done, then the soft-start will not be able to
ramp the output, and the output will later follow part of the
V
change the sequencing of the supplies, or use the
COMP/SD pin to disable V
Figure 6 shows a simple sequencer for this situation. If V
powers up first, Q
Q
on, the resistor divider R
on, which will turn off Q
powers up first, Q
will start-up as soon as V
point is 0.4V nominal, so a wide variety of NFET’s or NPN’s
or even some logic IC’s can be used as Q
must be low leakage when off (open-drain or open-collector)
V
R
OUT
IN
OUT
O
2
on, keeping the ISL6545 in shut-down. When V
=
ramp when it is applied. If this is not desired, then
CC
=
IN
----------------------------------
V
= 0.6V, then R
, but can also run off a separate supply or other
R
OUT
0.6V
to the upper MOSFET can share the same supply
S
OFFSET
OFFSET
•
0.6V
–
•
0.6V
(
---------------------------
R
S
CC
R
1
IN
1
to change the output voltage; that way the
+
(shortened to R
O
will be off, and R
will be on, turning Q
R
is not present by the time the
range between 5.5V and 6.5V is NOT
OFFSET
O
OFFSET
)
2
S
1
, and release the shut-down. If V
CC
and R
is part of the compensation circuit
OUT
comes up. The V
can be left open. Output
9
is chosen according to the
S
CC
until both supplies are ready.
2
is 1kΩ to 5kΩ (±1% for
determines when Q
O
is either 5V (±10%) or
3
below) is the lower one.
pulling to V
2
S
off; so the ISL6545
1
is the upper
or Q
DISABLE
2
CC
CC
; but Q
IN
ISL6545, ISL6545A
powers
will turn
CC
CC
1
turns
(EQ. 2)
turns
trip
2
is
CC
IN
so as not to interfere with the COMP output. Q
be placed near the COMP/SD pin.
The V
0.6V reference). It can be as high as 20V (for V
below V
voltage.
The first consideration for high V
voltage of 36V. The V
voltage - minus the diode drop), plus any ringing (or other
transients) on the BOOT pin must be less than 36V. If V
20V, that limits V
The second consideration for high V
(BOOT - V
BOOT = V
must be <24V. So based on typical circuits, a 20V maximum
V
ringing in their particular application.
Another consideration for high V
duty cycles (such as 20V in to 1.0V out, for 5% duty cycle)
require component selection compatible with that choice
(such as low r
filter). At the other extreme (for example, 20V in to 12V out),
the upper MOSFET needs to be low r
the duty cycle gets too high, it can affect the overcurrent
sample time. In all cases, the input and output capacitors
and both MOSFETs must be rated for the voltages present.
Switching Frequency
The switching frequency is either a fixed 300kHz or 600kHz,
depending on the part number chosen (ISL6545 is 300kHz;
ISL6545A is 600kHz; the generic name “ISL6545” may apply
to either in the rest of this document, except when choosing
the frequency). However, all of the other timing mentioned
(POR delay, OCP sample, soft-start, etc.) is independent of
the clock frequency (unless otherwise noted).
BOOT Refresh
In the event that the UGATE is on for an extended period of
time, the charge on the boot capacitor can start to sag,
raising the r
a circuit that detects a long UGATE on-time (nominal 100µs),
and forces the LGATE to go high for one clock cycle, which
will allow the boot capacitor some time to recharge.
Separately, the OCP circuit has an LGATE pulse stretcher
(to be sure the sample time is long enough), which can also
IN
is a good starting assumption; the user should verify the
IN
IN
range can be as low as ~1V (for V
). There are some restrictions for running high V
IN
CC
DS(ON)
+ V
) voltage; this must be less than 24V. Since
DS(ON)
FIGURE 6. SEQUENCER CIRCUIT
CC
CC
R
R
1
2
V
of the upper MOSFET. The ISL6545 has
+ ringing, that reduces to (V
IN
plus ringing to 16V.
IN
lower MOSFET, and a good LC output
(as seen on PHASE) plus V
R
Q
3
1
V
CC
IN
IN
to COMP/SD
Q
2
is duty cycle. Very low
is the maximum BOOT
IN
DS(ON)
is the maximum
OUT
. In addition, if
2
as low as the
OUT
IN
should also
+ ringing)
April 29, 2010
CC
just
FN6305.5
(boot
IN
IN
is