LTC3611EWP LINER [Linear Technology], LTC3611EWP Datasheet - Page 11

• Manufacturer Part Number: LTC3611EWP
• Description: 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
The basic LTC3611 application circuit is shown on the front
page of this data sheet. External component selection is
primarily determined by the maximum load current. The
LTC3611 uses the on-resistance of the synchronous power
MOSFET for determining the inductor current. The desired
amount of ripple current and operating frequency also
determines the inductor value. Finally, C
ability to handle the large RMS current into the converter
and C
output voltage ripple and transient specification.
V
The LTC3611 has an open-drain PGOOD output that
indicates when the output voltage is within ±10% of the
regulation point. The LTC3611 also has a V
allows the on-time to be adjusted. Tying the V
results in lower values for R
applications. The V
the on-time to maintain constant frequency operation in
applications where V
frequency shifts with changes in load current.
V
The V
valley current, which in turn determines the maximum
average output current that the LTC3611 can deliver. The
maximum output current is given by:
The I
Current Limit vs V
Characteristics.
An external resistor divider from INTV
set the voltage on the V
be simply tied to ground force a default value equivalent
to 0.7V. Do not float the V
ON
RNG
I
OUT(MAX)
and PGOOD
VALLEY(MAX)
Pin and I
OUT
RNG
is chosen with low enough ESR to meet the
pin is used to adjust the maximum inductor
= I
LIMIT
VALLEY(MAX)
is shown in the figure “Maximum Valley
RNG
ON
Adjust
OUT
Voltage” in the Typical Performance
pin also provides a means to adjust
RNG
changes and to correct minor
RNG
ON
pin from 1V to 1.4V, or it can
+ 1/2 ΔI
which is useful in high V
pin.
L
,
IN
CC
is selected for its
can be used to
ON
ON
pin high
pin that
OUT
Operating Frequency
The choice of operating frequency is a tradeoff between
efficiency and component size. Low frequency operation
improves efficiency by reducing MOSFET switching losses
but requires larger inductance and/or capacitance in order
to maintain low output ripple voltage.
The operating frequency of LTC3611 applications is de-
termined implicitly by the one-shot timer that controls the
on-time t
by the current into the I
pin according to:
Tying a resistor R
on-time inversely proportional to V
the I
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
To hold frequency constant during output voltage changes,
tie the V
when V
limit its input to the one-shot timer. If the pin is tied below
0.7V, the input to the one-shot is clamped at 0.7V. Similarly,
if the pin is tied above 2.4V, the input is clamped at 2.4V.
In high V
the comparator input is 2.4V results in a lower value for
R
frequency for several common output voltages.
ON
I
t
f =
ION
ON
. Figures 1a and 1b show how R
ON
V
=
=
OUT
pin is:
VON
ON
ON
R
V
OUT
V
I
ION
ON
VON
IN
pin to V
> 2.4V. The V
of the top MOSFET switch. The on-time is set
V
R
OUT
applications, tying V
ON
(10pF)
(10pF)
ON
OUT
from V
or to a resistive divider from V
ON
[Hz]
ON
pin and the voltage at the V
pin has internal clamps that
IN
to the I
ON
ON
IN
. The current out of
relates to switching
to INTV
LTC3611
ON
pin yields an
CC
11
so that
3611fb
OUT
ON