Datasheet

LTC3611
11
3611fd
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
IN
is selected for its
ability to handle the large RMS current into the converter
and C
OUT
is chosen with low enough ESR to meet the
output voltage ripple and transient specification.
V
ON
and PGOOD
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
ON
pin that
allows the on-time to be adjusted. Tying the V
ON
pin high
results in lower values for R
ON
which is useful in high V
OUT
applications. The V
ON
pin also provides a means to adjust
the on-time to maintain constant frequency operation in
applications where V
OUT
changes and to correct minor
frequency shifts with changes in load current.
V
RNG
Pin and I
LIMIT
Adjust
The V
RNG
pin is used to adjust the maximum inductor
valley current, which in turn determines the maximum
average output current that the LTC3611 can deliver. The
maximum output current is given by:
I
OUT(MAX)
= I
VALLEY(MAX)
+ 1/2 ΔI
L
The I
VALLEY(MAX)
is shown in the figure “Maximum Valley
Current Limit vs V
RNG
Voltage” in the Typical Performance
Characteristics.
An external resistor divider from INTV
CC
can be used to
set the voltage on the V
RNG
pin from 1V to 1.4V, or it can
be simply tied to ground force a default value equivalent
to 0.7V. Do not float the V
RNG
pin.
ApplicAtions inForMAtion
Operating Frequency
The choice of operating frequency is a trade-off 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
ON
, of the top MOSFET switch. The on-time is
set by the current into the I
ON
pin and the voltage at the
V
ON
pin according to:
t
ON
=
V
VON
I
ION
(10pF)
Tying a resistor R
ON
from V
IN
to the I
ON
pin yields an
on-time inversely proportional to V
IN
. The current out of
the I
ON
pin is:
I
ION
=
V
IN
R
ON
For a step-down converter, this results in approximately
constant frequency operation as the input supply varies:
f =
V
OUT
V
VON
R
ON
(10pF)
[Hz]
To hold frequency constant during output voltage changes,
tie the V
ON
pin to V
OUT
or to a resistive divider from V
OUT
when V
OUT
> 2.4V. The V
ON
pin has internal clamps that
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
OUT
applications, tying V
ON
to INTV
CC
so that
the comparator input is 2.4V results in a lower value for