LTC3611_15 Datasheet, PDF (11/26 Pages) Linear Technology – 10A, 32V Monolithic Synchronous Step-Down DC/DC Converter

LTC3611
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_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.
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_VON
V_OUT
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
3611fd
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