Datasheet

LTC3864
12
3864f
The LTC3864 is a current mode, constant frequency PWM
controller for an asynchronous step-down DC/DC regulator
with a P-channel power MOSFET acting as the main switch
and a Schottky power diode acting as the commutating
(catch) diode. The input range extends from 3.5V to 60V.
The output range can be programmed from 0.8V to all the
way up to V
IN
. The LTC3864 can transition from regulation
to 100% duty cycle when the input voltage drops below
the programmed output voltage. Additionally, the LTC3864
offers Burst Mode operation with 40µA quiescent current,
which delivers outstanding efficiency in light load opera-
tion. The LTC3864 is a low pin count, robust and easy to
use solution in applications which require high efficiency
and operate with widely varying high voltage inputs.
The typical application on the front page is a basic LTC3864
application circuit. The LTC3864 can sense the inductor
current through a high side series sense resistor, R
SENSE
,
placed between V
IN
and the source of the external P-
MOSFET. Once the required output voltage and operating
frequency have been determined, external component
selection is driven by load requirements, and begins with
the selection of inductor and R
SENSE
. Next, the power
MOSFET and catch diode are selected. Finally, input and
output capacitors are selected.
Output Voltage Programming
The output voltage is programmed by connecting a
feedback resistor divider from the output to the V
FB
pin
as shown in Figure 1. The output voltage in steady state
operation is set by the feedback resistors according to
the equation:
V
OUT
= 0.8V • 1+
R
FB2
R
FB1






To improve the transient response, a feedforward capacitor
C
FF
may be used. Great care should be taken to route the
V
FB
line away from noise sources, such as the inductor
or the GATE signal that drives the external P-MOSFET.
APPLICATIONS INFORMATION
Switching Frequency and Clock Synchronization
The choice of operating frequency is a trade-off between
efficiency and component size. Lowering the operating fre-
quency improves efficiency by reducing MOSFET switching
losses but requires larger inductance and/or capacitance
to maintain low output ripple voltage. Conversely, raising
the operating frequency degrades efficiency but reduces
component size.
The LTC3864 can free run at a user programmed switch-
ing frequency, or it can synchronize with an external
clock to run at the clock frequency. When the LTC3864
is synchronized, the GATE pin will phase synchronize
with the rising edge of the applied clock in order to turn
the external P-MOSFET on. The switching frequency of
the LTC3864 is programmed with the FREQ pin, and the
external clock is applied at the PLLIN/MODE pin. Table 1
highlights the different states in which the FREQ pin can
be used in conjunction with the PLLIN/MODE pin.
Table 1
FREQ PIN PLLIN/MODE PIN FREQUENCY
0V DC Voltage 350kHz
Floating DC Voltage 535kHz
Resistor to GND DC Voltage 50kHz to 850kHz
Any of the Above External Clock Phase Locked to External
Clock
LTC3864
V
FB
V
OUT
R
FB2
C
FF
R
FB1
3864 F01
Figure 1. Setting the Output Voltage