Datasheet

LTC3812-5
11
38125fc
Main Control Loop
The LTC3812-5 is a current mode controller for DC/DC
step-down converters. In normal operation, the top
MOSFET is turned on for a fi xed interval determined by
a one-shot timer (OST). When the top MOSFET is turned
off, the bottom MOSFET is turned on until the current
comparator I
CMP
trips, restarting the one-shot timer and
initiating the next cycle. Inductor current is determined by
sensing the voltage between the PGND and SW pins using
the bottom MOSFET on-resistance. The voltage on the I
TH
pin sets the comparator threshold corresponding to the
inductor valley current. The fast 25MHz error amplifi er EA
adjusts this voltage by comparing the feedback signal V
FB
to the internal 0.8V reference voltage. If the load current
increases, it causes a drop in the feedback voltage relative
to the reference. The I
TH
voltage then rises until the average
inductor current again matches the load current.
The operating frequency is determined implicitly by the top
MOSFET on-time and the duty cycle required to maintain
regulation. The one-shot timer generates an on time that is
proportional to the ideal duty cycle, thus holding frequency
approximately constant with changes in V
IN
. The nominal
frequency can be adjusted with an external resistor R
ON
.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Forcing a
voltage above 1.5V will turn on the device.
Pulse-Skipping Mode
The LTC3812-5 can operate in one of two modes selectable
with the FCB pin—pulse-skipping mode or forced
continuous mode (see Figure 1). Pulse-skipping mode is
selected when increased effi ciency at light loads is desired
(see Figure 2). In this mode, the bottom MOSFET is turned
off when inductor current reverses to minimize effi ciency
loss due to reverse current fl ow and gate charge switching.
At low load currents, I
TH
will drop below the zero current
level (1.2V) shutting off both switches. Both switches will
remain off with the output capacitor supplying the load
current until the I
TH
voltage rises above the zero current
Figure 1. Comparison of Inductor Current Waveforms for Pulse-Skipping Mode
and Forced Continuous Operation
OPERATION
Figure 2. Effi ciency in Pulse-Skipping/Forced Continuous Modes
LOAD (A)
0.01
40
EFFICIENCY (%)
50
60
70
80
0.1 1 10
38125 F02
30
20
10
0
90
100
V
IN
= 12V
V
IN
= 42V
PULSE
SKIP
FORCED
CONTINUOUS
DECREASING
LOAD
CURRENT
38125 F01
PULSE SKIPPING MODE
0A
0A
0A
0A
0A
0A
FORCED CONTINUOUS