LTC3812-5_15 Datasheet, PDF (11/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
OPERATION
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 fixed 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 amplifier 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 efficiency at light loads is desired
(see Figure 2). In this mode, the bottom MOSFET is turned
off when inductor current reverses to minimize efficiency
loss due to reverse current flow 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
100
PULSE
90 _SKIP
80
70
60
50
40
30
20
10
0
0.01
FORCED
CONTINUOUS
0.1
1
LOAD (A)
V_IN = 12V
V_IN = 42V
10
38125 F02
Figure 2. Efficiency in Pulse-Skipping/Forced Continuous Modes
PULSE SKIPPING MODE
FORCED CONTINUOUS
0A
0A
DECREASING
LOAD
CURRENT
0A
0A
0A
0A
38125 F01
Figure 1. Comparison of Inductor Current Waveforms for Pulse-Skipping Mode
and Forced Continuous Operation
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