LTC3810-5
12
38105fd
Main Control Loop
The LTC3810-5 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 SENSE
–
and SENSE
+
pins using a sense resistor or 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
.
For applications with stringent constant frequency re-
quirements, the LTC3810-5 can be synchronized with an
external clock. By programming the nominal frequency
the same as the external clock frequency, the LTC3810-5
behaves as a constant frequency part against the load and
supply variations.
Pulling the
SHDN
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 Skip Mode
The LTC3810-5 can operate in one of two modes selectable
with the MODE/SYNC pin—pulse skip mode or forced con-
tinuous mode (see Figure 1). Pulse skip 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
level to initiate another cycle. In this mode, frequency is
proportional to load current at light loads.
Pulse skip mode operation is disabled by comparator F
when the MODE/SYNC pin is brought below 0.8V, forcing
continuous synchronous operation. Forced continuous
mode is less efficient due to resistive losses, but has the
advantage of better transient response at low currents,
approximately constant frequency operation, and the ability
to maintain regulation when sinking current.
Figure 1. Comparison of Inductor Current Waveforms for Pulse Skip Mode
and Forced Continuous Operation
operaTion
Figure 2. Efficiency in Pulse Skip/
Forced Continuous Modes
DECREASING
LOAD
CURRENT
38105 F01
PULSE SKIP MODE
0A
0A
0A
0A
0A
0A
FORCED CONTINUOUS
LOAD (A)
0.01
40
EFFICIENCY (%)
50
60
70
80
0.1
1
10
38105 F02
30
20
10
0
90
100
V
IN
= 12V
V
IN
= 42V
PULSE
SKIP
FORCED
CONTINUOUS
