LTC3810EG LINER [Linear Technology], LTC3810EG Datasheet - Page 12

• Manufacturer Part Number: LTC3810EG
• Description: 100V Current Mode Synchronous Switching Regulator Controller
• Manufacturer: LINER [Linear Technology]

OPERATION
LTC3810
Main Control Loop
The LTC3810 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 bot-
tom MOSFET is turned on until the current comparator
I
next cycle. Inductor current is determined by sensing the
voltage between the SENSE
sense resistor or the bottom MOSFET on-resistance. The
voltage on the I
responding to the inductor valley current. The fast 25MHz
error amplifi er EA adjusts this voltage by comparing the
feedback signal V
age. If the load current increases, it causes a drop in the
feedback voltage relative to the reference. The I
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
frequency can be adjusted with an external resistor R
For applications with stringent constant frequency re-
quirements, the LTC3810 can be synchronized with an
external clock. By programming the nominal frequency
the same as the external clock frequency, the LTC3810
12
CMP
0A
0A
0A
trips, restarting the one-shot timer and initiating the
PULSE SKIP MODE
Figure 1. Comparison of Inductor Current Waveforms
for Pulse Skip Mode and Forced Continuous Operation
TH
pin sets the comparator threshold cor-
FB
to the internal 0.8V reference volt-
3810 F01
–
and SENSE
0A
0A
0A
FORCED CONTINUOUS
IN
+
. The nominal
pins using a
TH
voltage
ON
.
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 can operate in one of two modes selectable
with the MODE/SYNC pin—pulse skip mode or forced
continuous mode (see Figure 1). Pulse skip mode is se-
lected 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
level (1.2V) shutting off both switches. Both switches will
remain off with the output capacitor supplying the load
current until the I
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 effi cient 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.
100
90
80
70
60
50
40
30
20
10
Figure 2. Effi ciency in Pulse Skip Mode
and Forced Continuous Mode
0
0.01
V
IN
= 75V
TH
V
voltage rises above the zero current
IN
TH
0.1
= 25V
will drop below the zero current
LOAD (A)
V
IN
PULSE SKIP MODE
FORCED CONTINUOUS
= 75V
V
IN
1
= 25V
3810 F02
10
3810fb