LTC3812-5
12
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level to initiate another cycle. In this mode, frequency is
proportional to load current at light loads.
Pulse-skipping mode operation is disabled by comparator F
when the FCB 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.
Fault Monitoring/Protection
Constant on-time current mode architecture provides ac-
curate cycle-by-cycle current limit protection—a feature
that is very important for protecting the high voltage power
supply from output short-circuits. The cycle-by-cycle cur-
rent monitor guarantees that the inductor current will never
exceed the value programmed on the V
RNG
pin.
Foldback current limiting provides further protection if the
output is shorted to ground. As V
FB
drops, the buffered
current threshold voltage I
THB
is pulled down and clamped
to 1V. This reduces the inductor valley current level to
one-sixth of its maximum value as V
FB
approaches 0V.
Foldback current limiting is disabled at start-up.
Overvoltage and undervoltage comparators OV and UV
pull the PGOOD output low if the output feedback voltage
exits a ±10% window around the regulation point after the
internal 120μs power bad mask timer expires. Furthermore,
in an overvoltage condition, M1 is turned off and M2 is
turned on immediately and held on until the overvoltage
condition clears.
The LTC3812-5 provides an undervoltage lockout com-
parator for the INTV
CC
supply. The INTV
CC
UV threshold
is 4.2V to guarantee that the MOSFETs have suffi cient
gate drive voltage before turning on. If INTV
CC
is under
the UV threshold, the LTC3812-5 is shut down and the
drivers are turned off.
Strong Gate Drivers
The LTC3812-5 contains very low impedance drivers
capable of supplying amps of current to slew large MOSFET
gates quickly. This minimizes transition losses and allows
paralleling MOSFETs for higher current applications. A
60V fl oating high side driver drives the topside MOSFET
and a low side driver drives the bottom side MOSFET
(see Figure 3). The bottom side driver is supplied directly
from the INTV
CC
pin. The top MOSFET drivers are biased
from fl oating bootstrap capacitor C
B
, which normally is
recharged during each off cycle through an external diode
from INTV
CC
when the top MOSFET turns off. In pulse-
skipping mode operation, where it is possible that the
bottom MOSFET will be off for an extended period of time,
an internal timeout guarantees that the bottom MOSFET is
turned on at least once every 25μs for one on-time period
to refresh the bootstrap capacitor.
Figure 3. Floating TG Driver Supply and Negative BG Return
OPERATION
IC/Driver Supply Power
The LTC3812-5’s internal control circuitry and top and
bottom MOSFET drivers operate from a supply voltage
(INTV
CC
pin) in the range of 4.2V to 14V. The LTC3812-5
has two integrated linear regulator controllers to easily
generate this IC/driver supply from either the high voltage
input or from the output voltage. For best effi ciency the
supply is derived from the input voltage during start-up
and then derived from the lower voltage output as soon
as the output is higher than 4.7V. Alternatively, the supply
can be derived from the input continuously if the output is
< 4.7V or an external supply in the appropriate range can
be used. The LTC3812-5 will automatically detect which
mode is being used and operate properly.
BOOST
TG
SW
BG
PGND
INTV
CC
INTV
CC
LTC3812-5
M1
M2
+
+
V
IN
C
IN
V
OUT
C
OUT
D
B
C
B
L
38125 F03
