LTC3812-5_15 Datasheet, PDF (12/34 Pages) Linear Technology – 60V Current Mode Synchronous Switching Regulator Controller

LTC3812-5
OPERATION
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
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.
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 sufficient
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 floating 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 floating 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.
INTV_CC
LTC3812-5 INTV_CC
BOOST
TG
SW
BG
PGND
V_IN
+
D_B
C_IN
C_B
M1
L
V_OUT
+
M2
C_OUT
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Figure 3. Floating TG Driver Supply and Negative BG Return
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 efficiency 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.
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