LTC3814-5_15 Datasheet, PDF (10/30 Pages) Linear Technology – 60V Current Mode Synchronous Step-Up Controller

LTC3814-5
OPERATION
Main Control Loop
The LTC3814-5 is a current mode controller for DC/DC
step-up 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 bot-
tom 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
peak 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 (t_OFF) and the duty cycle required to
maintain regulation. The one-shot timer generates a top
MOSFET on-time that is inversely proportional to the I_OFF
current and proportional to the V_OFF voltage. Connecting
V_OUT to I_OFF and V_IN to V_OFF with a resistive divider keeps
the frequency approximately constant with changes in V_IN.
The nominal frequency can be adjusted with an external
resistor R_OFF.
Pulling the RUN/SS pin low forces the controller into its
shutdown state, turning off both M1 and M2. Forcing a
voltage above 0.9V will turn on the device.
Fault Monitoring/Protection
Constant off-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 overcurrent conditions. The
cycle-by-cycle current monitor guarantees that the induc-
tor current will never exceed the value programmed on
the V_RNG pin.
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 125μs power bad mask timer expires. Furthermore,
10
in an overvoltage condition, M2 is turned off and M1 is
turned on immediately and held on until the overvoltage
condition clears.
The LTC3814-5 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 LTC3814-5 is shut down and the
drivers are turned off.
Strong Gate Drivers
The LTC3814-5 contains very low impedance drivers ca-
pable 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 1). 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 an output
overvoltage condition, where it is possible that the bot-
tom 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 top MOSFET
on-time period to refresh the bootstrap capacitor.
INTV_CC
LTC3814-5 INTV_CC
BOOST
TG
SW
BG
PGND
V_IN
+
C_IN
D_B
L
C_B
M1 ₊
M2
V_OUT
C_OUT
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Figure 1. Floating TG Driver Supply and Negative BG Return
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