LTC3813 Datasheet, PDF (11/32 Pages) Linear Technology – 100V Current Mode Synchronous Step-Up Controller

LTC3813
OPERATION
Main Control Loop
The LTC3813 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 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 cor-
responding 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 volt-
age. 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.
For applications with stringent constant-frequency require-
ments, the LTC3813 can be synchronized with an external
clock. By programming the nominal frequency the same as
the external clock frequency, the LTC3813 behaves as a con-
stant-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.
Fault Monitoring/Protection
Constant off-time current mode architecture provides
accurate cycle-by-cycle current limit protection—a fea-
ture that is very important for protecting the high volt-
age power supply from output overcurrent conditions.
The cycle-by-cycle current monitor guarantees that the
inductor 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,
in an overvoltage condition, M1 is turned off and M2 is
turned on immediately and held on until the overvoltage
condition clears.
The LTC3813 provides two undervoltage lockout com-
parators—one for the INTV_CC/DRV_CC supply and one for
the input supply V_IN. The INTV_CC UV threshold is 6.2V to
guarantee that the MOSFETs have sufficient gate drive volt-
age before turning on. The V_IN UV threshold (UVIN pin) is
0.8V with 10% hysteresis which allows programming the
V_IN threshold with the appropriate resistor divider con-
nected to V_IN. If either comparator inputs are under the
UV threshold, the LTC3813 is shut down and the drivers
are turned off.
Strong Gate Drivers
The LTC3813 contains very low impedance drivers capable
of supplying amps of current to slew large MOSFET gates
quickly. This minimizes transition losses and allows paral-
leling MOSFETs for higher current applications. A 100V
floating high side driver drives the top side MOSFET and
a low side driver drives the bottom side MOSFET (see
Figure 1). The bottom side driver is supplied directly
from the DRV_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
DRV_CC
LTC3813
DRV_CC
BOOST
TG
SW
BG
BGRTN
0V TO –5V
V_IN
+
D_B
C_IN
L
C_B
M1 +
M2
V_OUT
C_OUT
3813 F01
Figure 1. Floating TG Driver Supply and Negative BG Return
3813fb
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