lt3724 Linear Technology Corporation, lt3724 Datasheet - Page 9

• Manufacturer Part Number: lt3724
• Description: High Voltage, Current Mode Switching Regulator Controller
• Manufacturer: Linear Technology Corporation

OPERATIO
The LT3724 is a PWM controller with a constant fre-
quency, current mode control architecture. It is designed
for low to medium power, switching regulator applica-
tions. Its high operating voltage capability allows it to step-
up or down input voltages up to 60V without the need for
a transformer. The LT3724 is used in nonsynchronous
applications, meaning that a freewheeling rectifier diode
(D1 of Function Diagram) is used instead of a bottom side
MOSFET. For circuit operation, please refer to the Func-
tional Diagram of the IC and Typical Application on the
front page of the data sheet. The LT3800 is a similar part
that uses synchronous rectification, replacing the diode
with a MOSFET in a step-down application.
Main Control Loop
During normal operation, the external N-channel MOSFET
switch is turned on at the beginning of each cycle. The
switch stays on until the current in the inductor exceeds a
current threshold set by the DC control voltage, V
is the output of the voltage control loop. The voltage
control loop monitors the output voltage, via the V
voltage, and compares it to an internal 1.231V reference.
It increases the current threshold when the V
below the reference voltage and decreases the current
threshold when the V
voltage. For instance, when an increase in the load current
occurs, the output voltage drops causing the V
to drop relative to the 1.231V reference. The voltage
control loop senses the drop and increases the current
threshold. The peak inductor current is increased until the
average inductor current equals the new load current and
the output voltage returns to regulation.
Current Limit/Short-Circuit
The inductor current is measured with a series sense
resistor (see the Typical Application on the front page).
When the voltage across the sense resistor reaches the
maximum current sense threshold, typically 150mV, the
TG MOSFET driver is disabled for the remainder of that
cycle. If the maximum current sense threshold is still
exceeded at the beginning of the next cycle, the entire cycle
is skipped. Cycle skipping keeps the inductor currents to
a reasonable value during a short-circuit, particularly
when V
discussed in the “Application Information” section.
IN
is high. Setting the sense resistor value is
U
(Refer to Functional Diagram)
FB
voltage is above the reference
FB
FB
voltage is
C
voltage
, which
FB
pin
For higher converter efficiency and less power dissipation
in the IC, V
such as the converter output. When an external supply
back drives the internal V
diode and the V
regulation voltage, the internal regulator is disabled and
goes into a low current mode. V
most of the internal IC functions and is also used to charge
the bootstrapped capacitor (C
The external MOSFET switch is biased from the
bootstrapped capacitor. While the external MOSFET switch
is off, an internal BJT switch, whose collector is connected
to the SW pin and emitter is connected to the PGND pin,
is turned on to pull the SW node to PGND and recharge the
bootstrap capacitor. The switch stays on until either the
V
An internal V
power for start-up under all operating conditions with
MOSFET gate charge loads up to 90nC. The regulator can
operate continuously in applications with V
to 60V, provided the V
charge currents do not create excessive power dissipation
in the IC. Safe operating conditions for continuous regu-
lator use are shown in Figure 1. In applications where
these conditions are exceeded, V
an external source after start-up. The LT3724 regulator
can, however, be used for “full time” use in applications
where short-duration V
tinuous voltages.
CC
Figure 1. V
/Boosted Supply
CC
70
60
50
40
30
20
10
CC
0
CC
can also be supplied from an external supply
Regulator Continuous Operating Conditions
regulator provides V
CC
MOSFET TOTAL GATE CHARGE (nC)
20
OPERATING
voltage is pulled to a diode above its
AREA
SAFE
IN
40
IN
CC
transients exceed allowable con-
voltage and/or MOSFET gate
regulator through an external
BOOST
60
CC
CC
) via an external diode.
is the bias supply for
must be derived from
80
IN
derived gate-drive
3724 F01
100
LT3724
IN
voltages up
3724fb
9