LT3844
9
3844fc
For more information www.linear.com/LT3844
operaTion
(Refer to Functional Diagram)
The LT3844 is a PWM controller with a constant frequency,
current mode control architecture. It is designed for low
to medium power, switching regulator applications. Its
high operating voltage capability allows it to step up
or down input voltages up to 60V without the need for
a transformer. The LT3844 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
Functional 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
C
,
which is the output of the voltage control loop. The voltage
control loop monitors the output voltage, via the V
FB
pin
voltage, and compares it to an internal 1.231V reference.
It increases the current threshold when the V
FB
voltage
is below the reference voltage and decreases the current
threshold when the V
FB
voltage is above the reference
voltage. For instance, when an increase in the load current
occurs, the output voltage drops causing the V
FB
volt-
age 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 T
ypical Application on the front page).
When the voltage across the sense resistor reaches the
maximum current sense threshold, typically 100mV, the
TG MOSFET driver is disabled for the remainder of that
cycle. If the maximum current sense threshold is still ex
-
ceeded 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
IN
is high. Setting the sense resistor value is discussed
in the “Application Information” section.
V
CC
/Boosted Supply
An internal V
CC
regulator provides V
IN
derived gate-drive
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
IN
voltages up
to 60V, provided the power dissipation of the regulator
does not exceed 250mW. The power dissipation is cal
-
culated as follows:
P
d(REG)
= (V
IN
– 8V) • f
SW
• Q
G
where Q
G
is the MOSFET gate charge.
In applications where these conditions are exceeded, V
CC
must be derived from an external source after start-up.
Maximum continuous regulator power dissipation may be
exceeded for short duration V
IN
transients.
For higher converter efficiency and less power dissipa-
tion in the IC, V
CC
can also be supplied from an external
supply such as the converter output. When an external
supply back drives the internal V
CC
regulator through an
external diode and the V
CC
voltage is pulled to a diode
above its regulation voltage, the internal regulator is dis-
abled and goes into a low current mode. V
CC
is the bias
supply for most of the internal IC functions and is also
used to charge the bootstrapped capacitor (C
BOOST
) via an
external diode. 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 start of the next cycle or until the bootstrapped
capacitor is fully charged.
MOSFET Driver
The LT3844 contains a high speed boosted driver to turn
on and off an external N-channel MOSFET switch. The
MOSFET driver derives its power from the boost capacitor
which is referenced to the SW pin and the source of the
MOSFET. The driver provides a large pulse of current to turn
on the MOSFET fast to minimize transition times. Multiple
MOSFETs can be paralleled for higher current operation.
