Datasheet

LT3464
6
3464f
The LT3464 uses a constant off-time control scheme in
conjunction with Burst Mode
®
operation to provide high
efficiency over a wide range of output current. Operation
can best be understood by studying the Block Diagram.
When the FB pin voltage is lower than the 1.25V reference,
the hysteretic comparator enables the power section,
causing the chip to start switching, thus charging the
output capacitor. When the output voltage increases
enough to overcome the hysteresis, the feedback com-
parator shuts off the power section leaving only low power
circuitry running until the output voltage falls again. This
cycle repeats, keeping the output voltage within a small
window. The switching action is as follows: The switch
turns on, and current through it starts to ramp up until the
OPERATIO
U
point where the current limit is reached, at which point the
switch turns off for a fixed amount of time. While the
switch is off the inductor is delivering current to the load.
When the off time expires, the switch turns on again until
the current limit is reached, and the cycle repeats.
This chip includes an internal power Schottky diode and a
PNP transistor for output disconnect. The PNP transistor
disconnects the load from the input during shutdown. The
PNP control circuitry is designed to keep the PNP out of
saturation across a wide range of current, to keep quies-
cent current to a minimum and to provide current limiting
to protect the chip during short-circuit conditions.
Burst Mode is a registered a trademark of Linear Technolgy Corporation.
BLOCK DIAGRA
W
S
R
Q
Q
DELAY
–
–
+
–
+
12mV
OUT
OUT
V
REF
1.25V
CTRL
V
IN SW
GND
OUT
ANTI
SAT
4
8
1
2
7 6 5 3
SHDN
FB
CAP
3464 BD
C
PL
0.1Ω