Datasheet

LTC3421
10
3421f
OPERATIO
U
The circuit connected to BURST should be able to sink or
source up to 2mA. Note that Burst Mode operaton is
inhibited during start-up and soft-start.
Note that if V
IN
is above V
OUT
– 0.3V, the part will exit Burst
Mode operation and the synchronous rectifier will be
disabled.
Note that if the load applied during forced Burst Mode
operation exceeds the current that can be supplied, the
output voltage will start to droop and the part will auto-
matically come out of Burst Mode operation and enter fixed
frequency mode, raising V
OUT
. The maximum current that
can be supplied in Burst Mode operation is given by:
Simplified Diagram of Automatic Burst Mode Control Circuit
–
+
24
TO
MODULATOR
CLAMP
0.5V TO 1V
SLEEP
3421 TA03
ERROR AMP/
SLEEP COMP
V
C
R
COMP
C
COMP
V
REF
±1%
–
+
–
+
9
BURST
0.9V/
1.1V
R
B
C
B
SSDONE
MODE
1 = Burst Mode
OPERATION
0 = PWM MODE
SSDONE
V
REF
–4%
UV
V
CC
1mA I
OUT
/3000
1
FB
I
VV
V
in amps
O MAX
OUT IN
IN
()
.
•
–
=
+
()
055
2
1
OUTPUT DISCONNECT AND INRUSH LIMITING
The LTC3421 is designed to allow true output disconnect
by eliminating body diode conduction of the internal
P-channel MOSFET rectifier. This allows V
OUT
to go to zero
volts during shutdown without drawing any current from
the input source. It also allows for inrush current limiting
at turn-on, minimizing surge currents seen by the input
supply. Note that to obtain the advantages of output