LT3463/LT3463A
PI FU CTIO S
SHDN1 (Pin 9):
Shutdown Pin for Switcher 1. Pull this pin
above 1.5V to enable Switcher 1. Pull below 0.3V to turn
it off. Do not leave this pin floating.
FB1 (Pin 10):
Feedback Pin for Switcher 1. Set the output
voltage by selecting values for R1 and R2.
GND (Pin 11):
Exposed Pad. Solder this exposed pad
directly to the local ground plane. This pad must be
electrically connected for proper operation.
BLOCK DIAGRA
V
IN
C1
3
V
IN
SHDN1
9
SHDN1
V
OUT1
R2
10
R1
1.25V
FB1
–
+
A1
SWITCHER 1
A2
LT3463: R
S1
= R
S2
= 0.1Ω
LT3463A: R
S1
= 0.1Ω, R
S2
= 0.063Ω
OPERATIO
The LT3463 uses a constant off-time control scheme to
provide high efficiency over a wide range of output cur-
rent. Operation can be best understood by referring to the
block diagram in Figure 1. When the voltage at the FB1 pin
is slightly above 1.25V, comparator A1 disables most of
the internal circuitry. Output current is then provided by
capacitor C2, which slowly discharges until the voltage at
the FB1 pin goes below the hysteresis point of A1 (typical
hysteresis at the FB1 pin is 8mV). A1 then enables the
internal circuitry, turns on power switch Q1, and the
4
W
U
U
U
U
L1
V
OUT1
V
OUT2
D3
C4
L2
V
IN
C2
C3
2
SW1
1
V
OUT1
D1
5
D2
D2
4
SW2
SHDN2 8
SHDN2
300ns
ONE-SHOT
Q1
Q2
300ns
ONE-SHOT
1.25V
V
REF
7
R3
+
R
S1
R
S2
25mV
+
25mV
+
–
FB2
6
R4
–
–
A4
SWITCHER 2
A3
V
OUT2
GND
11
3463 F01
Figure 1. Block Diagram
current in inductor L1 begins ramping up. Once the switch
current reaches 250mA, comparator A2 resets the one-
shot, which turns off Q1 for 300ns. Q1 turns on again and
the inductor currents ramp back up to 250mA, then A2
again resets the one-shot. This switching action continues
until the output voltage is charged up (until the FB1 pin
reaches 1.25V), then A1 turns off the internal circuitry and
the cycle repeats. The second switching regulator is an
inverting converter (which generates a negative output)
but the basic operation is the same.
3463f
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
