Datasheet

LT1940/LT1940L
6
1940fa
The LT1940 is a dual, constant frequency, current mode
buck regulator with internal 2A power switches. The two
regulators share common circuitry including input source,
voltage reference and oscillator, but are otherwise inde-
pendent. This section describes the operation of the
LT1940; refer to the Block Diagram.
If the RUN/SS (run/soft-start) pins are both tied to ground,
the LT1940 is shut down and draws 30µA from the input
source tied to V
IN
. Internal 2µA current sources charge
external soft-start capacitors, generating voltage ramps at
BLOCK DIAGRA
W
–
+
–
+
–
+
–
+
R
SQ
SLAVE
OSC
∑
INT REG
AND REF
MASTER
OSC
RUN/SS2
RUN/SS1
2µA
2µA
CLK1
CLK2
V
IN
1.25V
125mV
I
LIMIT
CLAMP
RUN/SS
PG
C
C
C
F
R
C
GND
ERROR
AMP
SLOPE
V
C
0.75V
0.5V
CLK
R1
C1
C
IN
SW
FB
BOOST
V
IN
IN
D2
C3
L1
D1
C1
R2
OUT
1940 F02
these pins. If either RUN/SS pin exceeds 0.6V, the internal
bias circuits turn on, including the internal regulator,
1.25V reference and 1.1MHz master oscillator. In this
state, the LT1940 draws 3.5mA from V
IN
, whether one or
both RUN/SS pins are high. Neither switching regulator
will begin to operate until its RUN/SS pin reaches ~0.8V.
The master oscillator generates two clock signals of
opposite phase.
The two switchers are current mode step-down regula-
tors. This means that instead of directly modulating the
Figure 2. Block Diagram of the LT1940 with Associated External Components (One of Two Switching Regulators Shown)