LT1940
BLOCK DIAGRA
2µA
RUN/SS2
INT REG
AND REF
MASTER
OSC
CLK1
CLK2
2µA
RUN/SS1
V
IN
IN
C
IN
∑
SLOPE
R
C1
SLAVE
OSC
SW
S
Q
C3
L1
OUT
BOOST
D2
CLK
+
–
V
C
0.5V
ERROR
AMP
FB
C
F
C
C
RUN/SS
PG
GND
Figure 2. Block Diagram of the LT1940 with Associated External Components (One of Two Switching Regulators Shown)
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
these pins. If either RUN/SS pin exceeds 0.5V, 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.
1940i
6
–
+
+
–
R
C
I
LIMIT
CLAMP
–
+
W
V
IN
0.9V
D1
C1
R1
R2
1.25V
125mV
1940 F02
LINER [ LINEAR TECHNOLOGY ]
