LT1945
BLOCK DIAGRA
V
IN
C1
V
IN
8
2
R5
80k
R6
80k
+
–
Q1
Q2
X10
R3
60k
V
OUT1
R1
(EXTERNAL)
R2
(EXTERNAL)
R4
280k
NFB1
1
OPERATIO
The LT1945 uses a constant off-time control scheme to
provide high efficiencies over a wide range of output
current. Operation can be best understood by referring to
the block diagram in Figure 1. Q1 and Q2 along with R3 and
R4 form a bandgap reference used to regulate the output
voltage. When the voltage at the NFB1 pin is slightly below
–1.23V, comparator A1 disables most of the internal
circuitry. Output current is then provided by capacitor C2,
which slowly discharges until the voltage at the NFB1 pin
goes above the hysteresis point of A1 (typical hysteresis
at the NFB1 pin is 8mV). A1 then enables the internal
circuitry, turns on power switch Q3, and the current in
inductors L1 and L2 begins ramping up. Once the switch
current reaches 350mA, comparator A2 resets the one-
shot, which turns off Q3 for 400ns. L2 continues to deliver
current to the output while Q3 is off. Q3 turns on again and
4
W
L1
C3
L2
V
OUT1
D1
SHDN1
SW1
C2
V
OUT2
C4
SW2
SHDN2
D2
L3
V
IN
10
6
4
V
IN
R6B
40k
A1
ENABLE
ENABLE
A1B
R5B
40k
+
V
OUT2
–
Q1B
400ns
ONE-SHOT
DRIVER
RESET
Q3
Q3B
DRIVER
RESET
R3B
30k
R4B
140k
42mV
400ns
ONE-SHOT
Q2B
X10
5
FB2
R1B
(EXTERNAL)
R2B
(EXTERNAL)
+
0.12Ω
A2
SWITCHER 1
3
GND
9
PGND
PGND
7
0.12Ω
+
42mV
–
–
A2B
SWITCHER 2
1945 BD
Figure 1. LT1945 Block Diagram
U
the inductor currents ramp back up to 350mA, then A2
again resets the one-shot. This switching action continues
until the output voltage is charged up (until the NFB1 pin
reaches –1.23V), then A1 turns off the internal circuitry
and the cycle repeats.
The second switching regulator is a step-up converter
(which generates a positive output) but the basic opera-
tion is the same.The LT1945 contains additional circuitry
to provide protection during start-up and under short-
circuit conditions. When the FB2 pin voltage is less than
approximately 600mV, the switch off-time is increased to
1.5µs and the current limit is reduced to around 250mA
(70% of its normal value). This reduces the average
inductor current and helps minimize the power dissipation
in the power switch and in the external inductor and diode.
1945f
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