LT1945 LINER [Linear Technology], LT1945 Datasheet - Page 4

• Manufacturer Part Number: LT1945
• Description: Dual Micropower DC/DC Converter with Positive and Negative Outputs
• Manufacturer: LINER [Linear Technology]

(EXTERNAL)
(EXTERNAL)
LT1945
BLOCK DIAGRA
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
R1
R2
V
V
IN
OUT1
NFB1
C1
1
8
Q1
R5
80k
U
V
IN
Q2
X10
R6
80k
R3
60k
R4
280k
+
–
2
W
SHDN1
A1
ONE-SHOT
SWITCHER 1
3
ENABLE
400ns
GND
L1
RESET
A2
+
–
10
DRIVER
Figure 1. LT1945 Block Diagram
C3
SW1
42mV
D1
Q3
0.12
L2
9
PGND
C2
V
PGND
OUT1
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
1.5 s 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.
0.12
7
Q3B
V
OUT2
42mV
C4
DRIVER
SW2
D2
+
–
6
A2B
SWITCHER 2
ONE-SHOT
ENABLE
400ns
L3
RESET
SHDN2
A1B
4
V
+
–
IN
140k
R6B
R3B
R4B
40k
30k
Q2B
X10
R5B
40k
Q1B
1945 BD
5
FB2
V
OUT2
V
IN
R1B
(EXTERNAL)
R2B
(EXTERNAL)
1945f