LT1945
W
BLOCK DIAGRA
C3
D2
L1
L2
L3
V
IN
V
V
V
IN
OUT1
OUT2
C4
D1
C2
C1
V
IN
SHDN1
SW1
SW2
SHDN2
8
2
10
6
4
V
IN
R5
80k
R6
80k
R6B
40k
R5B
40k
A1
A1B
+
+
ENABLE
ENABLE
V
OUT2
R1B
–
–
Q1B
(EXTERNAL)
FB2
Q1
400ns
400ns
Q2
Q2B
X10
5
Q3
Q3B
ONE-SHOT
ONE-SHOT
X10
R2B
(EXTERNAL)
DRIVER
DRIVER
R3
60k
R3B
30k
RESET
RESET
+
+
–
R4
280k
R4B
140k
0.12Ω
0.12Ω
V
OUT1
–
42mV
42mV
A2
A2B
R1
(EXTERNAL)
NFB1
SWITCHER 1
SWITCHER 2
1
R2
PGND PGND
GND
3
9
7
(EXTERNAL)
1945 BD
Figure 1. LT1945 Block Diagram
U
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
theblockdiagraminFigure1.Q1andQ2alongwithR3and
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
the inductor currents ramp back up to 350mA, then A2
againresetstheone-shot. Thisswitchingactioncontinues
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
inductorcurrentandhelpsminimizethepowerdissipation
in the power switch and in the external inductor and diode.
1945f
4
Linear [ Linear ]
