LT1944 Ver la hoja de datos (PDF) - Linear Technology
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Fabricante
LT1944 Datasheet PDF : 8 Pages
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LT1944
BLOCK DIAGRA
VIN
C1
8 VIN
L1
SHDN1
2
D1
SW1
10
VOUT1
C2
VOUT1
R1
(EXTERNAL)
R2
(EXTERNAL)
FB1
1
R5
R6
40k
40k
+
A1
ENABLE
–
Q1
Q2
X10
R3
30k
R4
140k
400ns
ONE-SHOT
RESET
A2
Q3
DRIVER
42mV
0.12Ω
VOUT2
C3
D2
SW2
6
Q3B
DRIVER
0.12Ω
42mV
L2
VIN
SHDN2
4
VIN
R6B R5B
40k 40k
A1B
ENABLE
400ns
ONE-SHOT
RESET
A2B
+
–
Q2B
X10
R3B
30k
R4B
140k
VOUT2
Q1B
FB2
5
R1B
(EXTERNAL)
R2B
(EXTERNAL)
GND
3
PGND PGND
9
7
1944 BD
Figure 1. LT1944 Block Diagram
U
OPERATIO
The LT1944 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 FB1 pin is slightly above
1.23V, comparator A1 disables most of the internal cir-
cuitry. Output current is then provided by capacitor C2,
which slowly discharges until the voltage at the FB1 pin
drops below the lower hysteresis point of A1 (typical
hysteresis at the FB pin is 8mV). A1 then enables the
internal circuitry, turns on power switch Q3, and the
current in inductor L1 begins ramping up. Once the switch
current reaches 350mA, comparator A2 resets the one-
shot, which turns off Q3 for 400ns. L1 then delivers
current to the output through diode D1 as the inductor
current ramps down. Q3 turns on again and the inductor
current ramps back up to 350mA, then A2 resets the one-
shot, again allowing L1 to deliver current to the output.
This switching action continues until the output voltage is
charged up (until the FB1 pin reaches 1.23V), then A1
turns off the internal circuitry and the cycle repeats. The
LT1944 contains additional circuitry to provide protection
during start-up and under short-circuit conditions. When
the FB1 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. The second switching
regulator operates in the same manner.
4
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