Datasheet
4
LT1944-1
BLOCK DIAGRA
W
Figure 1. LT1944-1 Block Diagram
OPERATIO
U
The LT1944-1 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 100mA, 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 100mA, 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
LT1944-1 contains additional circuitry to provide protec-
tion 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 70mA (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, but with a 175mA current limit and an off-time of
1.5µs. With this longer off-time, switcher 2 is ideal for very
low duty cycle applications (i.e. Li-Ion to 5V boost
converters).
–
+
–
+
8
400ns
ONE-SHOT
DRIVER
RESET
DRIVER
RESET
ENABLE
12mV
0.12Ω
A2
A1
Q3
9
3
R4
140k
R3
30k
R6
40k
R5
40k
Q2
X10
Q1
1
V
IN
FB1
2
SHDN1
10
SW1
PGNDGND
1944-1 BD
L1
C2
V
OUT1
V
IN
D1
R2
(EXTERNAL)
R1
(EXTERNAL)
V
OUT1
C1
–
+
–
+
1.5µs
ONE-SHOT
ENABLE
21mV
0.12Ω
A2B
A1B
Q3B
7
R4B
140k
R3B
30k
R6B
40k
R5B
40k
Q2B
X10
Q1B
5
V
IN
FB2
4
SHDN2
6
SW2
PGND
L2
C3
V
OUT2
V
IN
D2
R2B
(EXTERNAL)
R1B
(EXTERNAL)
V
OUT2
SWITCHER 2SWITCHER 1