lt1944-1ems-trpbf Linear Technology Corporation, lt1944-1ems-trpbf Datasheet - Page 4

• Manufacturer Part Number: lt1944-1ems-trpbf
• Description: Dual Micropower Step-up Dc/dc Converter
• Manufacturer: Linear Technology Corporation

(EXTERNAL)
(EXTERNAL)
LT1944
BLOCK DIAGRA
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
4
R1
R2
V
V
IN
OUT1
FB1
C1
1
GND
8
Q1
R5
40k
V
IN
U
3
Q2
X10
R6
40k
R3
30k
R4
140k
+
–
2
W
SHDN1
A1
ONE-SHOT
ENABLE
400ns
L1
RESET
A2
+
–
10
DRIVER
Figure 1. LT1944 Block Diagram
SW1
D1
42mV
Q3
0.12
C2
V
9
OUT1
PGND
PGND
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 1.5 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. The second switching
regulator operates in the same manner.
V
OUT2
0.12
7
Q3B
C3
42mV
SW2
D2
DRIVER
6
+
–
A2B
ONE-SHOT
ENABLE
400ns
L2
RESET
SHDN2
A1B
4
V
+
–
IN
140k
R6B
R3B
R4B
40k
30k
Q2B
X10
R5B
40k
Q1B
1944 BD
5
FB2
V
OUT2
V
IN
R1B
(EXTERNAL)
R2B
(EXTERNAL)