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參數(shù)資料
| 型號(hào)： | LQH3C4R7 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Dual Micropower DC/DC Converter with Positive and Negative Outputs |
| 中文描述： | 雙微功率DC / DC變換器的正，負(fù)輸出 |
| 文件頁(yè)數(shù)： | 4/8頁(yè) |
| 文件大?。?/td> | 142K |
| 代理商： | LQH3C4R7 |
4
LT1945
1945f
BLOCK DIAGRA
W
Figure 1. LT1945 Block Diagram
OPERATIOU
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
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
μ
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.
–
+
–
+
8
400ns
ONE-SHOT
DRIVER
RESET
RESET
ENABLE
42mV
0.12
A2
A1
Q3
9
3
R4
280k
R3
60k
R6
80k
R5
80k
Q2
X10
Q1
1
V
IN
NFB1
2
SHDN1
10
SW1
PGND
GND
1945 BD
L1
C3
C2
V
OUT1
V
IN
D1
R2
(EXTERNAL)
R1
(EXTERNAL)
V
OUT1
C1
–
+
–
+
400ns
ONE-SHOT
ENABLE
42mV
0.12
A2B
A1B
Q3B
7
R4B
140k
R3B
30k
R6B
40k
R5B
40k
Q2B
X10
Q1B
5
V
IN
FB2
4
SHDN2
6
SW2
PGND
L3
L2
C4
V
OUT2
V
IN
D2
R2B
(EXTERNAL)
R1B
(EXTERNAL)
V
OUT2
SWITCHER 1
SWITCHER 2
DRIVER
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