LT1432-3.3
3.3V High Efficiency Step-Down
Switching Regulator Controller
FEATURES
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DESCRIPTIO
Accurate Preset 3.3V Output
Up to 87% Efficiency
Optional Burst Mode
TM
Operation for Light Loads
Can Be Used with Many LTC Switching ICs
Accurate Ultra-Low-Loss Current Limit
Operates with Inputs from 4.5V to 30V
Shutdown Mode Draws Only 15碌A(chǔ)
Uses Small 30碌H Inductor
The LT
廬
1432-3.3 is a control chip designed to operate
with the LT1171/LT1271 family of switching regulators to
make a very high efficiency 3.3V step-down (buck) switch-
ing regulator. A minimum of external components is
needed.
Included is an accurate current limit which uses only
60mV sense voltage and uses 鈥渇ree鈥?PC board trace
material for the sense resistor. Logic controlled electronic
shutdown mode draws only 15碌A(chǔ) battery current. The
switching regulator operates down to 4.5V input.
The LT1432-3.3 has a logic controlled Burst Mode operation
to achieve high efficiency at very light load currents (0mA to
100mA) such as memory keep-alive. In normal switching
mode, the standby power loss is about 30mW, limiting
efficiency at light loads. In Burst Mode operation, standby
loss is reduced to approximately 11mW. Output current in
this mode is typically in the 5mA to 100mA range.
The LT1432-3.3 is available in 8-pin SO and PDIP pack-
ages. The LT1171/LT1271 is also available in surface
mount DD packages.
APPLICATI
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Laptop and Palmtop Computers
Portable Data-Gathering Instruments
, LTC and LT are registered trademarks of Linear Technology Corporation.
Burst Mode is a trademark of Linear Technology Corporation.
TYPICAL APPLICATI
D2
1N5818
V
IN
+
V
SW
C1
330碌F
35V
C6
0.02碌F
C5
0.03碌F
D1
MBR330p
鈥?/div>
V
IN
LT1271
10碌H
3A
100碌F
16V
FB
V
C
+
GND
D2**
OPTIONAL
OUTPUT
FILTER
+
V
C
V
IN
LT1432-3.3
DIODE
V
+
V
LIM
V
OUT
GND
脳
C2
390碌F
16V
V
OUT
***
3.3V
3A
EFFICIENCY (%)
C3
10碌F
TANT
+
L1
30碌H
R2*
0.013鈩?/div>
MODE LOGIC
220pF
MODE
<0.3V = NORMAL MODE
>2.5V = SHUTDOWN
OPEN = Burst Mode
OPERATION
*R2 IS MADE FROM PC BOARD
COPPER TRACES.
**OPTIONAL CONNECTION FOR D2.
鈥?/div>
FOR CIRCUITS WHICH DO NOT USE
EFFICIENCY IS HIGHER, BUT MINIMUM
V
IN
INCREASES. SEE APPLICATION
Burst Mode OPERATION, C5 MAY
INFORMATION SECTION.
BE PARALLEL WITH A 680鈩? 0.1碌F
IN SERIES TO GIVE WIDE PHASE MARGIN ***MAXIMUM CURRENT IS DETERMINED
BY THE CHOICE OF LT1071 FAMILY MAIN SWITCHER IC.
WITH DIFFERENT SWITCHING ICs AND
SEE APPLICATION INFORMATION SECTION.
OUTPUT CAPACITORS
LT1432-3.3 TA01
Figure 1. High Efficiency 5V Buck Converter
U
Efficiency
100
LT1271, L = 30碌H
V
IN
= 7V
90
D2 CONNECTED
TO OUTPUT
80
D2 CONNECTED
TO INPUT
70
Burst Mode OPERATION
(USE mA SCALE)
60
0
0
1A
20mA
2A
40mA
3A
60mA
LT1432-3.3 TA02
UO
UO
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