LT1933 Linear Technology, LT1933 Datasheet - Page 14

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LT1933

Manufacturer Part Number
LT1933
Description
600mA 500kHz Step-Down Switching Regulator in SOT-23
Manufacturer
Linear Technology
Datasheet

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LT1933
should be placed on the same side of the circuit board, and
their connections should be made on that layer. Place a
local, unbroken ground plane below these components,
and tie this ground plane to system ground at one location,
ideally at the ground terminal of the output capacitor C1.
The SW and BOOST nodes should be as small as possible.
Finally, keep the FB node small so that the ground pin and
ground traces will shield it from the SW and BOOST nodes.
Include two vias near the GND pin of the LT1933 to help
remove heat from the LT1933 to the ground plane.
High Temperature Considerations
The die temperature of the LT1933 must be lower than the
maximum rating of 125°C. This is generally not a concern
unless the ambient temperature is above 85°C. For higher
temperatures, care should be taken in the layout of the
circuit to ensure good heat sinking of the LT1933. The
maximum load current should be derated as the ambient
temperature approaches 125°C.
The die temperature is calculated by multiplying the LT1933
power dissipation by the thermal resistance from junction
to ambient. Power dissipation within the LT1933 can be
TYPICAL APPLICATIO S
14
3.6V TO 20V
4.5V TO 36V
OFF ON
OFF ON
V
V
IN
IN
C2
2.2µF
C2
2.2µF
5
4
5
4
1.8V Step-Down Converter
3.3V Step-Down Converter
V
SHDN
V
SHDN
IN
IN
GND
GND
2
2
LT1933
LT1933
D2
BOOST
BOOST
FB
FB
SW
SW
3
3
R2
10k
R2
10k
4.42k
16.5k
1
6
1
6
R1
R1
C3
0.1µF
C3
0.1µF
U
D1
D1
D2
10µH
22µH
L1
L1
1933 TA02a
1933 TA02b
C1
22µF
2x
C1
22µF
6.3V
V
1.8V/500mA
V
3.3V/500mA
OUT
OUT
estimated by calculating the total power loss from an
efficiency measurement and subtracting the catch diode
loss. The resulting temperature rise at full load is nearly
independent of input voltage. Thermal resistance depends
on the layout of the circuit board, but a value of 125°C/W
is typical.
Die temperature rise was measured on a two-layer, five by
five cm circuit board in still air. The LT1933 producing 5V
at 500mA showed a temperature rise of 28°C, allowing it
to deliver full load to 97°C ambient. Above this tempera-
ture the load current should be reduced. For 3.3V at
500mA the temperature rise is 24°C.
Other Linear Technology Publications
Application notes AN19, AN35 and AN44 contain more
detailed descriptions and design information for Buck
regulators and other switching regulators. The LT1376
data sheet has a more extensive discussion of output
ripple, loop compensation and stability testing. Design
Note DN100 shows how to generate a bipolar output
supply using a Buck regulator.
14.5V TO 36V
6.3V TO 36V
OFF ON
OFF ON
V
V
IN
IN
C2
2.2µF
C2
2.2µF
5
4
5
4
12V Step-Down Converter
V
SHDN
V
SHDN
5V Step-Down Converter
IN
IN
GND
GND
2
2
LT1933
LT1933
BOOST
BOOST
FB
FB
SW
SW
3
3
R2
10k
R2
10k
30.1k
86.6k
1
6
1
6
R1
R1
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C3
0.1µF
C3
0.1µF
D1
D1
D3, 6V
D2
33µH
47µH
L1
L1
D2
1933 TA02c
1933 TA02d
C1
22µF
6.3V
C1
10µF
V
5V/500mA
V
12V/450mA
OUT
OUT
1933f

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