LT1432CS8 datasheet(14/28 Pages) LINER

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LT1432

because these traces do not have errors caused by plating

operations. The suggested trace width for 1/2oz foil is

0.03" for each 1A of current limit to keep trace temperature

rise reasonable. 3A current limit would require the width

to be 0.09". 1oz foil can reduce trace width to 0.02" per

amp. Inductance in the trace is not critical so the trace can

be wound serpentine or any other shape that fits available

space. Kelvin connections should be used as shown in

Figure 14 to avoid errors due to termination resistance.

The length of the sense resistor trace can be calculated

from:

Length =

Inches

SENSE

()

W = width of copper trace (

≈0.03" per amp for 1/2oz

copper foil)

RCU = resistivity of PCB trace, expressed as Ω per square.

It is found by calculating the resistance of a section of trace

with equal length and width. For typical 1/2oz material,

RCU is approximately 1mΩ per square. In the example

shown above, with width = 2A times 0.03" = 0.06";

APPLICATI

I FOR ATIO

Figure 15. Adding Foldback Current Limiting

Current limiting maintains true switching action, but power

dissipation in the IC switch and catch diode will shift

depending on output voltage. At output voltages near the

correct regulated value, power will be distributed between

switch and the diode according to the usual calculations.

Under short circuit conditions, switch duty cycle will drop

to a very low value, and power will concentrate in the

diode, which will be running at near 100% duty cycle. If

continuous shorts must be tolerated, the catch diode must

be sized to handle the full current limit value, or foldback

current can be used.

Foldback Current Limiting

Foldback current limiting makes the short circuit current

limit somewhat lower than the full load current limit to

reduce component stress under short circuit conditions.

This is shown in Figure 15 with the addition of R3 and R4.

The voltage drop across R3 adds to the 60mV current limit

VSW

GND

VIN

LT1271

200

µF

35V

0.02

µF

0.1

µF

680

Ω

0.03

µF

MBR330p

4.7

µF

TANT

1N4148

µH

0.025

Ω

470

µF

16V

VOUT

220pF

<0.3V = NORMAL MODE

>2.5V = SHUTDOWN

OPEN = BURST MODE

VIN

6V – 25V

LT1432 F15

100

Ω

12.5k

VIN

MODE

VOUT

VLIM

V+

LT1432

GND

OPTIONAL

OUTPUT

FILTER

100

µF

16V

µH

DIODE

Length =

0.06 0.02

0.001

= 1.2 Inches

()

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