LTC2913 datasheet(17/26 Pages) LINEAR

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LT4356-1/LT4356-2

4356fa

Layout Considerations

To achieve accurate current sensing, Kelvin connection

to the current sense resistor (RSNS in Figure 9) is recom-

mended. The minimum trace width for 1oz copper foil is

0.02" per amp to ensure the trace stays at a reasonable

temperature. 0.03" per amp or wider is recommended.

Note that 1oz copper exhibits a sheet resistance of about

530μ

Ω/square.Smallresistancescancauselargeerrorsin

high current applications. Noise immunity will be improved

signiﬁcantly by locating resistive dividers close to the pins

with short VCC and GND traces.

Design Example

As a design example, take an application with the follow-

ing speciﬁcations: VCC = 8V to 14V DC with transient up

to 80V, VOUT ≤ 16V, current limit (ILIM) at 5A, low battery

detection at 6V, and 1ms of overvoltage early warning

(Figure 9).

First, calculate the resistive divider value to limit VOUT to

16V during an overvoltage event:

VREG =

1.25V • R1 + R2

()

= 16V

Set the current through R1 and R2 during the overvoltage

condition to 250μA.

R2 =

1.25V

250μA

= 5k

Choose 4.99k for R2.

R1 =

16V – 1.25V

() • R2

1.25V

= 58.88k

The closest standard value for R1 is 59k.

Next calculate the sense resistor, RSNS, value:

RSNS =

50mV

ILIM

50mV

= 10m

CTMR is then chosen for 1ms of early warning time:

CTMR =

1ms • 5μA

100mV

= 50nF

The closest standard value for CTMR is 47nF.

Finally, calculate R4 and R5 for the 6V low battery threshold

detection:

6V =

1.25V • R4 + R5

()

Choose 100k for R5.

R4 =

6V – 1.25V

() • R5

1.25V

= 380k

Select 383k for R4.

The pass transistor, Q1, should be chosen to withstand

the output short condition with VCC = 14V.

The total overcurrent fault time is:

tOC =

47nF • 0.85V

45.5μA

= 0.878ms

The power dissipation on Q1 equals to:

P =

14V • 50mV

10m

= 70W

These conditions are well within the Safe Operating Area

of IRLR2908.

APPLICATIONS INFORMATION

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