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LT1910

sn1910 1910fs

The turn-on current spike into CLOAD is estimated by:

PEAK

LOAD

GTH

•

–

•

where VTH is the MOSFET gate threshold voltage. VG is

obtained by plotting the equation:

GATE

on the graph of Gate Drive Current (IGATE) vs Gate Voltage

(VGATE) as shown in Figure 6. The value of VGATE at the

intersection of the curves for a given supply is VG. For

example, if V+ = 24V and R1 = 100k, then VG = 18.3V. For

VTH = 2V, C1 = 0.1µF and CLOAD = 1000µF, the estimated

IPEAK = 1.6A. The diode and the second resistor in the

network ensure fast current limit turn-off.

When turning off a capacitive load, the source of the

MOSFET can “hang up” if the load resistance does not

discharge CLOAD as fast as the gate is being pulled down.

If this is the case, a 15V zener may be added from gate to

source to prevent VGS(MAX) from being exceeded.

and CD delay the overcurrent trip for drain currents up to

approximately 10 • ISET, above which the diode conducts

and provides immediate turn-off (see Figure 7). To ensure

proper operation of the timer, CD must be ≤CT.

APPLICATIO S I FOR ATIO

Figure 7. Current Limit Delay Time

Adding Current Limit Delay

When capacitive loads are being switched or in very noisy

environments, it is desirable to add delay in the drain

current sense path to prevent false tripping (inductive

loads normally do not need delay). This is accomplished

by the current limit delay network shown in Figure 5. RD

GATE VOLTAGE (V)

300

400

500

1910 F06

200

100

600

700

800

V+ = 48V

IGATE =

VGATE/10

V+ = 24V

V+ = 12V

V+ = 8V

Figure 6. Gate Drive Current vs Gate Voltage

MOSFET DRAIN CURRENT (1 = SET CURRENT)

0.01

0.1

100

1910 F07

Printed Circuit Board Shunts

The sheet resistance of 1oz copper clad is approximately

5 • 10–4

Ω/square with a temperature coefficient of

0.39%/

°C. Since the LT1910 drain sense threshold has a

similar temperature coefficient (0.33%/

°C),thisoffersthe

possibility of nearly zero TC current sensing using the

“free” drain sense resistor made out of PC trace material.

A conservative approach is to use 0.02" of width for each

1A of current for 1oz copper. Combining the LT1910 drain

sense threshold with the 1oz copper resistance results in

a simple expression for width and length:

Width (1oz Cu) = 0.02" • ISET

Length (1oz Cu) = 2"

The width for 2oz copper would be halved while the length

would remain the same.

Bends may be incorporated into the resistor to reduce

space; each bend is equivalent to approximately 0.6 • the

width of a straight length. Kelvin connection should be

employed by running a separate trace from the ends of the

resistor back to the LT1910’s V+ and SENSE pins. See

Application Note 53 for further information on printed

circuit board shunts.

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