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LTC1154

Capacitive Loads

Large capacitive loads, such as complex electrical sys-

tems with large bypass capacitors, should be powered

using the circuit shown in Figure 3. The gate drive to the

power MOSFET is passed through an RC delay network,

R1 and C1, which greatly reduces the turn-on ramp rate of

the switch. And since the MOSFET source voltage follows

the gate voltage, the load is powered smoothly and slowly

from ground. This dramatically reduces the start-up cur-

rent flowing into the supply capacitor(s) which, in turn,

reduces supply transients and allows for slower activation

of sensitive electrical loads. (Diode, D1, provides a direct

path for the LTC1154 protection circuitry to quickly dis-

charge the gate in the event of an over-current condition).

APPLICATI

I FOR ATIO

Lamp Loads

The in-rush current created by a lamp during turn-on can

be 10 to 20 times greater than the rated operating current.

The circuit shown in Figure 4 shifts the current limit

threshold up by a factor of 11:1 (to 30A) for 100ms when

the bulb is first turned on. The current limit then drops

down to 2.7A after the in-rush current has subsided.

Figure 3. Powering Large Capacitive Loads

The RC network, RD and CD, in series with the drain sense

input should be set to trip based on the expected charac-

teristics of the load

after start-up. With this circuit, it is

possible to power a large capacitive load and still react

quickly to an over-current condition. The ramp rate at the

output of the switch as it lifts off ground is approximately:

dV/dt = (VGATE – VTH)/(R1 × C1)

And therefore the current flowing into the capacitor during

start-up is approximately:

ISTART-UP = CLOAD × dV/dt

Using the values shown in Figure 3, the start-up current is

less than 100mA and does not false-trigger the drain

sense circuitry which is set at 2.7A with a 1ms delay.

Figure 5. Over-Current Shutdown Time vs MOSFET Current

MOSFET CURRENT (1 = SET CURRENT)

0.01

100

LTC1154 • F05

0.1

Selecting RD and CD

Figure 5 is a graph of normalized over-current shutdown

time versus normalized MOSFET current. This graph is

used to select the two delay components, RD and CD,

which make up a simple RC delay between the drain sense

resistor and the drain sense input.

Figure 4. Lamp Driver with Delayed Protection

STATUS

GND

LTC1154

470

µF

MTP3055E

15V

12V

0.036

Ω

LTC1154 • F03

0.01

µF

100k

1N4148

0.33

µF

CLOAD

100

µF

OUT

STATUS

GND

LTC1154

470

µF

MTP3055EL

9.1V

12V

0.036

Ω

LTC1154 • F04

10k

0.1

µF

VN2222LL

100k

12V/1A

BULB

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