LTC1154

9

1154fb

Capacitive Loads

Large capacitive loads, such as complex electrical systems

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 current

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

supply transients and allows for slower activation of sensi-

tive electrical loads. (Diode, D1, provides a direct path for

the LTC1154 protection circuitry to quickly discharge the

gate in the event of an overcurrent condition).

APPLICATIONS INFORMATION

Lamp Loads

The inrush 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 inrush current has subsided.

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

istics of the load after start-up. With this circuit, it is possible

to power a large capacitive load and still react quickly to

an overcurrent condition. The ramp rate at the output of

the switch as it lifts off ground is approximately:

And therefore the current flowing into the capacitor during

start-up is approximately:

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 3. Powering Large Capacitive Loads

IN

EN

STATUS

GND

DS

G

SD

LTC1154

+

470μF

MTP3055E

15V

12V

0.036Ω

LTC1154 • F03

0.01μF

100k

R1

100k

R2

100k

D1

1N4148

C1

0.33μF

+

100μF

OUT

IN

EN

STATUS

GND

DS

G

SD

LTC1154

+

470μF

MTP3055EL

9.1V

12V

0.036Ω

LTC1154 • F04

10k

1M

0.1μF

VN2222LL

100k

12V/1A

BULB

Figure 4. Lamp Driver with Delayed Protection

Figure 5 is a graph of normalized overcurrent shutdown

time versus normalized MOSFET current. This graph is

which make up a simple RC delay between the drain sense

resistor and the drain sense input.

Figure 5. Overcurrent Shutdown Time vs MOSFET Current

MOSFET CURRENT (1 = SET CURRENT)

1

0.01

10

10

100

LTC1154 • F05

1

0.1