9

LTC1153

should be connected across the load, as shown in Figure

2, to safely divert the stored energy.

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 switch 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 re-

duces the start-up current flowing into the supply capaci-

tor/s which, in turn, reduces supply transients and allows

for slower activation of sensitive electrical loads. (Diode,

D1, provides a direct path for the LTC1153 protection

circuitry to quickly discharge the gate).

S

APPLICATI

I FOR ATIO

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

less than 100mA and does not false-trip the breaker.

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 trip threshold up by

a factor of 11:1 (to 30A) for 100ms while the bulb is turned

on. The trip threshold then drops down to 2.7A after the

inrush current has subsided.

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 (10

µs) to break the circuit if a short-circuit condi-

tion is encountered. 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:

BREAKER CURRENT (1 = SET CURRENT)

1

0.01

10

10

100

LTC1153 • F05

1

0.1

Figure 5. Trip Delay Time vs Breaker Current

Figure 5 is a graph of normalized breaker trip time versus

breaker current. This graph is used to select the two delay

between the drain sense resistor and the drain sense input.

Figure 3. Powering Large Capacitive Loads

IN

STATUS

GND

DS

G

SD

LTC1153

0.47

µF

+

470

µF

MTP3055E

15V

12V

0.036

Ω

LTC1153 • F03

0.01

µF

1OOk

R1

1OOk

R2

1OOk

D1

1N4148

C1

0.33

µF

+

100

µF

OUT

IN

STATUS

GND

DS

G

SD

LTC1153

0.33

µF

+

470

µF

MTP3055EL

9.1V

12V

0.036

Ω

LTC1153 • F04

10k

1M

0.1

µF

VN2222LL

100k

12V/1A

BULB

Figure 4. Lamp Driver with Delayed Protection