LTC1154
O U
W
U
PPLICATI
A
S I FOR ATIO
Capacitive Loads
Lamp 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,
reducessupplytransientsandallowsforsloweractivation
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).
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.
12V
+
470µF
10k
0.036Ω
IN
V
S
100k
EN
DS
G
VN2222LL
0.1µF
LTC1154
STATUS
GND
1M
SD
MTP3055EL
12V
+
9.1V
470µF
IN
V
S
0.036Ω
C
R
12V/1A
BULB
D
D
0.01µF
100k
EN
DS
G
D1
1N4148
LTC1154
LTC1154 • F04
STATUS
GND
R
100k
R
2
Figure 4. Lamp Driver with Delayed Protection
1
100k
MTP3055E
OUT
SD
C
1
Selecting RD and CD
0.33µF
15V
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.
+
C
LOAD
100µF
LTC1154 • F03
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:
10
1
dV/dt = (VGATE – VTH)/(R1 × C1)
0.1
0.01
Andthereforethecurrentflowingintothecapacitorduring
start-up is approximately:
1
10
100
ISTART-UP = CLOAD × dV/dt
MOSFET CURRENT (1 = SET CURRENT)
LTC1154 • F05
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
9
Linear [ Linear ]
