LTC1154
APPLICATI
S I FOR ATIO
U
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.
12V
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).
12V
IN
EN
LTC1154
STATUS
GND
G
SD
C
1
0.33µF
15V
R
1
100k
V
S
C
D
0.01µF
DS
D1
1N4148
R
2
100k
MTP3055E
OUT
R
D
100k
+
470µF
0.036Ω
Figure 3. Powering Large Capacitive Loads
OVER-CURRENT SHUTDOWN TIME (1 = RC)
The RC network, R
D
and C
D
, 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 = (V
GATE
– V
TH
)/(R1
×
C1)
And therefore the current flowing into the capacitor during
start-up is approximately:
I
START-UP
= C
LOAD
×
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.
W
U
UO
+
470µF
IN
EN
LTC1154
STATUS
GND
G
1M
SD
9.1V
12V/1A
BULB
LTC1154 • F04
V
S
DS
10k
100k
0.036Ω
VN2222LL
0.1µF
MTP3055EL
Figure 4. Lamp Driver with Delayed Protection
Selecting R
D
and C
D
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, R
D
and C
D
,
which make up a simple RC delay between the drain sense
resistor and the drain sense input.
10
+
C
LOAD
100µF
LTC1154 • F03
1
0.1
0.01
1
10
100
MOSFET CURRENT (1 = SET CURRENT)
LTC1154 • F05
Figure 5. Over-Current Shutdown Time vs MOSFET Current
9
LINER [ LINEAR TECHNOLOGY ]
