Datasheet

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
fl owing 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 fi rst turned on. The current limit then drops down to
2.7A after the inrush current has subsided.
The RC network, R
D
and C
D
, in series with the drain sense
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:
dV/dt = (V
GATE
– V
TH
)/(R1 • C1)
And therefore the current fl owing 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.
Figure 3. Powering Large Capacitive Loads
IN
EN
STATUS
GND
V
S
DS
G
SD
LTC1154
+
470μF
MTP3055E
15V
12V
0.036Ω
LTC1154 • F03
C
D
0.01μF
R
D
100k
R1
100k
R2
100k
D1
1N4148
C1
0.33μF
+
C
LOAD
100μF
OUT
IN
EN
STATUS
GND
V
S
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
Selecting R
D
and C
D
Figure 5 is a graph of normalized overcurrent 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.
Figure 5. Overcurrent Shutdown Time vs MOSFET Current
MOSFET CURRENT (1 = SET CURRENT)
1
0.01
OVERCURRENT SHUTDOWN TIME (1 = RC)
10
10 100
LTC1154 • F05
1
0.1