LTC1154 Linear Technology, LTC1154 Datasheet - Page 9

• Manufacturer Part Number: LTC1154
• Description: High-Side Micropower MOSFET Driver
• Manufacturer: Linear Technology

A
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).
The RC network, R
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:
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.
PPLICATI
dV/dt = (V
I
START-UP
IN
EN
STATUS
GND
Figure 3. Powering Large Capacitive Loads
LTC1154
= C
GATE
LOAD
O
– V
12V
D
DS
SD
V
U
G
S
and C
TH
S
dV/dt
)/(R1 C1)
D
I FOR ATIO
100k
C
0.01 F
U
R
, in series with the drain sense
D
1
1N4148
D1
+
C
0.33 F
100k
R
1
W
2
470 F
100k
R
D
15V
+
MTP3055E
0.036
U
OUT
C
100 F
LTC1154 • F03
LOAD
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.
Selecting R
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
which make up a simple RC delay between the drain sense
resistor and the drain sense input.
Figure 5. Over-Current Shutdown Time vs MOSFET Current
IN
EN
STATUS
GND
Figure 4. Lamp Driver with Delayed Protection
0.01
0.1
10
1
LTC1154
D
1
and C
MOSFET CURRENT (1 = SET CURRENT)
12V
DS
SD
V
D
G
S
10
10k
100k
1M
+
VN2222LL
LTC1154 • F05
9.1V
470 F
0.1 F
100
LTC1154
MTP3055EL
0.036
D
12V/1A
BULB
LTC1154 • F04
and C
9
D
,