LTC1255CS8 Linear Technology, LTC1255CS8 Datasheet - Page 8

• Manufacturer Part Number: LTC1255CS8
• Description: IC MOSFET DVR HI-SIDE DUAL 8SOIC
• Manufacturer: Linear Technology

Specifications of LTC1255CS8

• Configuration: High-Side
• Input Type: Non-Inverting
• Delay Time: 180µs
• Number Of Configurations: 2
• Number Of Outputs: 2
• Voltage - Supply: 9 V ~ 24 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Peak: -
• High Side Voltage - Max (bootstrap): -

APPLICATIO S I FOR ATIO
LTC1255
of sensitive electrical loads. (Resistor R2, and the diode
D1, provide a direct path for the LTC1255 protection
circuitry to quickly discharge the gate in the event of an
overcurrent condition.)
The RC network, R
drain sense input should be set to trip based on the
expected characteristics of the load after startup, i.e., 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:
Therefore, the current flowing into the capacitor during
startup is approximately:
Using the values shown in Figure 3, the startup current is
less than 100mA and does not false trigger the drain sense
circuitry which is set at 2.7A with a 1ms delay.
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 a short period
of time while the bulb is turned on. The current limit then
drops down to 2.7A after the in-rush current has subsided.
8
12V
dV/dt = (V
I
STARTUP
IN1
Figure 4. Lamp Driver With Delayed Protection
1/2 LTC1255
GND
= C
V
GATE
S
LOAD
DS1
G1
– V
DELAY
U U
TH
dV/dt
)/(R1 C1)
10k
and C
100k
1M
VN2222LL
DELAY
W
0.1 F
, in series with the
+
LTC1255 • F04
470 F
9.1V
U
MTP3055EL
R
0.036
SENSE
12V/1A
BULB
Selecting R
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
C
drain sense input and the drain sense resistor.
The Y axis of the graph is normalized to one RC time
constant. The X axis is normalized to the set current.
(The set current is defined as the current required to
develop 100mV across the drain sense resistor.)
Note that the shutdown time is shorter for increasing
levels of MOSFET current. This ensures that the total
energy dissipated by the MOSFET is always within the
bounds established by the manufacturer for safe opera-
tion. (See MOSFET data sheet for further S.O.A.
information.)
Using a Speed-Up Diode
Another way to reduce the amount of time that the
power MOSFET is in a short-circuit condition is to
“bypass” the delay resistor with a small signal diode as
shown in Figure 6. The diode will engage when the drop
across the drain sense resistor exceeds about 0.7V,
providing a direct path to the sense pin and dramatically
reducing the amount of time the MOSFET is in an
overload condition. The drain sense resistor value is
selected to limit the maximum DC current to 4A.
DELAY
Figure 5. Normalized Delay Time vs MOSFET Current
, which make up a simple RC delay between the
0.01
0.1
10
NORMALIZED MOSFET CURRENT (1 = SET CURRENT)
1
DELAY
0.1
and C
1
DELAY
10
LTC1255 • F05
100
DELAY
and