LTC4367 datasheet(15/24 Pages) AD

15 / 24 page

LTC4381

Rev. 0

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APPLICATIONS INFORMATION

Calculating Transient Stress

P√t for a prototypical transient waveform is calculated

using Equation 9 and Figure 3.

Let

a = VREG – VIN

b = VPK – VIN

(VIN = Nominal Input Voltage)

Then

P t

= ILOAD •

b – a

(

τ 2a2ln

+ 3a2 + b2 – 4ab

⎛

⎝⎜

⎞

⎠⎟

⎡

⎣

⎢

⎤

⎦

⎥

(9)

For the transient conditions of VPK = 100V, VIN = 12V,

VREG = 28.5V, tr = 10µs, τ = 1ms, and a load current of

1A, P√t is 1.4W√s which can be handled by the MOSFET.

The P√t of other transient waveshapes is evaluated by

integrating the MOSFET power over root of time. LTspice®

can be used to simulate timer behavior for more complex

transients and cases where overvoltage and overcurrent

faults coexist, as well as the peak temperature rise of

the MOSFET.

Calculating Short-Circuit Stress

SOA stress must also be calculated for a short-circuit

condition. Short-circuit P√t is given by Equation 10.

P t

= ΔVDS •

ΔVSNS

RSNS

⎛

⎝⎜

⎞

⎠⎟

• tTMR

(10)

where ∆VDS is the voltage across the MOSFET, ∆VSNS is

the current limit threshold and tTMR is the overcurrent

timer interval, given by Equation 5 and Equation 6.

For VIN = 15V, ∆VDS = 12V (VOUT = 3V), ∆VSNS = 50mV,

RSNS = 12mΩ, RDRN = 100kΩ and CTMR = 68nF, P√t is

4.95W√s – somewhat higher than the transient SOA cal-

culated in the previous example.

Figure 4. Automotive Cold Crank Ride Through

Limiting Inrush Current and GATE Pin Compensation

The LTC4381 limits the inrush current to any load capac-

itance by controlling the GATE pin voltage slew rate.

Connect an external capacitor, C2, from GATE to ground

to reduce the inrush current at the expense of slower

turn-off time. The gate capacitor is set using Equation 11.

= IGATE(UP) •

COUT

IINRUSH

(11)

The LTC4381 needs a minimum of 47nF capacitance (C2)

and a 33Ω (R2) resistor in series at the GATE pin to stabi-

lize the current limit amplifier during an overcurrent event.

C2 also limits self enhancement of the MOSFET. A 10Ω

resistor, R3, is connected to the gate of the MOSFET to

suppress parasitic oscillations.

Automobile Cold Crank Ride Through

During cold crank, the battery potential drops from the

12V nominal to as low as 3V for up to 40ms. The LTC4381

needs at least 4V at the VCC pin to function correctly. The

low quiescent current requirement of the part allows an

RC filter with reasonable values to be placed at the VCC pin

to ride through cold crank as shown in Figure 4.

RSNS

40m

10k

RDRN

100k

6.8µF

CTMR

220nF

47nF

DRN

VCC

SEL

GND

TMR

SRC

SNS

OUT

LTC4381-2

GATE

GFET

4381 F04

VIN

12V

(NOMINAL 3V

AT COLD

CRANK)

12V/1A OUTPUT

CLAMPED AT 28.5V

COUT

22µF

FLT

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