LTC4367 datasheet(11/24 Pages) AD

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LTC4381

Rev. 0

For more information www.analog.com

The LTC4381 limits the voltage and current delivered

to the load during supply transient or output overload

events. The N-channel MOSFET provides a low resistance

path from the input to the load during normal operation. In

overvoltage conditions it limits the output to a threshold

voltage below the clamped gate voltage. The total fault

timer period is set to ride through short-duration faults,

while longer events cause the output to shut off and pro-

tect the MOSFET from damage.

Start-Up

Figure 1 shows a 12V, 1A application which limits the

output to approximately 28.5V. When power is first

applied with VCC ≥ 4V and ON ≥ 1.05V, there is a delay of

about 10ms before the GATE pin begins charging C2 and

MOSFET’s gate terminal with a fixed 20µA current source.

The internal MOSFET operates as a source follower, ramp-

ing the output up at a rate of IGATE(UP)/C2. Inrush current

in the load capacitance COUT is given by Equation 1.

IINRUSH = IGATE(UP) •

COUT

(1)

where IGATE(UP) is typically 20µA.

Eventually, the GATE pin charges to the point where VIN ≈

VOUT and stops only when ∆VGATE (VGATE – VOUT) reaches

its regulation point of 11.5V, fully enhancing the MOSFET.

Overcurrent Fault Protection

The LTC4381 features an adjustable current limit that pro-

tects against short circuits and excessive load current.

During an overcurrent event, the GATE pin is regulated to

limit the current sense voltage across the SNS and OUT

pins (∆VSNS) to 50mV when OUT is above 3V. In the case

of a severe short at the output, where OUT is less than

1.5V, the current sense voltage is 62mV. Output current is

thereby limited to ∆VSNS/RSNS. Current limit may control

the startup ramp rate in extreme cases, such as if COUT

is unusually large or if current limit is set to an unusually

low value, and artificially reduces COUT’s inrush current

below the value previously calculated.

APPLICATIONS INFORMATION

Overvoltage Fault Protection

The LTC4381 limits the voltage at the output during an

overvoltage at the input. For the LTC4381-1/LTC4381-2

illustrated in Figure 1, an internal clamp limits the output

to either 28.5V or 47V, depending on the state of the SEL

pin. With the SEL pin grounded as shown, the output

is clamped at 28.5V. Tying the SEL pin high causes the

output to clamp at 47V.

The GATE pin may also be limited by the compliance of

the internal 20µA current source, to VCC + 13.5V. In the

LTC4381-3/LTC4381-4 the GATE pin clamp is entirely

disconnected, leaving only the VCC + 13.5V compliance

limit. This arrangement allows the output to be effectively

clamped at any voltage from 14.5V to 72V, by clamping

VCC to between 4V and 61.5V.

VCC Pin

The LTC4381 can withstand an input surge voltage of up to

100V. If the maximum expected surge voltage is less than

80V, the VCC pin can be connected directly to the input

supply. If the surge voltage is between 80V to 100V, the

VCC pin must be protected by filtering or clamping since

its operating range is from 4V to 80V for LTC4381-1/

LTC4381-2 and 4V to 72V for LTC4381-3/LTC4381-4.

For short duration spikes and transients exceeding 80V,

filtering is the most sensible means of protecting the VCC

pin. R1 and C1 provide filtering in Figure 1. Owing to the

LTC4381’s low ICC, values up to 100k may be used for R1

without seriously impairing the lower end of the operating

voltage range. For long duration surges such as auto-

motive load dump, C1 becomes prohibitively large and

Zener D1 is the most effective means of limiting the VCC

Figure 1. 12V/1A, Output Limited to 28.5V

RSNS

40m

10k

RDRN

100k

4.7µF

CTMR

220nF

47nF

COUT

22µF

68V

DRN

VCC

SEL

GND

4381 F01

TMR

SRC

SNS

OUT

LTC4381-2

GATE

GFET

CMHZ5266B

12V/1A OUTPUT

CLAMPED AT 28.5V

VIN

12V

FLT

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