LTC4367 datasheet(13/22 Pages) AD

13 / 22 page

LTC4368

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Since the MOSFET acts like a source follower, the slew

rate at VOUT equals the slew rate at GATE. Therefore, the

inrush current due to the capacitance on VOUT is given by:

IINRUSH =

COUT

CGATE

• IGATE(UP)

For example, a 1A inrush current into a 100µF output

capacitance requires a GATE capacitance of (using

IGATE(UP) = 35µA):

CGATE =

35µA • COUT

IINRUSH

CGATE =

35µA • 100µF

= 3.5nF

The 3.3nF CGATE capacitor in the application circuit of

Figure 7 limits the inrush current to just over 1A. RGATE

prevents CGATE from slowing down the reverse polarity

protection circuits. It also stabilizes the fast pull-down

circuits and prevents chatter during fault conditions. Set

RGATE to 22k for most applications.

Forward Overcurrent Fault

Forward overcurrent protection prevents large currents

from flowing from VIN to VOUT. This threshold current is

determined by the external sense resistor (RSENSE) and an

internal comparator (Figure 7, U1) with a 50mV threshold:

IOC,FWD =

50mV

RSENSE

For the example of Figure 7, if 2.5A flows to the output

across the 20mΩ sense resistor, the external MOSFETs

(M1, M2) are immediately (8µs) turned off. This discon-

nects the load from the input supply.

Note that during initial startup, the output capacitance

(COUT) charges from ground to VIN. To prevent this

capacitive inrush current (IINRUSH) from falsely trigger-

ing the forward overcurrent comparator, place an inrush

limiting capacitor (CGATE) on the GATE pin (see Limiting

Inrush Current During Turn On). This inrush current plus

the output current must be less than the desired forward

overcurrent threshold:

IOC,FWD > IINRUSH + IOUT

For the example of Figure 7, the 3.3nF GATE capacitor

and the 100µF output capacitor limit the inrush current

(IINRUSH) to approximately 1A. This means that the output

current (IOUT) must be less than 1.5A during turn on in

order to avoid a forward overcurrent fault during turn on.

Once VOUT has ramped to its final value, the output cur-

rent is limited to 2.5A.

Once a forward overcurrent fault is triggered, there are

two application choices for turning the external MOSFETs

back on:

1. Automatically restart by placing an external capaci-

tor on the RETRY pin. An internal cool-down timer

will charge/discharge this capacitor 31 times with

a 5.5ms/nF total delay. At the end of this delay, the

external MOSFETs are turned back on, thus reconnect-

ing the load to the input supply. The 0.22µF capaci-

tor (CRETRY) in the application of Figure 7 yields a

1200ms cool-down timer delay. Note that the adjust-

able cool-down period provides the user with a means

of keeping the external MOSFETs within the rated SOA

(safe operating area). See Figure 8 timing diagram.

Figure 7. Overcurrent Comparators Monitor 2.5A/–0.15A

Current Faults

VIN

24V

IOUT < 2.5A

IOUT

FDS3992

100V DUAL

COUT

100µF

RETRY

4368 F07

VOUT

LTC4368-2

SENSE

CRETRY

0.22µF

SHDN

GATE

2.5A

–0.15A

RSENSE

0.02

CGATE

3.3nF

INRUSH

CONTROL:

~1A

RGATE

22k

VIN

TURN MOSFETS

BACK ON

TURN MOSFETS

BACK ON AFTER

31 CYCLES

TURN OFF

MOSFETS

START

TIMER

OC FORWARD TIMER/LATCH

RESET FORWARD

OC LATCH

–

3mV

–

100mV

50mV

VIN

VOUT

31 CYCLES

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