LTC2913 datasheet(14/22 Pages) AD

14 / 22 page

LTC4368

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

2. Latch off the MOSFETs by grounding the RETRY pin

(no external RETRY capacitor needed). This latches

the forward overcurrent fault. The external MOSFETs

are kept in the off condition until the SHDN input pin is

toggled low then high (tCLEAR pulse width < tLOWPWR).

See Figure 9 timing diagram.

Reverse Overcurrent Protection

Reverse overcurrent protection prevents large currents

from flowing from VOUT to VIN. There are two options for

reverse overcurrent protection thresholds. The LTC4368-1

(–50mV) bidirectional circuit breaker allows load current

to flow in either direction: from VIN to VOUT or from VOUT

to VIN. The LTC4368-2 provides diode-like behavior by

making the reverse overcurrent threshold (–3mV) sig-

nificantly smaller than the forward overcurrent thresh-

old (+50mV). The reverse overcurrent fault threshold is

determined by the external sense resistor (RSENSE) and

an internal comparator (Figure 7, U2). For the LTC4368-2

application of Figure 7:

IOC,REV =

–3mV

RSENSE

–3mV

20mΩ

= –0.15A

If –0.15A flows from the output across the 20mΩ sense

resistor, the external MOSFETs (M1,M2) are immediately

(8µs) turned off.

To turn the MOSFETs back on, an internal comparator

(Figure 7, U3) detects when VOUT drops 100mV below

VIN:

VOUT < VIN – 100mV

Once this condition is met, the gates of the external

MOSFETs are turned on again to reconnect the input sup-

ply to the load. See timing diagrams of Figure 10. Note

that if the LTC4368-1 option is used, the reverse current

threshold becomes:

IOC,REV =

–50mV

RSENSE

SENSE–VOUT

RETRY

tp(GATE)

FAULT

tRETRY

31 CYCLES

(1200ms COOL-DOWN PERIOD)

GATE

50mV

EXTERNAL N-CHANNEL

MOSFETs TURN OFF

EXTERNAL N-CHANNEL

MOSFETs TURN BACK ON

0mV

IINRUSH

LIMITED

STEADY STATE LOAD

4368 F08

Figure 8. Forward Overcurrent Fault with 0.22µF RETRY Capacitor

4368 F08

SENSE–VOUT

SHDN

FAULT

tCLEAR < t < tLOWPWR

GATE

50mV

EXTERNAL N-CHANNEL

MOSFETs TURN OFF

EXTERNAL N-CHANNEL

MOSFETs TURN BACK ON

0mV

IINRUSH

LIMITED

STEADY STATE LOAD

tp(GATE)

Figure 9. Forward Overcurrent Fault with RETRY Pin Grounded

4368 F10

SENSE–VOUT

VOUT

tp(GATE)

FAULT

GATE –VOUT

–3mV

EXTERNAL N-CHANNEL

MOSFETs TURN BACK ON

0mV

IINRUSH

LIMITED

STEADY STATE LOAD

FAULT

VIN – 100mV

11V

Figure 10. Reverse Overcurrent Fault: SENSE – VOUT < –3mV

(LTC4368-2)

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