LT1913 datasheet(16/22 Pages) AD

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LTC4368

Rev. B

For more information www.analog.com

APPLICATIONS INFORMATION

Slow Shutdown

The SHDN input turns off the external MOSFETs in a

slow, controlled manner. When SHDN is asserted low, a

90µA current sink slowly begins to turn off the external

MOSFETs.

Once the voltage at the GATE pin falls below the voltage

at the VOUT pin, the current sink is throttled back and a

feedback loop takes over. This loop forces the GATE volt-

age to track VOUT, thus keeping the external MOSFETs off

as VOUT decays. Note that when VOUT < 2.5V, the GATE

pin is pulled all the way to ground.

Slow gate turn off reduces load current slew rates and

mitigates voltage spikes due to parasitic inductances. To

further decrease GATE pin slew rate, place a capacitor

(CHOTSWAP, see Figure 11) across the gate and source

terminals of the external MOSFETs. The waveforms of

Figure 15 were captured using the Si7942 Dual N-channel

MOSFETs, and a 2A load with 100µF output capacitor.

VIN

GREEN (TOP): MOSFET GATE

BLUE (BOTTOM): MOSFET SOURCE

100ns/DIV

10V/DIV

4368 F13

Figure 13. Hot Swapping VIN to +48V

SHDN

GATE

VOUT

100µF, 6Ω LOAD ON VOUT

DUAL Si7942 MOSFET

VIN = 12V

400µs/DIV

5V/DIV

4368 F15

GND

Figure 15. Slow Shutdown: GATE Tracks VOUT as VOUT Decays

4368 F14

GATE

MOSFET OFF

MOSFET ON

VIN

VIN(UVLO)

VUV

tD(ON)

Figure 14. Recovery Timing During Power-On

OV = GND, UV = SHDN = VIN

during the fast transients. To further improve positive

hot swap, place the optional CHOTSWAP = 6.8nF capacitor

across the gate/source of the external MOSFETs. For even

more hot swap protection, add a diode (MBR0540) across

RGATE (connect cathode to CGATE). Make sure this diode

has a reverse breakdown of at least 40V.

Recovery Delay Timer

The LTC4368 has a recovery delay timer that filters noise

at VIN and helps prevent chatter at VOUT. After either an OV

or UV fault has occurred, the input supply must return to

the desired operating voltage window for typically 32ms

(tD(ON)) in order to turn the external MOSFET back on, as

illustrated in Figure 5 and Figure 6. Going out of and then

back into fault in fewer than 32ms will keep the MOSFET

off continuously. Similarly, coming out of shutdown (SHDN

low to high) triggers an 800µs startup delay timer (tSTART,

see Figure 16).

The recovery delay timer is also active while the LTC4368

is powering up. The 32ms timer starts once VIN rises

above VIN(UVLO) and VIN lies within the user selectable

UV/OV power good window. See Figure 14.

GATE

VOUT

tD(SLOW)

GATE = VOUT

tSTART

tSHDN(F)

ΔVGATE

SHDN

4368 F16

FAULT

Figure 16. Slow Shutdown Timing

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