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LTC1421/LTC1421-2.5

APPLICATIONS INFORMATION

When CON1 and CON2 are both forced to ground for more

than 20ms, the LTC1421 assumes that the board is fully

connected to the host and power-up can begin. When

VCCLO and VCCHI exceed the 2.45V undervoltage lockout

threshold, the 20

µA current reference is connected from

RAMP to GND, the charge pumps are turned on and CPON

is forced high (time point 4). VOUTHI and VOUTLO begin to

ramp up. When VOUTLO exceeds the reset threshold volt-

age, PWRGD will immediately be forced high (time point

5). After a 200ms delay, RESET will be pulled high and

DISABLE will be pulled low, enabling the data bus (time

point 6).

Ground Sense Comparator

When POR is pulled low for more than 20ms, GATELO and

GATEHI are pulled to ground and VOUTLO and VOUTHI will

be discharged. If POR is pulled back high while VOUTLO

and VOUTHI are still ramping down, the discharge will

continue. When they drop below the VTRIP point, a power-

up sequence will begin automatically. The trip point poten-

tial for LTC1421 is set at 0.1V and 2.5V for LTC1421-2.5.

In applications, where either VOUTLO or VOUTHI might be

forced above 100mV before power-up, the LTC1421-2.5

should be used. This could occur when leakage through

the body diode of the logic chips keeps VOUTLO high or in

the case where logic lines are precharged.

In other applications, where outputs need to drop to near

ground potential before ramping up again to ensure proper

initial state for the logic chips, the LTC1421 should be

used.

Power-On Reset Timing

The POR input is used to completely cycle the power

supplies on the board or to reset the electronic circuit

breaker feature. The POR pin can be connected to a

grounded push button, toggle switch or a logic signal

from the host. When POR is pulled low for more than

20ms, a power-on reset sequence begins (Figure 15,

Figure 15. Power-On Reset Timing

VCCHI

VCCLO

DISABLE

FAULT

POR

CON1

CON2

VOUTLO

VOUTHI

CPON

VTH2

1421 F15

VTH1

20ms

200ms

µs

GATEHI

GATELO

RESET

PWRGD

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