LTC4413 datasheet(22/32 Pages) AD

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LTC4421

Rev. 0

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APPLICATIONS INFORMATION

Minimizing Bias Current Draw from Lower Priority

Supplies

In order to minimize current draw from lower priority

power sources, the LTC4421 draws the vast majority of

its supply current from the highest priority available sup-

ply. When SHDN is high and EXTVCC is connected to the

system output voltage, the LTC4421 consumes 600µA

from the supply powering the output and only 10µA to

26µA from the other supplies. When SHDN is low, the 6µA

(typical) is drawn from the highest of the V1, V2, EXTVCC

and OUT1 voltages, and the supply current in each of the

other, lower voltage pins is a miniscule 250nA.

Digital Status Outputs VALID1, VALID2, CH1, CH2

The LTC4421 provides open-drain pull-down digital out-

puts to provide system status information. The VALID1

and VALID2 pins pull low when their respective V1 and V2

input supplies have been validated. The CH1 and CH2 pins

pull low when their input supply is connected to the output

voltage. Connect large value pull-up resistors between

these pins and INTVCC to provide the logic high, taking

care not to exceed the 500µA maximum current draw

from INTVCC. The pull-downs are capable of driving low

power LED’s, but they cannot be pulled up to INTVCC in

that case due to the LED current required. When using

LED’s, power the pull-up from a supply voltage up to 36V.

These outputs can be used in conjunction with the DISABLE

pins in a variety of application circuits to change V1, V2 pri-

ority over time. For example, consider what happens when

CH2 is connected to DISABLE1. Once V2 is connected to

the output, it will continue to power the output regardless of

V1’s validity. In effect, V2 became the higher priority supply,

but only after it connected to the output. This configuration

can be used in systems where, after switching to the sec-

ondary supply, it is desirable to run the secondary supply

to full discharge before re-connecting to the primary. For

proper operation at power up, it is essential that Channel 1

becomes valid before Channel 2.

In Figure 10, logic gates U1 and U2 disable channel 1

whenever V2 is valid, enabled and does not have a latched

overcurrent fault. Disabling channel 1 disconnects V1

from the output and allows V2 to connect to the output.

This configuration permanently flips V1 and V2 priority,

so that V2 is always the higher priority input.

In Figure 11, inverter U1 is used to connect the logically

inverted VALID2 signal to DISABLE1. This configuration

prevents Channel 1 from connecting to the output when-

ever Channel 2 is invalid. This prevents the primary input

from powering the output unless a valid secondary supply

is available to power the output when the primary fails.

Input and Output Short Circuits and Supply Transient

Protection

When an input supply powering the output is shorted

to ground, the LTC4421 senses reverse current through

the sense resistor. When the reverse voltage developed

DIS2

VALID2

FAULT2

DISABLE1

DISABLE2

RFLT2

100k

RVLD2

100k

LTC4421

LTC4421 F10

RVLD2

100k

DISABLE1

VALID2

LTC4421

LTC4421 F11

Figure 10. Flip Priority. Two External Logic Gates Are Used to

Flip Priorities of Channel 1 and Channel 2

Figure 11. Valid Secondary Required. Preventing the Primary

Input Supply from Powering the Output Unless a Valid Secondary

Supply Is Present

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