LTC4449 datasheet(15/48 Pages) AD

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LTC7872

Rev. 0

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d. During a startup sequence until the SS pin charges up

past 1.2V.

e. When any channel is in overcurrent fault status.

f. When the IC is over temperature.

The OVLOW and OVHIGH thresholds are set using an exter-

nal resistor divider off VLOW and VHIGH, respectively.

When the voltage at the pin exceeds the comparator

threshold of 1.2V, a 5μA hysteresis current is sourced

out of the respective pin and the FAULT signal goes low

after a 120μs delay. The UVHIGH threshold is also set using

an external resistor divider off VHIGH. When the voltage

at the pin falls below the comparator threshold of 1.2V, a

5μA hysteresis current is sunk into the UVHIGH pin and the

FAULT signal goes low after a 120μs delay. The amount of

hysteresis can be adjusted by changing the total imped-

ance of the resistor divider, while the resistor ratio sets

the UV/OV trip point.

Besides flagging the FAULT pin, the UV/OV compara-

tors also affect the operation of the controller, as shown

in Table 1. When the OVLOW comparator crosses its

1.2V threshold:

a. In buck mode, the controller stops switching.

b. In boost mode, the controller continues to switch.

c. ITH and SS are unaffected in both buck and boost

modes. Whenever a fault is detected, discharge the

SS pin as needed externally.

When the OVHIGH comparator crosses its 1st threshold

of 1.2V:

a. The controller stops switching in both buck and

boost modes.

b. ITH and SS are unaffected in both buck and boost

modes. Whenever a fault is detected, discharge the

SS pin as needed externally.

When the OVHIGH comparator crosses its 2nd threshold

of 2.4V:

a. The controller stops switching in both buck and

boost modes.

A phase-locked loop (PLL) is available on the LTC7872

to synchronize the internal oscillator to an external clock

source that is connected to the SYNC pin. The PLL loop

filter network is integrated inside the LTC7872. The phase

locked loop is capable of locking to any frequency within

the range of 60kHz to 750kHz. The frequency setting

resistor should always be present to set the controller’s

initial switching frequency before locking to the external

clock. The controller operates in the user selected mode

when it is synchronized.

Undervoltage Lockout

The LTC7872 has two functions that help protect the con-

troller in case of undervoltage conditions. Two precision

UVLO comparators constantly monitor the V5 and DRVCC

voltages to ensure that adequate voltages are present. The

switching action is stopped when V5 or DRVCC is below

the undervoltage lockout threshold. To prevent oscillation

when there is a disturbance on the V5 or DRVCC, the UVLO

comparators have precision hysteresis.

Another way to detect an undervoltage condition is to

monitor the VHIGH supply. Because the RUN pin has a

precision turn-on reference of 1.22V, one can use a resis-

tor divider to VHIGH to turn on the IC when VHIGH is high

enough. An extra 4μA of current flows out of the RUN pin

once the RUN pin voltage passes 1.22V. The RUN com-

parator itself has about 80mV of hysteresis. Additional

hysteresis for the RUN comparator can be programmed

by adjusting the values of the resistive divider. For accu-

rate VHIGH undervoltage detection, VHIGH needs to be

higher than 5V.

Fault Flag (FAULT, OVHIGH, OVLOW and UVHIGH Pins)

The FAULT pin is connected to the open-drain of an inter-

nal N-channel MOSFET. It can be pulled high with an exter-

nal resistor connected to a voltage up to 6V, such as V5

or an external bias voltage. The FAULT pin is pulled low

when at least one of the following conditions is met:

a. The RUN pin is below its turn on threshold.

b. When V5 or DRVCC is below its UVLO threshold.

c. Any of the three OV/UV comparators has been tripped.

OPERATION

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