LTC4449 datasheet(16/48 Pages) AD

16 / 48 page

LTC7872

Rev. 0

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where:

VZERO is the IMON voltage when average output current

is zero; VZERO = 1.25V typically

K = 40 if the ILIM voltage is 0V or 1/4 VV5

K = 20 if the ILIM voltage is float, 3/4 VV5 or VV5

IL(ALL) is the total average inductor current including

all four channels

RSENSE is the current sensing resistor value.

An external voltage can be applied to the SETCUR pin

to regulate the maximum average inductor current. The

SETCUR pin voltage should be set as:

VSETCUR =

K •IL MAX

(

) •RSENSE

where:

IL(MAX) is the maximum total average inductor current

including all four channels

The SETCURP and SETCURN are internally generated

voltages based on the SETCUR pin:

SETCURP = 1.25V + VSETCUR

SETCURN = |1.25V – VSETCUR|

SETCURP, SETCURN, and IMON are the three inputs to

the current regulation loop error amplifier with SETCURP

and SETCURN acting as the reference. When the IMON pin

voltage approaches SETCURP or SETCURN, the ITH pin

control is taken over by the current loop error amplifier

from the voltage loop error amplifier.

In either buck or boost mode, both the maximum positive

average current and the maximum negative average cur-

rent are regulated. There is a 16µA current flowing out of

the SETCUR pin so that a single resistor to SGND can set

both the positive average current loop and negative aver-

age current loop. The sourcing current from the SETCUR

pin is programmable through the SPI interface. For bat-

tery charging applications, SETCUR can be programmed

dynamically on-the-fly to set the charging currents to the

batteries in either buck or boost mode. SETCUR can be

used at start-up to limit the in-rush current in both buck

mode and boost mode.

b. Both ITH and IMON pins are driven into high imped-

ance. This feature allows the users to isolate one

LTC7872 from a multiphase system in case a fault is

detected on one particular IC.

c. The SS pin is unaffected.

When the UVHIGH comparator crosses its 1.2V threshold:

a. In buck mode, the controller stops switching after a

120μs delay, and the SS pin pulls to SGND.

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

SS pin is unaffected.

c. ITH is unaffected in both buck and boost modes.

Table 1. OV/UV Faults

FAULT

MODE SWITCHING ITH PINS

IMON

OVLOW 1.2V

Threshold

Buck

Stops

No Effect No Effect

No Effect

Boost

Continues

No Effect No Effect

No Effect

OVHIGH 1.2V

Threshold

Buck

Stops

No Effect No Effect

No Effect

Boost

Stops

No Effect No Effect

No Effect

OVHIGH 2.4V

Threshold

Buck

Stops

Hi-Z

No Effect

Boost

Stops

Hi-Z

No Effect

UVHIGH 1.2V

Threshold

Buck

Stops

No Effect No Effect Pulls to SGND

Boost

Continues

No Effect No Effect

No Effect

Current Monitoring and Regulation

(IMON, SETCUR Pins)

The inductor current can be sensed using either its DCR or

a RSENSE resistor. The current monitoring pin, IMON, out-

puts a voltage that is proportional to the average induc-

tor current of the four channels sensed by the LTC7872.

The operational range of IMON is 0.4V to 2.5V. When the

average inductor current is zero, the IMON pin voltage

rests at 1.25V. As the inductor current increases in buck

mode, the IMON voltage proportionally increases; As the

inductor current increases in boost mode, the IMON volt-

age proportionally decreases. Use the following equation

to calculate the voltages on IMON:

VIMON = VZERO +

K •IL ALL

(

) •RSENSE

; Buck Mode

VIMON = VZERO –

K •IL ALL

(

) •RSENSE

; Boost Mode

OPERATION

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