LTC4449 datasheet(17/48 Pages) AD

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LTC7872

Rev. 0

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immediately when the regulated VFBLOW/VFBHIGH volt-

age is within ±10% of the reference window. However,

there is an internal 40µs power bad mask when regulated

VFBLOW/VFBHIGH voltage goes out of the ±10% window.

The PGOOD pin is allowed to be pulled up by an external

resistor to sources of up to 6V.

Programmable VHIGH, VLOW Margining

As shown in the Figure 1, the LTC7872 has a SPI con-

trolled 7-bit D/A converter current source. Through the

SPI interface, the LTC7872 receives a 7-bit DAC code and

converts this value to a bidirectional analog output cur-

rent. The current is connected to the VFBLOW pin in buck

mode or the VFBHIGH pin in boost mode. By connecting

the DAC current to the feedback node of a voltage regula-

tor, in buck mode, VLOW voltage is programmed with the

equation:

VLOW = 1.2V • (1 + RB/RA) – IDAC • RB

In boost mode, VHIGH voltage is programmed with

the equation:

VHIGH = 1.2V • (1 + RD/RC) – IDAC • RD

There are two different registers for VLOW and VHIGH pro-

gramming, MFR_IDAC_VLOW and MFR_IDAC_VHIGH.

The current DAC selects the register value based on the

buck or boost mode. The current DAC’s LSB is 1µA. The

MSB determines the current direction. When MSB is 0,

IDAC is sourcing current (reducing VLOW or VHIGH), which

is positive current flowing out of the feedback pin. When

MSB is 1, IDAC is sinking current (increasing VLOW or

VHIGH), which is negative current flowing into the feed-

back pin.

Figure 1.

CURRENT DAC

7872 F01

IDAC

BUCK MODE

BOOST MODE

VFBLOW

VLOW

VFBHIGH

VHIGH

IDAC

Current DAC for VLOW/VHIGH Programming

To defeat the average current programming operation,

tie the SETCUR pin to V5 or voltage higher than 1.25V.

Buck and Boost Modes (BUCK Pin)

The LTC7872 can be dynamically and seamlessly switched

from buck mode to boost mode and vice versa via the

BUCK pin. Tie this pin to V5 to select buck mode and

to ground to select boost mode operation. This pin has

an internal pull up resistor that defaults to buck mode if

left floating. There are two separate error amplifiers for

VHIGH or VLOW regulation. Having two error amplifiers

allows fine tuning of the loop compensation for the buck

and boost modes independently to optimize transient

response. When buck mode is selected, the correspond-

ing error amplifier is enabled, and ITHLOW voltage con-

trols the peak inductor current. The other error amplifier

is disabled and ITHHIGH is parked at its zero current level.

In boost mode, ITHHIGH is enabled while ITHLOW is parked

at its zero current level. During a buck to boost or a boost

to buck transition, the internal soft-start is reset. Resetting

soft-start and parking the ITH pin at the zero current level

ensures a smooth transition to the newly selected mode.

Refer to Table 2 for a summary.

To further minimize any transients, SETCUR can be pro-

grammed to zero current level before switching between

boost and buck modes.

Table 2. ITH PIN Parking Conditions

Mode When Parked Comments

ITHHIGH

Buck

Normal

Operation

OVHIGH 2.4V Threshold Overrides Park

Boost

Prebiased

Turn-on

OVHIGH 2.4V Threshold Overrides Park

ITHLOW

Buck

Prebiased

Turn-on

OVHIGH 2.4V Threshold and OVLOW

Override Park

Boost

Normal

Operation

OVHIGH 2.4V Threshold Overrides Park

Power Good (PGOOD Pin)

When the regulated VFBLOW/VFBHIGH voltage is not within

±10% of the 1.2V reference voltage, the PGOOD pin is

pulled low. The PGOOD pin is also pulled low when the

RUN pin is below 1.2V or when the LTC7872 is in the

soft-start or UVLO. The PGOOD pin will flag power good

OPERATION

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