LTC4449 datasheet(14/48 Pages) AD

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LTC7872

Rev. 0

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pin to be used to program a soft-start by connecting an

external capacitor from the SS pin to SGND. An internal

1μA pull-up current charges this capacitor, creating a volt-

age ramp on the SS pin. As the SS voltage rises linearly

from 0V to 1.2V (and beyond), the VLOW voltage rises

smoothly from zero to its final value. When the RUN pin

is pulled low to disable the controller, or when V5 drops

below its undervoltage lockout threshold, the SS pin is

pulled low by an internal MOSFET. When in undervolt-

age lockout, the controller is disabled and the external

MOSFETs are held off. External circuitry can be added to

discharge the soft-start capacitor during fault conditions

to ensure a soft-start when the faults are cleared.

Frequency Selection, Spread Spectrum, and Phase-

Locked Loop (FREQ and SYNC Pins)

The selection of switching frequency is a trade-off between

efficiency and component size. Low frequency opera-

tion increases efficiency by reducing MOSFET switching

losses, but requires larger inductance and/or capacitance

to maintain low output ripple voltage.

If the SYNC pin is tied to SGND, the FREQ pin can be

used to program the controller’s operating frequency

from 67kHz to 725kHz. There is a precision 20μA current

flowing out of the FREQ pin so that the user can program

the controller’s switching frequency with a single resis-

tor to SGND. A curve is provided later in the Applications

Information section showing the relationship between

the voltage on the FREQ pin and switching frequency

(Figure 7).

Switching regulators can be particularly troublesome for

applications when electromagnetic interface (EMI) is a

concern. To improve EMI, the LTC7872 can operate in

spread spectrum mode, which is enabled by tying the

SYNC pin to V5. This feature varies the switching fre-

quency at low frequency rate (switching frequency/512,

by default) with a triangular frequency modulation of

±12%. For example, if the LTC7872’s frequency is pro-

grammed to switch at 200kHz, enabling spread spectrum

will modulate the frequency between 176kHz and 224kHz

at a 0.4kHz rate. These spread spectrum parameters are

programmed by the MFR_SSFM register.

Soft-Start (Buck Mode)

By default, the start-up of the VLOW voltage is normally

controlled by an internal soft-start ramp. The internal soft-

start ramp represents a noninverting input to the error

amplifier. The VFBLOW pin is regulated to the lowest of the

error amplifier’s three noninverting inputs (the internal

soft-start ramp, the SS pin or the internal 1.2V reference).

As the ramp voltage rises from 0V to 1.2V over approxi-

mately 1ms, the VLOW voltage rises smoothly from its

prebiased value to its final set value. Certain applications

can require the start-up of the converter into a non-zero

load voltage, where residual charge is stored on the VLOW

capacitor at the onset of converter switching. In order to

prevent the VLOW from discharging under these condi-

tions, the top and bottom MOSFETs are disabled until

soft-start is greater than VFBLOW.

Soft-Start (Boost Mode)

The same internal soft-start capacitor and external soft-

start capacitor are also active if the controller starts with

boost mode of operation. The error amplifier for boost

mode also tries to regulate to the lowest reference during

start-up. However, the topology of the boost converter

limits the effectiveness of this soft-start mechanism until

the boost output voltage reaches its input voltage level.

Therefore, it is recommended that the controller starts in

buck mode of operation.

Shutdown and Start-Up (RUN and SS Pins)

The LTC7872 can be shut down using the RUN pin.

Pulling the RUN pin below 1.22V shuts down the main

control loop for the controller and most internal circuits,

including the DRVCC and V5 regulators. Releasing the

RUN pin allows an internal 2μA current to pull up the

pin and enable the controller. Alternatively, the RUN pin

may be externally pulled up or driven directly by logic. Be

careful not to exceed the absolute maximum rating of 6V

on this pin. The start-up of the controller’s VLOW voltage

is controlled by the voltage on the SS pin. When the volt-

age on the SS pin is less than the 1.2V internal reference,

the LTC7872 regulates the VFBLOW voltage to the SS pin

voltage instead of the 1.2V reference. This allows the SS

OPERATION

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