LTC7899 datasheet(11/50 Pages) AD

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Data Sheet

LTC7899

analog.com

Rev. 0

11 of 50

PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS

Figure 3. Pin Configuration

Table 3. Pin Descriptions

NAME

DESCRIPTION

PLLIN/

SPREAD

External Synchronization Input to Phase Detector and Spread Spectrum Enable. When

an external clock is applied to this pin, the phase-locked loop will force the rising BG

signal to be synchronized with the rising edge of the external clock. When not

synchronizing to an external clock, tie this input to INTVCC to enable spread spectrum

dithering of the oscillator or to VIN− to disable spread spectrum.

MODE

Mode Select Input. This input determines how the LTC7899 operates at light loads.

Connect MODE to VIN- to select the Burst Mode operation. An internal 100kΩ resistor to

VIN- also invokes Burst Mode operation when MODE is floating. Connect MODE to INTVCC

to force continuous inductor current operation. Tying MODE to INTVCC through a 100kΩ

resistor selects the pulse-skipping operation.

FREQ

Frequency Control Pin for the Internal Voltage Controlled Oscillator (VCO). Connect

FREQ to VIN- for a fixed frequency of 370kHz. Connect FREQ to INTVCC for a fixed

frequency of 2.25MHz. Program frequencies between 100kHz and 2.5MHz by using a

resistor between FREQ and VIN-. Minimize the capacitance on FREQ.

External Soft Start Input. SS regulates the VFBB voltage to the lesser of 1.2V or the

voltage on the SS pin. An internal 9µA pull-up current source is connected to SS. A

capacitor to VIN- at SS sets the ramp time to the final regulated output voltage. The

ramp time is equal to 1 ms for every 7.5nF of capacitance.

PHASMD

Control Input to Phase Selector. This determines the CLKOUT phase relationships with

respect to BG. Pulling this pin to VIN− forces CLKOUT to be out of phase 90° with respect

to BG. Connecting this pin to INTVCC forces CLKOUT to be out of phase 120° with

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