LTC7897 datasheet(11/53 Pages) AD

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Data Sheet

LTC7897

analog.com

Rev. A

11 of 53

PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS

Figure 3. Pin Configuration

Table 3. Pin Descriptions

NAME

DESCRIPTION

PLLIN/

SPREAD

External Synchronization Input to Phase Detector/Spread Spectrum Enable. When an

external clock is applied to PLLIN/SPREAD, the phase-locked loop forces the rising

TGxx signal to synchronize 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 GND to disable spread spectrum dithering.

MODE

Mode Select Input. This input determines how the LTC7897 operates at light loads.

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

to GND 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 GND 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 GND. Minimize the capacitance on FREQ.

TRACK/SS

External Tracking/Soft-Start Input. TRACK/SS regulates the VFB voltage to the lesser of

0.8V or the voltage on the TRACK/SS pin. An internal 9µA pull-up current source is

connected to TRACK/SS. A capacitor to GND at TRACK/SS sets the ramp time to the

final regulated output voltage. The ramp time is equal to 1 ms for every 11.25nF of

capacitance. Alternatively, a resistor divider on another voltage supply connected to

TRACK/SS allows the output to track the other supply during start-up.

TEST

Test Pin. This pin must be soldered to PCB GND.

VPRG

Output Voltage Control Pin. VPRG sets the adjustable output mode using external

feedback resistors or the fixed 12V or 5V output mode. Floating VPRG programs the

FREQ

MODE

PLLIN/SPREAD

TEST 5

VPRG

SENSE+

VFB

ITH

ILIM

PGOOD

TGUP

TRACK/SS

GND

LTC7897

TOP VIEW

(Not to Scale)

VIN

DRVCC

TGDN

NOTES

1. EXPOSED PAD (PIN 29). THE EXPOSED PAD MUST

BE SOLDERED TO PCB GROUND FOR RATED

ELECTRICAL AND THERMAL PERFORMANCE.

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