LTC7803 datasheet(9/36 Pages) AD

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LTC7803

Rev 0

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PIN FUNCTIONS

TRACK/SS (Pin 1/Pin 3): External Tracking and Soft-Start

Input. The LTC7803 regulates the VFB voltage to the lesser

of 0.8V or the voltage on the TRACK/SS pin. An internal

12.5µA pull-up current source is connected to this pin.

A capacitor to ground at this pin sets the ramp time to

the final regulated output voltage. The ramp time is equal

to 0.65ms for every 10nF of capacitance. Alternatively, a

resistor divider on another voltage supply connected to

the TRACK/SS pin allows the LTC7803 output to track the

other supply during start-up.

SENSE+ (Pin 2/Pin 4): The Positive (+) Input to the

Differential Current Comparators. The ITH pin voltage and

controlled offsets between the SENSE– and SENSE+ pins

in conjunction with RSENSE set the current trip threshold.

SENSE– (Pin 3/Pin 5): The Negative (–) Input to the

Differential Current Comparators. When SENSE– is

greater than INTVCC, the SENSE– pin supplies current to

the current comparator. When SENSE– is 3.2V or greater,

it supplies the majority of the sleep mode quiescent cur-

rent instead of VIN, further reducing the input-referred

quiescent current.

VFB (Pin 4/Pin 6): Error Amplifier Feedback Input. Connect

an external resistor divider between the output voltage

and the VFB pin to set the regulated output voltage.

ITH (Pin 5/Pin 7): Error Amplifier Output and Switching

Regulator Compensation Point. The current comparator

trip point increases with this control voltage. Place com-

pensation components between the ITH pin and GND.

RUN (Pin 6/Pin 8): Run Control Input. Forcing this pin

below 1.2V disables switching of the corresponding

controller. Forcing this pin below 0.7V shuts down the

LTC7803, reducing quiescent current to approximately

1.2µA. This pin can be tied to VIN for always-on operation.

FREQ (Pin 7/Pin 9): Frequency Control pin for the inter-

nal VCO. Connecting the pin to GND forces the VCO to

a fixed low frequency of 375kHz. Connecting the pin

to INTVCC forces the VCO to a fixed high frequency of

2.25MHz. Frequencies between 100kHz and 3MHz can

be programmed using a resistor between FREQ and GND.

Minimize the capacitance on this pin.

PLLIN/SPREAD (Pin 8/Pin 10): External Synchronization

Input and Spread Spectrum Selection. When an external

clock is applied to this pin, the phase-locked loop will

force the rising TG signal to be synchronized with the

rising edge of the external clock. When an external clock

is present, the regulators operate in pulse-skipping mode

if it is selected by the MODE pin, or in forced continuous

mode otherwise. When not synchronizing to an external

clock, tie this input to INTVCC to enable spread spectrum

dithering of the oscillator or to ground to disable spread

spectrum.

MODE (Pin 9/Pin 11): Mode Select Input. This input

determines how the LTC7803 operates at light loads.

Pulling this pin to ground selects Burst Mode operation.

An internal 100k resistor to ground also invokes Burst

Mode operation when the pin is floating. Tying this pin

to INTVCC forces continuous inductor current operation.

Tying this pin to INTVCC through a 100k resistor selects

pulse-skipping operation.

INTVCC (Pin 10/Pin 12): Output of the Internal 5.15V Low

Dropout Regulator (LDO). The driver and control circuits

are powered by this supply. Must be decoupled to GND

with a minimum of 4.7μF ceramic or tantalum capacitor.

EXTVCC (Pin 11/Pin 13): External Power Input to an

Internal LDO Connected to INTVCC. This LDO supplies

INTVCC power, bypassing the internal LDO powered from

VIN whenever EXTVCC is higher than 4.7V. See INTVCC

Regulators in the Applications Information section. Do not

exceed 30V on this pin. Tie this pin to GND if the EXTVCC

LDO is not used.

VIN (Pin 12/Pin 14): Main Bias Input Supply Pin. A bypass

capacitor should be tied between this pin and GND.

BG (Pin 13/Pin 15): High Current Gate Drive for Bottom

(Synchronous) N-Channel MOSFET. Voltage swing at this

pin is from GND to INTVCC.

BOOST(Pin 14/Pin 16): Bootstrapped Supply to the Top

Side Floating Driver. Connect a capacitor between the

BOOST and SW pin. Also connect a low leakage Schottky

diode between the BOOST and INTVCC pins.

(MSOP/QFN)

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