LT8705 datasheet(8/38 Pages) LINER

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LTC3784

3784fb

For more information www.linear.com/LTC3784

pin FuncTions

FREQ (Pin 1/Pin 4): Frequency Control Pin for the Internal

VCO. Connecting the pin to GND forces the VCO to a fixed

low frequency of 350kHz. Connecting the pin to INTVCC

forces the VCO to a fixed high frequency of 535kHz. The

frequency can be programmed from 50kHz to 900kHz

by connecting a resistor from the FREQ pin to GND. The

resistor and an internal 20μA source current create a volt-

age used by the internal oscillator to set the frequency.

Alternatively, this pin can be driven with a DC voltage to

vary the frequency of the internal oscillator.

PHASMD (Pin 2/Pin 5): This pin can be floated, tied to

SGND, or tied to INTVCCtoprogramthephaserelationship

between the rising edges of BG1 and BG2, as well as the

phase relationship between BG1 and CLKOUT.

CLKOUT (Pin 3/Pin 6): A Digital Output Used for Daisy-

chainingMultipleLTC3784ICsinMultiphaseSystems.The

PHASMDpinvoltagecontrolstherelationshipbetweenBG1

and CLKOUT. This pin swings between SGND and INTVCC.

PLLIN/MODE (Pin 4/Pin 7): External Synchronization

Input to Phase Detector and Forced Continuous Mode

Input. When an external clock is applied to this pin, the

phase-locked loop will force the rising edge of BG1 to be

synchronized with the rising edge of the external clock.

When an external clock is applied to this pin, the OVMODE

pin is used to determine how the LTC3784 operates at light

load. When not synchronizing to an external clock, this

input determines how the LTC3784 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 floated. Tying this pin

to INTVCC forces continuous inductor current operation.

Tying this pin to a voltage greater than 1.2V and less than

INTVCC – 1.3V selects pulse-skipping operation. This can

be done by adding a 100k resistor between the PLLIN/

MODE pin and INTVCC.

SGND (Pin 5/Pin 8): Signal Ground. All small-signal

components and compensation components should

connect to this ground, which in turn connects to PGND

at a single point.

RUN (Pin 6/Pin 9): Run Control Input. Forcing this pin

below 1.28V shuts down the controller. Forcing this pin

below 0.7V shuts down the entire LTC3784, reducing

quiescent current to approximately 4µA. An external

resistor divider connected to VIN can set the threshold

for converter operation. Once running, a 4.5µA current is

sourced from the RUN pin allowing the user to program

hysteresis using the resistor values.

SS (Pin 7/Pin 10): Output Soft-Start Input. A capacitor to

ground at this pin sets the ramp rate of the output voltage

during start-up.

SENSE2– , SENSE1– (Pin 8, Pin 28/Pin 11, Pin 3): Nega-

tive Current Sense Comparator Input. The (–) input to the

current comparator is normally connected to the negative

terminal of a current sense resistor connected in series

with the inductor.

SENSE2+, SENSE1+ (Pin 9, Pin 27/Pin 12, Pin 2): Posi-

tive Current Sense Comparator Input. The (+) input to the

current comparator is normally connected to the positive

terminalofacurrentsenseresistor.Thecurrentsenseresis-

tor is normally placed at the input of the boost controller in

serieswiththeinductor.Thispinalsosuppliespowertothe

current comparator. The common mode voltage range on

SENSE+ and SENSE– pins is 2.3V to 60V (65V abs max).

VFB (Pin 10/Pin 13): Error Amplifier Feedback Input. This

pin receives the remotely sensed feedback voltage from

an external resistive divider connected across the output.

ITH (Pin 11/Pin 14): Current Control Threshold and Error

AmplifierCompensationPoint.Thevoltageonthispinsets

the current trip threshold.

(QFN/SSOP)

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