LTC3255 datasheet(7/28 Pages) LINER

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LTC7820

7820fc

For more information www.linear.com/LTC7820

PIN FUNCTIONS

UV (Pin 8): Undervoltage Comparator Input. If the UV

pin voltage is lower than 0.9V, the PGOOD pin is pulled

down while the controller keeps switching. If the UV pin

voltage is higher than 1V and no faults exist, PGOOD pin

is released. Connect to INTVCC if not used.

ISENSE+ (Pin 27): Current Sense Comparator Positive

Input. Kelvin connected to the positive node of the cur-

rent sensing resistor. The current sensing resistor has to

be connected to the drain of the very top MOSFET. When

the voltage between ISENSE+ pin and ISENSE– pin is higher

than 50mV, the controller indicates an overcurrent fault

by pulling the FAULT pin down. The ISENSE+ pin is also

used to source 93mA current to the VLOW pin during the

capacitor’s pre-balancing time at power-up in voltage

divider applications. Connect directly to the drain of the

very top MOSFET if not used.

ISENSE– (Pin 28): Current Sense Comparator Negative

Input. Kelvin connected to the negative node of the current

sensing resistor. Short to ISENSE+ if not used.

RUN (Pin 6): Run Control Input. Forcing RUN below 1.14V

shutsdownthecontroller.WhenRUNishigherthan1.22V,

internal circuitry starts up. There is a 1µA pull-up current

flowing out of RUN pin when the RUN pin voltage is below

1.14V and additional 5µA current flowing out of RUN pin

when the Run pin voltage is above 1.22V.

TIMER (Pin 4): Charge Balance and Fault Timer Control

Input. A capacitor between this pin and ground sets the

amount of time to charge VLOW to VHIGH_SENSE/2 voltage

during power-up. It also sets the short-circuit retry time.

See the Application Information section for details.

FAULT (Pin 9): Open Drain Output Pin. FAULT is pulled to

ground when the VLOW_SENSE voltage is out of its window

thresholds or the voltage between ISENSE+ and ISENSE– is

higher than 50mV. FAULT pin is also pulled to ground

under INTVCC UVLO.

PGOOD (Pin 7): Open Drain Output Pin. PGOOD is pulled

to ground if there are any faults or if the UV pin indicates

an undervoltage condition.

HYS_PRGM (Pin 3): A resistor connected between this

pin and ground will program the two thresholds of the

window comparator that monitors the voltage difference

between VHIGH_SENSE/2 and VLOW_SENSE. There is a 10µA

current flowing out of this pin.

G4 (Pin 15): High Current Gate Drive for the Bottom

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

pin is from ground to INTVCC.

G3 (Pin 17): High Current Gate Drive for the Third Upper

Most N-Channel MOSFET. This is the output of the floating

driver with a voltage swing from BOOST3 to SW3.

G2 (Pin 21): High Current Gate Drive for the Second Upper

most N-Channel MOSFET. This is the output of the floating

driver with a voltage swing from BOOST2 to VLOW.

G1 (Pin 24): High Current Gate Drive for the Upper most

N-Channel MOSFET. This is the output of the floating driver

with a voltage swing from BOOST1 to SW1.

SW1/SW3 (Pin 25/Pin 16): Switch Node Connections.

BOOST1, BOOST2, BOOST3 (Pins 23, 22, 18): Boot-

strapped supplies to the floating drivers. Capacitors are

connected between these BOOST pins and their respective

SWn and VLOW pins.

EXTVCC (Pin 11): External Power Input to EXTVCC LDO.

This LDO supplies INTVCC power whenever EXTVCC is

higher than 6.5V and VCC is higher than 7V. Do not exceed

40V on this pin.

INTVCC(Pin12):OutputoftheInternalLinearLowDropout

Regulator.Thedriverandcontrolcircuitsarepoweredfrom

thisvoltagesource.Mustbebypassedtopowergroundwith

a minimum of 4.7µF ceramic or other low ESR capacitor.

VCC (Pin 14): Power Supply for Internal Circuitry and

INTVCC Linear Regulator. A bypass capacitor should be

tied between this pin and the power ground.

VHIGH_SENSE (Pin 1): Kelvin Sensing Input. Monitor the

voltage of the drain of the top MOSFET.

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