LTC7813 datasheet(9/36 Pages) LINER

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LTC7821

Rev A

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PIN FUNCTIONS

MODE/PLLIN(Pin1):ModeSelectionorExternalSynchro-

nization Input to Phase Detector. When external synchro-

nization is not used, this pin selects the operating modes

and can be tied to SGND, to INTVCC or left floating. If the

pin is connected to SGND, it enables forced continuous

modewhileaconnectiontoINTVCCenablespulse-skipping

mode. Floating the pin enables Burst Mode operation.

For external sync, apply a clock signal to this pin. The

integrated PLL along with its internal compensation net-

work will synchronize the internal oscillator to this clock.

Forced continuous mode will be enabled.

CLKOUT (Pin 2): Clock Output Pin. This pin outputs a

clock 180° out of phase with the main operating clock of

the LTC7821.

RUN (Pin 3): Run Control Input. A voltage above 1.3V

turns the controller ON. There is a 1μA pull-up current

on this pin when its voltage is below 1.3V.

FAULT (Pin 4): Open Drain Output pin. When the signal

goes low, it indicates one of the following conditions:

(a) In the capacitor balancing phase, capacitors CFLY or

CMID (see Typical Application) are not charged to

VIN/2. A low FAULT indicates an abnormal condition

that is preventing CFLY or CMID from being be charged

up to VIN/2.

(b) During normal operation, the voltage deviates from

VIN/2 by a window amount set by the voltage on the

HYS_PRGM pin.

thePTCresistorconnectedasthelowerlegofaresistor

divider trips the TEMP pin threshold.

During any of these condition, the TRACK/SS pin will also

be pulled low.

PGOOD (Pin 5): Power Good Pin. This is an open drain

output. PGOOD is pulled to ground when the voltage of

the VFB pin is not within ±7.5% of its set point after an

internal 50μs mask timer expires. It will also be pulled low

when FAULT is tripped.

TIMER (Pin 6): Charge Balancing Timer Input. A capaci-

tor connected from this pin to ground sets the amount

of time allocated to charge CFLY and CMID to VIN/2 during

the capacitor balancing phase. It also sets the auto-retry

timeout, should the capacitors fail to reach this voltage

within the set time. Capacitors CFLY and CMID begin and

end charging when the TIMER voltage is between 0.5V

and 1.2V, respectively. If the capacitor is balanced before

the TIMER voltage reaches 1.2V, this voltage is reset to

ground and normal operation begins. However, if the bal-

ance is not reached when the voltage reaches 1.2V, then

the charging of the capacitors stops and the auto-retry

timeout period begins. The TIMER capacitor will now slew

at half the rate until it reaches 4V and then resets to zero

and begins to slew at 1x rate. Once it reaches 0.5V, the CFLY

and CMID begin to charge again and the process repeats.

TRACK/SS (Pin 7): Output Voltage Tracking and Soft-Start

Input. The LTC7821 regulates the VFBvoltagetothelowest

of three voltages: 0.8V, the voltage on the EXT_REF pin or

the voltage on the TRACK/SS pin. An internal 10μA pull-

up current source is connected to this pin. A capacitor to

ground at this pin sets the ramp time to the final regulated

outputvoltage.Alternatively,aresistordividerfromanother

voltage supply connected to this pin allows the LTC7821

output voltage to track the other supply during start-up.

EXT_REF (Pin 8): External Reference Input. A voltage ap-

plied to this pin forces the VFB to regulate to this voltage.

Internal clamps set at 0.4V and 0.93V limit the lower and

upper bounds of VFB regulation. Connecting this pin to

INTVCC will cause the internal reference to be used for

output voltage regulation.

HYS_PRGM (Pin 9): There is a 10μA current flowing out

of this pin. A voltage created by connecting a resistor

from this pin to ground sets an equal amount of window

threshold around VIN/2toawindowcomparator.Whenthe

voltage at MIDSENSE is not within this window threshold,

FAULT will be pulled low and switching will stop. CFLY and

CMID will be rebalanced to half of VIN before resuming

normal operation.

ITH(Pin10):CurrentControlThresholdandErrorAmplifier

Compensation Point. The current comparator threshold

increases with its ITH control voltage.

FREQ (Pin 11): Frequency Set Pin. There is a 10μA current

flowing out of this pin. A resistor to ground sets a voltage

which in turn programs the frequency.

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