LTC7813 datasheet(13/36 Pages) LINER

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LTC7821

Rev A

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OPERATION

current at which ICMP resets the RS latch is controlled by

the voltage on the ITH pin, which is the output of the error

amplifier EA. The VFB pin receives the voltage feedback

signal, which is compared to the internal reference voltage

by the EA. When the load current increases, it causes a

slight decrease in VFB relative to the 0.8V reference, which

in turn causes the ITH voltage to increase until the aver-

age inductor current matches the new load current. After

MOSFETs M1 and M3 have turned OFF, MOSFETs M2 and

M4 are turned ON until either the inductor current starts

to reverse, as indicated by the reverse current comparator

IREV, or the beginning of the next cycle.

During the switching of M1/M3 and M2/M4, capacitor

CFLY is alternately connected in series with or parallel to

CMID. The voltage at MID will be approximately at VIN/2.

INTVCC/EXTVCC Power

Power for the bootstrap drivers and bottom MOSFET and

mostinternalcircuitryisderivedfromtheINTVCCpin.When

the EXTVCC pin is grounded or tied to a voltage less than

7V,aninternal5.8VlinearregulatorsuppliesINTVCCpower

from VIN. If EXTVCC is taken above 7V, this linear regula-

tor is turned OFF and another 5.8V linear regulator turns

ON to provide the INTVCC power from EXTVCC. Using the

EXTVCC pin allows the INTVCC power to be derived from

a high efficiency external source, resulting in an overall

increase in system efficiency.

Bootstrap Capacitor Refresh

Each of the three uppermost MOSFET drivers is biased

from its respective floating bootstrap capacitor, CB1 to

CB3, which are refreshed during switching through a

charge pump configuration consisting of diodes D1 to

D3 and the external MOSFETs.

During the charge balancing phase or light load condition

when switching may stop for extended amount of time,

the voltage across the bootstrap capacitor may decrease

sufficiently that the gate drive voltage is not optimal. Un-

dervoltage detectors monitor the voltage across each of

the bootstrap capacitors. When any of them goes below

3V, an internal current source of 1mA will be turned ON

to charge that bootstrap capacitor through the upper plate

of the capacitor. A 1mA sinking source that is connected

to the bottom plate of the bootstrap capacitor will also be

turned ON to sink away this current. This ensures a net

zero residual current at the bottom plate capacitor node,

hence avoiding any impact on the bias condition of that

node. When CB1 and CB2/CB3 reach 4.3V and 4.47V

respectively, the refreshing stops. When CB2 and CB3

need to be refreshed, all switching stops.

Shutdown and Start-Up (RUN and TRACK/SS Pins)

WhentheRUNpinisbelow1.3V,theINTVCClinearregulator

along with all the internal circuitry that is powered from

this supply, is disabled. The main control loop will also

be disabled. Releasing the RUN pin will allow the internal

1μA current source to pull this pin up, thus enabling the

part. The RUN pin can also be driven directly by logic but

ensure that this voltage does not exceed the Absolute

Maximum Rating of 6V.

The slew rate of the output voltage VOUT can be controlled

by the voltage on the TRACK/SS pin. When the voltage

on the TRACK/SS is less than the internal reference of

0.8V (or EXT_REF if this feature is invoked), the LTC7821

regulates the VFB voltage to the TRACK/SS voltage instead

of to the reference. This allows the TRACK/SS pin to be

used to program the soft-start period by connecting an

external capacitor from the TRACK/SS pin to SGND. An

internal 10μA pull-up current charges this capacitor, creat-

ing a voltage ramp on the TRACK/SS pin. As the TRACK/

SS voltage rises linearly from 0V to the reference voltage

(and beyond), the output voltage VOUT rises smoothly

from zero to the final value. Alternatively, the TRACK/SS

pin can be used to cause the start-up of VOUT to track that

of another supply. Typically this requires connecting to the

TRACK/SS pin an external resistor divider from the other

supply to ground (see Application Information section).

Light Load Current Operation (Burst Mode Operation,

Pulse-Skipping Mode, or Continuous Conduction)

The LTC7821 can be enabled to enter high efficiency Burst

Modeoperation,constant-frequencypulse-skippingmode,

or forced continuous conduction mode. To select forced

continuous operation, tie the MODE/PLLIN pin to a DC

voltage below 0.6V (e.g., SGND). To select pulse-skipping

mode of operation, tie the MODE/PLLIN pin to INTVCC. To

select Burst Mode operation, float the MODE/PLLIN pin.

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