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LTC7813

7813f

For more information www.linear.com/LTC7813

operaTion

Main Control Loop

The LTC7813 uses a constant frequency, current mode

control architecture. Channel 1 is a buck (step-down)

controller, and channel 2 is a boost (step-up) controller.

During normal operation, the external top MOSFET for

the buck channel (the external bottom MOSFET for the

boost controller) is turned on when the clock for that

channel sets the RS latch, and is turned off when the

main current comparator, ICMP, resets the RS latch. The

peak inductor current at which ICMP trips and resets the

latch is controlled by the voltage on the ITH pin, which is

the output of the error amplifier, EA. The error amplifier

compares the output voltage feedback signal at the VFB

pin (which is generated with an external resistor divider

connected across the output voltage, VOUT, to ground)

to the internal 0.800V reference voltage (1.2V reference

voltage for the boost). When the load current increases,

it causes a slight decrease in VFB relative to the reference,

which causes the EA to increase the ITH voltage until the

average inductor current matches the new load current.

After the top MOSFET for the buck (the bottom MOSFET for

the boost) is turned off each cycle, the bottom MOSFET is

turned on (the top MOSFET for the boost) until either the

inductor current starts to reverse, as indicated by the cur-

rent comparator IR, or the beginning of the next clock cycle.

DRVCC/EXTVCC/INTVCC Power

Power for the top and bottom MOSFET drivers is derived

from the DRVCC pin. The DRVCC supply voltage can be

programmed from 5V to 10V through control of the

DRVSET pin. When the EXTVCC pin is tied to a voltage

below its switchover voltage (4.7V or 7.7V depending

on the DRVUV voltage), the VBIAS LDO (low dropout

linear regulator) supplies power from VBIAS to DRVCC. If

EXTVCC is taken above its switchover voltage, the VBIAS

LDO is turned off and an EXTVCC LDO is turned on. Once

enabled, the EXTVCC LDO supplies power from EXTVCC

to DRVCC. Using the EXTVCC pin allows the DRVCC power

to be derived from a high efficiency external source such

as the LTC7813 buck regulator output.

Each top MOSFET driver is biased from the floating boot-

strap capacitor, CB,whichnormallyrechargesduringeach

cycle through an internal switch whenever SW goes low.

For buck channel 1, if the input voltage decreases to a

voltage close to its output, the loop may enter dropout

and attempt to turn on the top MOSFET continuously. The

dropoutdetectordetectsthisandforcesthetopMOSFEToff

for about one-twelfth of the clock period every tenth cycle

to allow CB to recharge, resulting in about 99% duty cycle.

TheINTVCCsupplypowersmostoftheotherinternalcircuits

in the LTC7813. The INTVCC LDO regulates to a fixed value

of 5V and its power is derived from the DRVCC supply.

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

The two channels of the LTC7813 can be independently

shut down using the RUN1 and RUN2 pins. Pulling a RUN

pin below 1.22V shuts down the main control loop for

that channel. Pulling both pins below 0.7V disables both

controllers and most internal circuits, including the DRVCC

and INTVCC LDOs. In this state, the LTC7813 draws only

3.6μA of quiescent current.

Releasing a RUN pin allows a small 150nA internal current

to pull up the pin to enable that controller. Each RUN pin

maybeexternallypulledupordrivendirectlybylogic. Each

RUN pin can tolerate up to 65V (absolute maximum), so it

can be conveniently tied to VBIASinalways-onapplications

where one or both controllers are enabled continuously

and never shut down.

The start-up of each controller’s output voltage VOUT

is controlled by the voltage on the TRACK/SS pin

(TRACK/SS1 for channel 1, SS2 for channel 2). When the

voltage on the TRACK/SS pin is less than the 0.8V internal

reference for the buck and the 1.2V internal reference for

the boost, the LTC7813 regulates the VFB voltage to the

TRACK/SS pin voltage instead of the corresponding refer-

ence voltage. This allows the TRACK/SS pin to be used to

program a soft-start by connecting an external capacitor

from the TRACK/SS pin to GND. An internal 10μA pull-up

current charges this capacitor creating a voltage ramp on

the TRACK/SS pin. As the TRACK/SS voltage rises linearly

from 0V to 0.8V/1.2V (and beyond up to about 4V), the

output voltage VOUT rises smoothly from zero (VIN for the

boost) to its final value.

(Refer to the Functional Diagrams)

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