LTC7802 datasheet(12/34 Pages) AD

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LTC7802-3.3

Rev. 0

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OPERATION

Main Control Loop

The LTC7802-3.3 is a dual step-down (buck) synchro-

nous controller utilizing a constant frequency, peak cur-

rent mode architecture. The two controller channels oper-

ate 180° out of phase which reduces the required input

capacitance and power supply induced noise. During

normal operation, the external top MOSFET is turned on

when the clock for that channel sets the SR latch, causing

the inductor current to increase. The main switch is turned

off when the main current comparator, ICMP, resets the SR

latch. After the top MOSFET is turned off each cycle, the

bottom MOSFET is turned on which causes the inductor

current to decrease until either the inductor current starts

to reverse, as indicated by the current comparator IR, or

the beginning of the next clock cycle.

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 ampli-

fier compares the output voltage feedback signal at the

VFB pin, (which is generated with a resistor divider con-

nected across the output voltage, VOUT, to ground) to the

internal 0.8V reference voltage. 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.

Power and Bias Supplies (VIN, EXTVCC, and INTVCC)

The INTVCC pin supplies power for the top and bottom

MOSFET drivers and most of the internal circuitry. LDOs

(low dropout linear regulators) are available from both the

VIN and EXTVCC pins to provide power to INTVCC, which

has a regulation point of 5.1V. When the EXTVCC pin is

left open or tied to a voltage less than 4.7V, the VIN LDO

supplies power to INTVCC. If EXTVCC is taken above 4.7V,

the VIN LDO is turned off and the EXTVCC LDO is turned

on. Once enabled, the EXTVCC LDO supplies power to

INTVCC. Using the EXTVCC pin allows the INTVCC power

to be derived from a high efficiency external source such

as one of the LTC7802-3.3 switching regulator outputs.

Each top MOSFET driver is biased from the floating boot-

strap capacitor CB, which normally recharges during each

cycle through an external diode when the switch voltage

goes low.

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 dropout detector

detects this and forces the top MOSFET off for a short

time every tenth cycle to allow CB to recharge, resulting in

a 99% duty cycle at 350kHz operation and approximately

98% duty cycle at 2MHz operation.

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

The two channels of the LTC7802-3.3 can be indepen-

dently shut down using the RUN1 and RUN2 pins. Pulling

a RUN pin below 1.1V shuts down the main control loop

for that channel. Pulling both RUN pins below 0.7V dis-

ables both controllers and most internal circuits, including

the INTVCC LDOs. In this shutdown state, the LTC7802-

3.3 draws only 1.5μA of quiescent current.

The RUN pins may be externally pulled up or driven

directly by logic. Each pin can tolerate up to 40V (abso-

lute maximum), so it can be conveniently tied to VIN in

always-on applications where one or both controllers are

enabled continuously and never shut down. Additionally,

a resistive divider from VIN to a RUN pin can be used to

set a precise input undervoltage lockout so that the power

supply does not operate below a user adjustable level.

The start-up of each channel’s output voltage VOUT is con-

trolled by the voltage on the corresponding TRACK/SS

pin. When the voltage on the TRACK/SS pin is less than

the 0.8V internal reference voltage, the LTC7802-3.3 regu-

lates the VFB voltage to the TRACK/SS pin voltage instead

of the 0.8V reference voltage. This allows the TRACK/SS

pin to be used as a soft-start which smoothly ramps the

output voltage on start-up, thereby limiting the input sup-

ply inrush current. An external capacitor from the TRACK/

SS pin to GND is charged by an internal 12.5μA pull-up

current, creating a voltage ramp on the TRACK/SS pin. As

(Refer to Functional Diagram)

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