LTC1876 datasheet(11/36 Pages) LINER

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LTC1876

1876fa

Main Control Loop

The LTC1876 uses a constant frequency, current mode

scheme to provide excellent line and load regulation for all

its outputs. The step-down controllers have two of its

switch drivers operating at 180 degrees out of phase from

each other. During normal operation, each top MOSFET is

turned on when the clock for that channel sets the RS latch,

and turned off when the main current comparator, I1,

resets the RS latch. The peak inductor current at which I1

resets the RS latch is controlled by the voltage on the ITH

pin, which is the output of each error amplifier EA. The

VOSENSE 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 de-

crease in VOSENSE relative to the 0.8V reference, which in

turn causes the ITH voltage to increase until the average

inductor current matches the new load current. After the

top MOSFET has turned off, the bottom MOSFET is turned

on until either the inductor current starts to reverse, as

indicated by current comparator I2, or the beginning of the

next cycle.

The top MOSFET drivers are biased from floating boot-

strap capacitor CB, which normally is recharged during

each off cycle through an external diode when the top

MOSFET turns off. As VIN decreases to a voltage close to

VOUT, the loop may enter dropout and attempt to turn on

the top MOSFET continuously. The dropout detector de-

tects this and forces the top MOSFET off for about 500ns

every tenth cycle to allow CB to recharge.

The main control loop is shut down by pulling the RUN/SS

pin low. Releasing RUN/SS allows an internal 1.2

µA

current source to charge soft-start capacitor CSS. When

CSS reaches 1.5V, the main control loop is enabled with the

ITH voltage clamped at approximately 30% of its maximum

value. As CSS continues to charge, the ITH pin voltage is

gradually released allowing normal, full-current opera-

tion. When both RUN/SS1 and RUN/SS2 are low, all

LTC1876 controller functions are shut down, and the

STBYMD pin determines if the standby 5V and 3.3V

regulators are kept alive.

AUX Regulator

The auxiliary boost regulator is completely independent

from other LTC1876 circuits. It can be operated even

though the LTC1876 step-down controllers are in shut-

down. The operation of the boost regulator is similar to the

controllers. The oscillator, OSCAUX, sets the RS latch and

turns on the monolithic power switch. A voltage propor-

tional to the switch current is added to a stabilizing ramp

and the resulting sum is fed into the positive terminal of the

PWM comparator, A1AUX. When this voltage exceeds the

level at the negative input of A1AUX, the SR latch is reset,

turning off the power switch. The level at the negative input

of A1AUX is set by the error amplifier EAAUX and is simply

an amplified version of the difference between the feed-

back voltage and the reference voltage. Hence the error

amplifier sets the correct peak current level to keep the

output in regulation. To protect the power switch from

excessive current, a 1A minimum limit is internally set.

When the switch reaches this limit, it will force the latch to

reset, turning it off. Applying a voltage less than 0.5V on

the shutdown pin will put the boost regulator in shutdown.

Low Current Operation

The FCB pin is a multifunction pin providing two functions:

1) to provide regulation for a secondary winding by

temporarily forcing continuous PWM operation on both

controllers; and 2) select between

two modes of low

current operation. When the FCB pin voltage is below 0.8V,

the controller forces continuous PWM current operation.

In this mode, the top and bottom MOSFETs are alternately

turned on to maintain the output voltage independent of

direction of inductor current. When the FCB pin is below

VINTVCC – 2V but greater than 0.8V, the controller enters

Burst Mode operation. Burst Mode operation sets a mini-

mum output current level before turning off the top switch

and turns off the synchronous MOSFET(s) when the

inductor current goes negative. This combination of re-

quirements will, at low currents, force the ITH pin below a

voltage threshold that will temporarily inhibit turn-on of

both output MOSFETs until the output voltage drops

slightly. There is 60mV of hysteresis in the burst compara-

tor B tied to the ITH pin. This hysteresis produces output

signals to the MOSFETs that turn them on for several

(Refer to Functional Diagram)

OPERATIO

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