LTC3707 Ver la hoja de datos (PDF) - Linear Technology

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LTC3707

High Efficiency, 2-Phase Synchronous Step-Down Switching Regulator

Linear Technology 

LTC3707

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OPERATIO (Refer to Functional Diagram)

Main Control Loop

The LTC3707 uses a constant frequency, current mode

step-down architecture with the two controller channels

operating 180 degrees out of phase. During normal opera-

tion, 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 con-

trolled 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 in-

creases, it causes a slight decrease 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 compara-

tor 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 1.2µA

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

LTC3707 controller functions are shut down, and the

STBYMD pin determines if the standby 5V and 3.3V

regulators are kept alive.

Low Current Operation

The FCB pin is a multifunction pin providing two func-

tions: 1) an analog input to provide regulation for a

secondary winding by temporarily forcing continuous

PWM operation on both controllers and 2) a logic input

to select between two modes of low current operation.

When the FCB pin voltage is below 0.800V, the controller

forces continuous PWM current mode 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 0.80V, the controller en-

ters Burst Mode operation. Burst Mode operation sets a

minimum output current level before inhibiting the top

switch and turns off the synchronous MOSFET(s) when

the inductor current goes negative. This combination of

requirements 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.

There is 60mV of hysteresis in the burst comparator B

tied to the ITH pin. This hysteresis produces output

signals to the MOSFETs that turn them on for several

cycles, followed by a variable “sleep” interval depending

upon the load current. The resultant output voltage ripple

is held to a very small value by having the hysteretic

comparator after the error amplifier gain block.

Constant Frequency Operation

When the FCB pin is tied to INTVCC, Burst Mode operation

is disabled and the forced minimum output current re-

quirement is removed. This provides constant frequency,

discontinuous (preventing reverse inductor current) cur-

rent operation over the widest possible output current

range. This constant frequency operation is not as efficient

as Burst Mode operation, but does provide a lower noise,

constant frequency operating mode down to approxi-

mately 1% of designed maximum output current. Voltage

should not be applied to the FCB pin prior to the application

of voltage to the VIN pin.

3707f

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