LTC3732 datasheet(10/28 Pages) LINER

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LTC3732

3732f

OPERATIO

(Refer to Functional Diagram)

FCB pin is below VCC – 1.5V but greater than 0.6V, the

controller performs as a Burst Mode switching regulator.

Burst Mode operation sets a minimum output current level

before turning off the top switch and turns off the synchro-

nous MOSFET(s) when the inductor current goes nega-

tive. This combination of requirements will, at low current,

force the ITH pin below a voltage threshold that will

temporarily shut off both output MOSFETs until the output

voltage drops slightly. There is a burst comparator having

60mV of hysteresis tied to the ITH pin. This hysteresis

results in 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.

B) Stage Shedding Operation

When the FCB pin is tied to the VCC pin, Burst Mode

operation is disabled and the forced minimum inductor

current requirement is removed. This provides constant

frequency, discontinuous current operation over the wid-

est possible output current range. At approximately 10%

of maximum designed load current, the second and third

output stages are shut off and the phase 1 controller alone

is active in discontinuous current mode. This “stage

shedding” optimizes efficiency by eliminating the gate

charging losses and switching losses of the other two

output stages. Additional cycles will be skipped when the

output load current drops below 1% of maximum de-

signed load current in order to maintain the output voltage.

This stage shedding operation is not as efficient as Burst

Mode operation at very light loads, but does provide lower

noise, constant frequency operating mode down to very

light load conditions.

C) Continuous Current Operation

Tying the FCB pin to ground will force continuous current

operation. This is the least efficient operating mode, but

may be desirable in certain applications. The output can

source or sink current in this mode. When forcing con-

tinuous operation and sinking current, this current will be

forced back into the main power supply, potentially

boosting the input supply to dangerous voltage levels—

BEWARE!

voltage. When the load current increases, it causes a slight

decrease in the EAIN pin voltage relative to the 0.6V

reference, which in turn causes the ITH voltage to increase

until each inductor’s average current matches one third of

the new load current (assuming all three current sensing

resistors are equal). In Burst Mode operation and stage

shedding mode, after each 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 is normally recharged during

each off cycle, through an external Schottky diode. When

VIN decreases to a voltage close to VOUT, however, the loop

may enter dropout and attempt to turn on the top MOSFET

continuously. The dropout detector counts the number of

oscillator cycles that the bottom MOSFET remains off and

periodically forces a brief on period to allow CB to re-

charge.

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

pin low. Releasing RUN/SS allows an internal 1.5

µA

current source to charge soft-start capacitor CSS. When

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

internally buffered ITH voltage is clamped but allowed to

ramp as the voltage on CSS continues to ramp. This “soft-

start” clamping prevents abrupt current from being drawn

from the input power source. When the RUN/SS pin is low,

all functions are kept in a controlled state. The RUN/SS pin

is pulled low when the VCC input voltage is below 4V or

when the IC die temperature rises above 150

°C.

Low Current Operation

The FCB pin is a multifunction pin: 1) an analog compara-

tor input to provide regulation for a secondary winding by

forcing temporary forced PWM operation and 2) a logic

input to select between three modes of operation.

A) Burst Mode Operation

When the FCB pin voltage is below 0.6V, the controller

performs as a continuous, PWM current mode synchro-

nous switching regulator. The top and bottom MOSFETs

are alternately turned on to maintain the output voltage

independent of direction of inductor current. When the

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