LT3755 datasheet(13/26 Pages) LINER

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LT3791-1

37911fa

For more information www.linear.com/LT3791-1

operaTion

Buck Region (VIN > VOUT)

Switch M4 is always on and switch M3 is always off during

this mode. At the start of every cycle, synchronous switch

M2 is turned on first. Inductor current is sensed when

synchronous switch M2 is turned on. After the sensed

inductor current falls below the reference voltage, which

is proportional to VC, synchronous switch M2 is turned off

and switch M1 is turned on for the remainder of the cycle.

Switches M1 and M2 will alternate, behaving like a typical

synchronous buck regulator. The duty cycle of switch M1

increases until the maximum duty cycle of the converter

in buck operation reaches DMAX(BUCK, TG1), given by:

DMAX(BUCK,TG1) = 100% – D(BUCK-BOOST)

where D(BUCK-BOOST) is the duty cycle of the buck-boost

switch range:

D(BUCK-BOOST) = 8%

Figure 3 shows typical buck operation waveforms. If VIN

approaches VOUT, the buck-boost region is reached.

Buck-Boost Region (VIN ~ VOUT)

When VIN is close to VOUT, the controller is in buck-boost

operation.Figure4andFigure5showtypicalwaveformsin

thisoperation.Everycyclethecontrollerturnsonswitches

M2 and M4, then M1 and M4 are turned on until 180° later

when switches M1 and M3 turn on, and then switches

M1 and M4 are turned on for the remainder of the cycle.

Boost Region (VIN < VOUT)

Switch M1 is always on and synchronous switch M2 is

always off in boost operation. Every cycle switch M3 is

turned on first. Inductor current is sensed when synchro-

nous switch M3 is turned on. After the sensed inductor

currentexceedsthereferencevoltagewhichisproportional

to VC, switch M3 turns off and synchronous switch M4

is turned on for the remainder of the cycle. Switches M3

and M4 alternate, behaving like a typical synchronous

boost regulator.

The duty cycle of switch M3 decreases until the minimum

duty cycle of the converter in boost operation reaches

DMIN(BOOST,BG2), given by:

DMIN(BOOST,BG2) = D(BUCK-BOOST)

where D(BUCK-BOOST) is the duty cycle of the buck-boost

switch range:

D(BUCK-BOOST) = 8%

Figure 6 shows typical boost operation waveforms. If VIN

approaches VOUT, the buck-boost region is reached.

Low Current Operation

TheLT3791-1isrecommendedtoruninforcedcontinuous

conduction mode at heavy load by pulling the CCM pin

higher than 1.5V. In this mode the controller behaves as

a continuous, PWM current mode synchronous switching

regulator. In boost operation, switch M1 is always on,

switch M3 and synchronous switch M4 are alternately

turned on to maintain the output voltage independent

of the direction of inductor current. In buck operation,

synchronous switch M4 is always on, switch M1 and syn-

chronous switch M2 are alternately turned on to maintain

the output voltage independent of the direction of inductor

current. In the forced continuous mode, the output can

source or sink current.

However, reverse inductor current from the output to the

input is not desired for certain applications. For these ap-

plications, the CCM pin must be connected to C/10 (pin 4)

with a pull-up resistor to INTVCC (see front page Typical

Application). Therefore, the CCM pin will be pulled lower

than 0.3V for discontinuous conduction mode by the C/10

pin when the output current is low. In this mode, switch

M4 turns off when the inductor current flows negative.

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