LTC3443 datasheet(9/16 Pages) LINER

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LTC3443

sn3443 3443fs

similar to a typical synchronous buck regulator. As the

control voltage increases, the duty cycle of switch A

increases until the maximum duty cycle of the converter in

Buck mode reaches DMAX_BUCK, given by:

DMAX_BUCK = 100 – D4SW %

where D4SW = duty cycle % of the four switch range.

D4SW = (150ns • f) • 100 %

where f = operating frequency, Hz.

Beyond this point the “four switch,” or Buck/Boost region

is reached.

Buck/Boost or Four Switch (VIN ~ VOUT)

When the internal control voltage, VCI, is above voltage V2,

switch pair AD remain on for duty cycle DMAX_BUCK, and

the switch pair AC begins to phase in. As switch pair AC

phases in, switch pair BD phases out accordingly. When

the VCI voltage reaches the edge of the Buck/Boost range,

at voltage V3, the AC switch pair completely phase out the

BD pair, and the boost phase begins at duty cycle D4SW.

The input voltage, VIN, where the four switch region begins

is given by:

ns f

OUT

150

–(

• )

The point at which the four switch region ends is given by:

VIN = VOUT(1 – D) = VOUT(1 – 150ns • f) V

Boost Region (VIN < VOUT)

Switch A is always on and switch B is always off during

this mode. When the internal control voltage, VCI, is above

voltage V3, switch pair CD will alternately switch to

provide a boosted output voltage. This operation is typical

to a synchronous boost regulator. The maximum duty

cycle of the converter is limited to 88% typical and is

reached when VCI is above V4.

Burst Mode OPERATION

Burst Mode operation is when the IC delivers energy to the

output until it is regulated and then goes into a sleep mode

where the outputs are off and the IC is consuming only

28µA. In this mode the output ripple has a variable

frequency component that depends upon load current.

During the period where the device is delivering energy to

the output, the peak current will be equal to 800mA typical

and the inductor current will terminate at zero current for

each cycle. In this mode the typical maximum average

output current is given by:

OUT MAX BURST

OUT

()

.•

≈

Burst Mode operation is user controlled, by driving the

MODE/SYNC pin high to enable and low to disable.

The peak efficiency during Burst Mode operation is less

than the peak efficiency during fixed frequency because

the part enters full-time 4-switch mode (when servicing

the output) with discontinuous inductor current as illus-

trated in Figures 3 and 4. During Burst Mode operation, the

control loop is nonlinear and cannot utilize the control

voltage from the error amp to determine the control mode,

therefore full-time 4-switch mode is required to maintain

the Buck/Boost function. The efficiency below 1mA

becomes dominated primarily by the quiescent current

and not the peak efficiency. The equation is given by:

Efficiency Burst

( bm) • ILOAD

≈

µ+

25 A ILOAD

where (ηbm) is typically 80% during Burst Mode

operation.

OPERATIO

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