LTC3533 datasheet(9/16 Pages) LINER

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LTC3533

3533f

OPERATION

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 control voltage, VC, is above voltage V2, switch

pair AD remain on for duty cycle DMAX_BUCK, and switch

pair AC begins to phase in. As switch pair AC phases in,

switch pair BD phases out accordingly. When VC 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:

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

The point at which the four switch region ends is given

by:

ns f

OUT

−

150

(• )

where f = operating frequency, Hz.

Boost Region (VIN < VOUT)

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

this mode. When the control voltage, VC, is above volt-

age 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 90% typical and is reached

when VC is above V4.

BURST MODE OPERATION

Burst Mode operation reduces the LTC3533’s quiescent

current consumption at light loads and improves overall

conversion efﬁciency, increasing battery life. During Burst

Mode operation the LTC3533 delivers energy to the out-

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

where the outputs are off and the quiescent current drops

to 40µA. In this mode the output ripple has a variable

frequency component that depends upon load current,

and will typically be about 2% peak-to-peak. Burst Mode

operation ripple can be reduced slightly by using more

output capacitance. Another method of reducing Burst

Mode operation ripple is to place a small feed-forward

capacitor across the upper resistor in the VOUT feedback

divider network (as in Type III compensation).

During the period where the device is delivering energy

to the output, the peak switch current will rise to 450mA

typical and the inductor current will terminate at zero cur-

rent for each cycle. In this mode, the typical maximum

average output currents are given by:

IMAX(BURST)BUCK ≈ 225mA; VOUT < VIN

IMAX(BURST)BOOST ≈ 225mA • (VIN/VOUT); VOUT > VIN

IMAX(BURST)BUCK-BOOST ≈ 350mA; VOUT ≈ VIN, since the

input and output are connected together for most of the

cycle.

The efﬁciency below 1mA becomes dominated primarily

by the quiescent current. The Burst Mode operation ef-

ﬁciency is given by:

Efficiency

LOAD

≅

•

where

η is typically 90% during Burst Mode operation

Programmable Automatic Burst Mode Operation

Burst Mode operation can be automatic or digitally con-

trolled with a single pin. In automatic mode, the LTC3533

enters Burst Mode operation at the programmed threshold

and returns to ﬁxed frequency operation when the load

demand increases. The load current at which the mode

transition occurs is programmed using a single external

resistor from BURST to ground, according to the follow-

ing equations:

Enter Burst Mode Operation I

BURST

Exit Burst Mode Operation I

BURST

Where RBURST is in kΩ and IBURST is the load transition

current in Amps. Do not use values of RBURST greater

than 1M

Ω.

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