LTC3442 datasheet(9/16 Pages) LINER

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LTC3532

3532fc

VIN

SW1

GND

SW2

+–

VOUT

250mA

0mA

3532 F03

VIN

≈

OPERATIO

Figure 3. Inductor Charge Cycle During Burst Mode Operation

VIN

SW1

GND

SW2

–+

VOUT

250mA

0mA

3532 F04

VOUT

≈ –

Figure 4. Inductor Disharge Cycle During Burst Mode Operation

Note that 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 cur-

rent as illustrated 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 cur-

rent. The Burst Mode operation efﬁciency is given by:

EFFICIENCY ≅

n• ILOAD

35μA + ILOAD

where n is typically 88% during Burst Mode operation.

Automatic Burst Mode Operation Control

Burst Mode operation can be automatic or manually con-

trolled with a single pin. In automatic mode, the IC will

enter Burst Mode operation at light load and return to ﬁxed

frequency operation at heavier loads. The load current at

which the mode transition occurs is programmed using

a single external resistor from the BURST pin to ground,

according to the following equations:

Enter Burst Mode Operation: I =

10.5V

RBURST

Leave Burst Mode Operation: I =

RBURST

where RBURST is in kΩ and IBURST is the load transition

current in Amps. For automatic operation, a ﬁlter capaci-

tor should also be connected from BURST to ground to

prevent ripple on BURST from causing the IC to oscillate

in and out of Burst Mode operation. The equation for the

minimum capacitor value is:

CBURST(MIN) ≥

COUT •VOUT

60,000V

where CBURST(MIN) and COUT are in μF. In the event that

a load transient causes the feedback pin to drop by more

than 4% from the regulation value while in Burst Mode

operation, the IC will immediately switch to ﬁxed frequency

mode and an internal pull-up will be momentarily applied

to BURST, rapidly charging the BURST capacitor. This

prevents the IC from immediately reentering Burst Mode

operation once the output achieves regulation.

Manual Burst Mode Operation

For manual control of Burst Mode operation, the RC net-

work connected to BURST can be eliminated. To force ﬁxed

frequency mode, BURST should be connected to VOUT. To

force Burst Mode operation, BURST should be grounded.

When commanding Burst Mode operation manually, the

circuit connected to BURST should be able to sink up to

2mA. For optimum transient response with large dynamic

loads, the operating mode should be controlled manually

by the host. By commanding ﬁxed frequency operation

prior to a sudden increase in load, output voltage droop can

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