KB3440EMS datasheet(10/20 Pages) KINGBOR

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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 ( bm) is typically 79% during Burst Mode opera-

tion for an ESR of the inductor of 50m . For 200m

inductor ESR, the peak efficiency ( bm) drops to 75%.

Burst Mode Operation to Fixed Frequency Transient

Response

When transitioning from Burst Mode operation to fixed

frequency, the system exhibits a transient since the modes

of operation have changed. For most systems this tran-

sient is acceptable, but the application may have stringent

input current and/or output voltage requirements that

dictate a broad-band voltage loop to minimize the tran-

sient. Lowering the DC gain of the loop will facilitate the

task (10M FB to VC) at the expense of DC load regulation.

Type 3 compensation is also recommended to broad band

the loop and roll off past the two pole response of the LC

of the converter (see Closing the Feedback Loop).

VIN

SW1

GND

SW2

+–

VOUT

400mA

0mA

3440 F03

VIN

SW1

GND

SW2

–+

VOUT

400mA

0mA

3440 F04

VOUT

–

Figure 3. Inductor Charge Cycle During Burst Mode Operation

Figure 4. Inductor Discharge Cycle During Burst Mode Operation

kB3440

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