LTC3440 查看數據表（PDF） - Linear Technology

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LTC3440

Micropower Synchronous Buck-Boost DC/DC Converter

Linear Technology 

LTC3440

U

OPERATIO

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 ≈ (ηbm) • ILOAD

25μA + ILOAD

where (ηbm) is typically 79% during Burst Mode opera-

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

inductor ESR, the peak efficiency (ηbm) 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

VOUT

7

6

A

dI

dT

≈

VIN

L

D

+

–

3

4

L

SW1

SW2

B

C

400mA

0mA

T1

5

GND

3440 F03

Figure 3. Inductor Charge Cycle During Burst Mode Operation

VIN

VOUT

7

6

A

dI

dT

≈

–

VOUT

L

D

–

+

3

4

SW1

L

SW2

B

C

400mA

0mA

5

GND

T2

3440 F04

Figure 4. Inductor Discharge Cycle During Burst Mode Operation

3440fb

10

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