LTC3440 Datasheet, PDF (10/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

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 + I_LOAD
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 V_C) 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).
V_IN
V_OUT
7
6
A
dI
dT
≈
V_IN
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
V_IN
V_OUT
7
6
A
dI
dT
≈
–
V_OUT
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
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