Datasheet

10
LTC3440
3440fb
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) •I
LOAD
≈
μ+
η
25 A 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).
7
V
IN
A
3
SW1
5
GND
4
SW2
L
+–
6
V
OUT
D
C
400mA
I
INDUCTOR
0mA
3440 F03
T1
B
dI
dT
V
IN
L
≈
7
V
IN
A
3
SW1
5
GND
4
SW2
L
–+
6
V
OUT
D
C
400mA
I
INDUCTOR
0mA
3440 F04
T2
B
dI
dT
V
OUT
L
≈ –
Figure 3. Inductor Charge Cycle During Burst Mode Operation
Figure 4. Inductor Discharge Cycle During Burst Mode Operation
OPERATIO
U