ltc3440 Linear Technology Corporation, ltc3440 Datasheet - Page 10

• Manufacturer Part Number: ltc3440
• Description: Micropower Synchronous Buck-boost Dc/dc Converter
• Manufacturer: Linear Technology Corporation

OPERATIO
LTC3440
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:
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%.
10
Efficiency Burst
U
≈
( bm) • I
25 A I
η
μ +
A
B
A
B
Figure 4. Inductor Discharge Cycle During Burst Mode Operation
V
Figure 3. Inductor Charge Cycle During Burst Mode Operation
V
7
LOAD
IN
7
LOAD
IN
SW1
SW1
3
3
+
–
dT
dI
dT
dI
≈ –
GND
GND
≈
L
L
5
5
V
V
L
IN
OUT
L
–
+
SW2
SW2
4
4
D
V
C
D
C
V
OUT
6
OUT
6
400mA
400mA
0mA
0mA
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
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).
T1
T2
C
) at the expense of DC load regulation.
3440 F03
3440 F04
3440fb