Datasheet
LTC3557/LTC3557-1
22
35571fc
APPLICATIONS INFORMATION
This technique provides a signifi cant effi ciency advantage
over the use of a 5V buck to drive the battery charger. With
a simple 5V buck output driving V
OUT
, battery charger
effi ciency is approximately:
η
CHARGER
=η
BUCK
•
V
BAT
5V
where η
BUCK
is the effi ciency of the high voltage buck
regulator and 5V is the output voltage of the buck regulator.
With a typical buck effi ciency of 87% and a typical battery
voltage of 3.8V, the total battery charger effi ciency is
approximately 66%. Assuming a 1A charge current, this
works out to nearly 2W of power dissipation just to charge
the battery!
With the V
C
control technique, battery charger effi ciency
is approximately:
η
CHARGER
=η
BUCK
•
V
BAT
0.3V + V
BAT
With the same assumptions as above, the total battery
charger effi ciency is approximately 81%. This example
works out to just 900mW of power dissipation. For
applications, component selection and board layout
information beyond those listed here please refer to the
respective LT3480, LT3481 or LT3505 data sheet.
V
IN
R
T
1
2
MBRM140
0.1µF
10µF
C
OUT
UP TO
1.2A
V
OUT
BAT
35571 F05
Li-Ion
Si2333DS
Si2333DS
(OPT)
6.8µH
1N4148
3
4
6
BOOST
V
C
26 3 25
23
21
22
LTC3557
LTC3557-1
WALL ACPR
GATE
V
OUT
BAT
SHDN
150k
806k
BZT52C16T
20k
68nF
1µF
HV
IN
8V TO 36V
49.9k
10.0k
SW
7
5, 9 8
FB
LT3505
LT3505
HIGH VOLTAGE
BUCK CIRCUITRY
GND V
C
+
Figure 5. LT3505 Buck Control Using V
C
(2.2MHz Switching with Frequency Foldback)
BAT (V)
2.5
V
OUT
(V)
3.5
4.0
4.5
35571 F06
3.0
2.5
3
3.5
4
5.0
4.5
I
O
= 0.0A
I
O
= 0.75A
I
O
= 1.5A
BAT
BAT (V)
2.5
V
OUT
(V)
3.5
4.0
4.5
35571 F07
3.0
2.5
3
3.5
4
5.0
4.5
I
O
= 0.0A
I
O
= 0.6A
BAT
Figure 6. LTC3557 V
OUT
Voltage vs Battery Voltage
with the LT3480
Figure 7. LTC3557-1V
OUT
Voltage vs Battery Voltage
with the LT3505