LTC3557-1 Linear Technology, LTC3557-1 Datasheet - Page 22

• Manufacturer Part Number: LTC3557-1
• Description: USB Power Manager
• Manufacturer: Linear Technology

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LTC3557/LTC3557-1
APPLICATIONS INFORMATION
22
Figure 7. LTC3577-1V
with the LT3505
Figure 6. LTC3577 V
with the LT3480
5.0
4.5
4.0
3.5
3.0
2.5
5.0
4.5
4.0
3.5
3.0
2.5
2.5
2.5
8V TO 36V
3
3
Figure 5. LT3505 Buck Control Using V
HV
OUT
OUT
IN
BAT (V)
BAT (V)
Voltage vs Battery Voltage
3.5
3.5
Voltage vs Battery Voltage
1μF
BUCK CIRCUITRY
HIGH VOLTAGE
LT3505
4
I
I
I
BAT
4
68nF
20k
O
O
O
I
I
BAT
O
O
= 0.0A
= 0.75A
= 1.5A
= 0.0A
= 0.6A
35571 F07
35571 F06
806k
BZT52C16T
4.5
4.5
150k
3
4
6
V
RUN/SS
GND
IN
LT3505
5, 9
BOOST
C
(2.2MHz Switching with Frequency Foldback)
V
V
SW
C
C
8
FB
26
LTC3557-1
WALL ACPR
LTC3557
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
effi ciency is approximately:
where η
regulator and 5V is the output voltage of the buck regu-
lator. 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
is approximately:
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 applica-
tions, component selection and board layout information
beyond those listed here please refer to the respective
LT3480, LT3481 or LT3505 data sheet.
1
2
7
3
η
η
0.1μF
CHARGER
CHARGER
25
MBRM140
GATE
V
BAT
OUT
BUCK
6.8μH
C
23
21
22
control technique, battery charger effi ciency
1N4148
=
=
is the effi ciency of the high voltage buck
49.9k
10.0k
η
η
Si2333DS
BUCK
BUCK
10μF
•
•
. 0 3
V
5
BAT
Si2333DS
(OPT)
+
V
V V
UP TO
1.2A
V
BAT
35571 F05
+
Li-Ion
C
OUT
BAT
V
BAT
OUT
OUT
, battery charger
35571f