LTC3557_15 Datasheet, PDF (22/28 Pages) Linear Technology – USB Power Manager with Li-Ion Charger and Three Step-Down Regulators

LTC3557/LTC3557-1
APPLICATIONS INFORMATION
HV_IN
8V TO 36V
3
1
V_IN BOOST
1μF
68nF
150k
4
LT3505
SHDN SW
2
BZT52C16T
20k ^806k
LT3505
HIGH VOLTAGE
BUCK CIRCUITRY
6 R_T
GND
7
FB
V_C
5, 9 8
1N4148
0.1μF _6.8μH
MBRM140
49.9k
10μF
10.0k
Si2333DS
UP TO
1.2A
26 3 25
V_C WALL ACPR
V_OUT
23
LTC3557
LTC3557-1
GATE
21
BAT ²²
V_OUT
C_OUT
Si2333DS
(OPT)
+
BAT
Li-Ion
35571 F05
Figure 5. LT3505 Buck Control Using V_C (2.2MHz Switching with Frequency Foldback)
5.0
4.5
4.0
3.5
3.0
2.5
2.5
I_O = 0.0A
I_O = 0.75A
I_O = 1.5A
BAT
3
3.5
4
4.5
BAT (V)
35571 F06
Figure 6. LTC3557 V_OUT Voltage vs Battery Voltage
with the LT3480
5.0
4.5
4.0
3.5
3.0
2.5
2.5
I_O = 0.0A
I_O = 0.6A
BAT
3
3.5
4
4.5
BAT (V)
35571 F07
Figure 7. LTC3557-1V_OUT Voltage vs Battery Voltage
with the LT3505
22
This technique provides a significant efficiency advantage
over the use of a 5V buck to drive the battery charger. With
a simple 5V buck output driving V_OUT, battery charger
efficiency is approximately:
η_CHARGER
=
η_BUCK
•
V_BAT
5V
where η_BUCK is the efficiency of the high voltage buck
regulator and 5V is the output voltage of the buck regulator.
With a typical buck efficiency of 87% and a typical battery
voltage of 3.8V, the total battery charger efficiency 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 efficiency
is approximately:
η_CHARGER
=
η_BUCK
•
V_BAT
0.3V + V_BAT
With the same assumptions as above, the total battery
charger efficiency 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.
35571fc