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參數(shù)資料
| 型號： | LTC3711 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | 4A, High Efficiency, Standalone Li Battery Charger |
| 中文描述： | 第4A，高效，獨立鋰電池充電器 |
| 文件頁數(shù)： | 14/20頁 |
| 文件大?。?/td> | 241K |
| 代理商： | LTC3711 |
14
LTC4007
4007i
Highest possible voltage rating on the capacitor will mini-
mize problems. Consult with the manufacturer before use.
Alternatives include new high capacity ceramic (at least
20
μ
F) from Tokin, United Chemi-Con/Marcon, et al. Other
alternative capacitors include OS-CON capacitors from
Sanyo.
The output capacitor (C3) is also assumed to absorb
output switching current ripple. The general formula for
capacitor current is:
I
V
V
V
L
f
RMS
BAT
BAT
DCIN
=
(
)
1
( )( )
0 29
.
1
–
For example:
V
DCIN
= 19V, V
BAT
= 12.6V, L1 = 10
μ
H, and
f = 300kHz, I
RMS
= 0.41A.
EMI considerations usually make it desirable to minimize
ripple current in the battery leads, and beads or inductors
may be added to increase battery impedance at the 300kHz
switching frequency. Switching ripple current splits be-
tween the battery and the output capacitor depending on
the ESR of the output capacitor and the battery imped-
ance. If the ESR of C3
is 0.2
and the battery impedance
is raised to 4
with a bead or inductor, only 5% of the
current ripple will flow in the battery.
Inductor Selection
Higher operating frequencies allow the use of smaller
inductor and capacitor values. A higher frequency gener-
ally results in lower efficiency because of MOSFET gate
charge losses. In addition, the effect of inductor value on
ripple current and low current operation must also be
considered. The inductor ripple current
I
L
decreases
with higher frequency and increases with higher V
IN
.
=
( )( )
I
V
V
L
OUT
OUT
IN
1
1–
Accepting larger values of
I
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is
I
L
= 0.4(I
MAX
). In no case should
I
L
exceed 0.6(I
MAX
) due to limits imposed by I
REV
and
CA1. Remember the maximum
I
L
occurs at the maxi-
mum input voltage. In practice 10
μ
H is the lowest value
recommended for use.
Lower charger currents generally call for larger inductor
values. Use Table 4 as a guide for selecting the correct
inductor value for your application.
Table 4
MAX AVERAGE
CURRENT (A)
MINIMUM INDUCTOR
VALUE (
μ
H)
40
±
20%
56
±
20%
20
±
20%
30
±
20%
15
±
20%
20
±
20%
10
±
20%
15
±
20%
INPUT VOLTAGE (V)
≤
20
>20
≤
20
>20
≤
20
>20
≤
20
>20
1
1
2
2
3
3
4
4
Charger Switching Power MOSFET
and Diode Selection
Two external power MOSFETs must be selected for use
with the charger: a P-channel MOSFET for the top (main)
switch and an N-channel MOSFET for the bottom (syn-
chronous) switch.
The peak-to-peak gate drive levels are set internally. This
voltage is typically 6V. Consequently, logic-level threshold
MOSFETs must be used. Pay close attention to the BV
DSS
specification for the MOSFETs as well; many of the logic
level MOSFETs are limited to 30V or less.
Selection criteria for the power MOSFETs include the “ON”
resistance R
DS(ON)
, total gate capacitance QG, reverse
transfer capacitance C
RSS
, input voltage and maximum
output current. The charger is operating in continuous
mode at moderate to high currents so the duty cycles for
the top and bottom MOSFETs are given by:
Main Switch Duty Cycle = V
OUT
/V
IN
Synchronous Switch Duty Cycle = (V
IN
– V
OUT
)/V
IN
.
APPLICATIOU
W
U
U
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