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參數(shù)資料
| 型號(hào)： | 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(dú)立鋰電池充電器 |
| 文件頁數(shù)： | 13/20頁 |
| 文件大?。?/td> | 241K |
| 代理商： | LTC3711 |
13
LTC4007
4007i
sense voltage is reduced, causing some careful consider-
ation of the ripple current. Input referred maximum com-
parator threshold is 117mV, which is the same ratio of 1.4x
the DC target. Input referred I
REV
threshold is scaled back
to –24mV. The current at which the switcher starts will be
reduced as well so there is some risk of boost activity.
These concerns can be addressed by using a slightly larger
inductor to compensate for the reduction of tolerance to
ripple current.
Charger Voltage Programming
Pins CHEM and C3C4 are used to program the charger final
output voltage. The CHEM pin programs Li-Ion battery
chemistry for 4.1V/cell (low) or 4.2V/cell (high). The C3C4
pin selects either 3 series cells (low) or 4 series cells
(high). It is recommended that these pins be shorted to
ground (logic low) or left open (logic high) to effect the
desired logic level. Use open-collector or open-drain out-
puts when interfacing to the CHEM and 3C4C pins from a
logic control circuit.
Table 3. Charger Voltage Programming
V
FINAL
(V)
12.3
12.6
16.4
16.8
3C4C
LOW
LOW
HIGH
HIGH
CHEM
LOW
HIGH
LOW
HIGH
Setting the Timer Resistor
The charger termination timer is designed for a range of
1hour to 3 hour with a
±
15% uncertainty. The timer is
programmed by the resistor R
RT
using the following
equation:
t
TIMER
= 2
27
R
RT
175pF
It is important to keep the parasitic capacitance on the R
T
pin to a minimum. The trace connecting R
T
to R
RT
should
be as short as possible.
Soft-Start
The LTC4007 is soft started by the 0.12
μ
F capacitor on the
ITH pin. On start-up, ITH pin voltage will rise quickly to
0.5V, then ramp up at a rate set by the internal 40
μ
A pull-
up current and the external capacitor. Battery charging
APPLICATIOU
W
U
U
current starts ramping up when ITH voltage reaches 0.8V
and full current is achieved with ITH at 2V. With a 0.12
μ
F
capacitor, time to reach full charge current is about 2ms
and it is assumed that input voltage to the charger will
reach full value in less than 2ms. The capacitor can be
increased up to 1
μ
F if longer input start-up times are
needed.
Input and Output Capacitors
The input capacitor (C2) is assumed to absorb all input
switching ripple current in the converter, so it must have
adequate ripple current rating. Worst-case RMS ripple
current will be equal to one half of output charging current.
Actual capacitance value is not critical. Solid tantalum low
ESR capacitors have high ripple current rating in a rela-
tively small surface mount package, but caution must be
used when tantalum capacitors are used for input or
output bypass High input surge currents can be created
when the adapter is hot-plugged to the charger or when a
battery is connected to the charger. Solid tantalum capaci-
tors have a known failure mechanism when subjected to
very high turn-on surge currents. Only Kemet T495 series
of “Surge Robust” low ESR tantalums are rated for high
surge conditions such as battery to ground.
The relatively high ESR of an aluminum electrolytic for C1,
located at the AC adapter input terminal, is helpful in
reducing ringing during the hot-plug event. Refer to AN88
for more information.
Figure 7. t
TIMER
vs R
RT
R
RT
(k
)
100
0
t
T
(
20
60
80
100
200
140
200
300
350
4007 F07
40
160
180
120
150
250
400 450
500
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