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| 型号： | LTC1960CUHF#PBF |
| 厂商： | Linear Technology |
| 文件页数： | 22/28页 |
| 文件大小： | 0K |
| 描述： | IC BATT CHRGR/SELECTR DL 38-QFN |
| 标准包装： | 52 |
| 功能： | 充电管理 |
| 电源电压： | 6 V ~ 28 V |
| 工作温度： | 0°C ~ 70°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 38-WFQFN 裸露焊盘 |
| 供应商设备封装： | 38-QFN（5x7） |
| 包装： | 管件 |
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LTC1960
APPLICATIONS INFORMATION
= REF
? V ISET
V REF ? T ? ∑
? V ISET ? R SET
V REF ? T ? ∑
R VSET ( C B1 || C B2 )
V REF ? T ? ∑
R VSET ? V VSET
0.29(V BAT ) ? 1 ? BAT ?
? V DCIN ?
I RMS =
V SET /I SET Capacitors
Capacitor C7 is used to filter the delta-sigma modulation
frequency components to a level which is essentially DC.
Acceptable voltage ripple at I SET is about 10mV P-P . Since
the period of the delta-sigma switch closure, T ?∑ , is about
10μs and the internal IDAC resistor, R SET , is 18.77k, the
ripple voltage can be approximated by:
V ? T ? ∑
R SET ? C7
Then the equation to extract C7 is:
C7 =
= 0.8/0.01/18.77k(10μs) @ 0.043μF
In order to prevent overshoot during start-up transients,
the time constant associated with C7 must be shorter than
the time constant of C5 at the I TH pin. If C7 is increased
to improve ripple rejection, then C5 should be increased
proportionally and charger response time to average cur-
rent variation will degrade.
Capacitor C B1 and C B2 are used to filter the VDAC delta-
sigma modulation frequency components to a level which
is essentially DC. C B2 is the primary filter capacitor and
C B1 is used to provide a zero in the response to cancel
the pole associated with C B2 . Acceptable voltage ripple
at V SET is about 10mV P-P . Since the period of the delta-
sigma switch closure, T ?∑ , is about 11μs and the internal
VDAC resistor, R VSET , is 7.2kΩ, the ripple voltage can be
approximated by:
? V VSET =
Then the equation to extract C B1 || C B2 is:
C B1 || C B2 =
C B2 should be 10× to 20× C B1 to divide the ripple voltage
present at the charger output. Therefore C B1 = 0.01μF and
C B2 = 0.1μF are good starting values. In order to prevent
overshoot during start-up transients the time constant as-
sociated with C B2 must be shorter than the time constant
of C5 at the I TH pin. If C B2 is increased to improve ripple
rejection, then C5 should be increased proportionally and
charger response time to voltage variation will degrade.
Input and Output Capacitors
In the 4A Lithium Battery Charger (Typical Application
section), the input capacitor (C IN ) 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 a high ripple current rating in a
relatively 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
C15, located at the AC adapter input terminal, is helpful
in reducing ringing during the hot-plug event.
Highest possible voltage rating on the capacitor will
minimize problems. Consult with the manufacturer before
use. Alternatives include new high capacity ceramic (at
least 20μF) from Tokin, United Chemi-Con/Marcon, et al.
Other alternative capacitors include OSCON capacitors
from Sanyo.
The output capacitor (C OUT ) is also assumed to absorb
output switching current ripple. The general formula for
capacitor current is:
? V ?
(L1)(f)
For example:
V DCIN = 19V, V BAT = 12.6V, L1 = 10μH, and f = 300kHz,
I RMS = 0.41A.
1960fb
22
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