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参数资料
| 型号： | LTC4012IUF-2#TRPBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 633 kHz SWITCHING FREQ-MAX, PQCC20 |
| 封装： | 4 X 4 MM, 0.75 MM HEIGHT, LEAD FREE, PLASTIC, MO-220WGGD-1, QFN-20 |
| 文件页数： | 14/28页 |
| 文件大小： | 381K |
| 代理商： | LTC4012IUF-2#TRPBF |
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4012fa
LTC4012/
LTC4012-1/LTC4012-2
Theoutputcapacitorshownacrossthebatteryandground
must also absorb PWM output ripple current. The general
formula for this capacitor current is:
I
V
L f
RMS
BAT
CLP
PWM
=
0 29
1
.
–
For example, IRMS = 0.22A with:
VBAT = 12.6V
VCLP = 19V
L1 = 10H
fPWM = 550kHz
High capacity ceramic capacitors (20F or more) available
from a variety of manufacturers can be used for input/out-
put capacitors. Other alternatives include OS-CON and
POSCAP capacitors from Sanyo.
Low ESR solid tantalum capacitors have high ripple cur-
rent rating in a relatively small surface mount package,
but exercise caution when using tantalum for input or
output bulk capacitors. High input surge current can be
created when the adapter is hot-plugged to the charger
or when a battery is connected to the charger. Solid tan-
talum capacitors have a known failure mechanism when
subjected to very high surge currents. Select tantalum
capacitors that have high surge current ratings or have
been surge tested.
EMI considerations usually make it desirable to minimize
ripple current in battery leads. Adding Ferrite beads or
inductors can increase battery impedance at the nominal
550KHzswitchingfrequency.Switchingripplecurrentsplits
between the battery and the output capacitor in inverse
relation to capacitor ESR and the battery impedance. If
the ESR of the output capacitor is 0.2
Ω and the battery
impedance is raised to 4
Ω with a ferrite bead, only 5%
of the current ripple will flow to the battery.
Inductor Selection
Higher switching frequency generally results in
lower efficiency because of MOSFET gate charge
losses, but it allows smaller inductor and capacitor
values to be used. A primary effect of the inductor
value L1 is the amplitude of ripple current created.
The inductor ripple current IL decreases with higher
inductance and PWM operating frequency:
I
V
L f
L
BAT
CLP
PWM
=
–
1
Accepting larger values of IL allows the use of low in-
ductance, but results in higher output voltage ripple and
greater core losses. Lower charge currents generally call
for larger inductor values.
TheLTC4012limitsmaximuminstantaneouspeakinductor
current during every PWM cycle. To avoid unstable switch
waveforms, the ripple current must satisfy:
I
mV
R
I
L
SENSE
MAX
<
2 150
–
so choose:
L
V
f
mV
R
I
CLP
PWM
SENSE
MAX
1
0 125
150
>
.
–
For C-grade parts, a reasonable starting point for setting
ripple current is IL = 0.4 IMAX. For I-grade parts, use
IL = 0.2 IMAX only if the IC will actually be used to
chargebatteriesoverthewiderI-gradetemperaturerange.
The voltage compliance of internal LTC4012 circuits also
imposes limits on ripple current. Select RIN (in Figure 1)
to avoid average current errors in high ripple designs. The
following equation can be used for guidance:
R
I
A
R
I
A
SENSE
L
IN
SENSE
L
50
20
≤
applications inForMation
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