錚?/div>
For example, with V
CC
= 16V, V
BAT
= 8.4V, L1 = 30碌H and
f = 200kHz, I
RMS
= 0.2A.
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 200kHz
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 impedance.
If the ESR of C
OUT
is 0.2鈩?and the battery impedance is
raised to 4鈩?with a bead of inductor, only 5% of the current
ripple will flow in the battery.
Soft Start
The LT1510 is soft started by the 0.1碌F capacitor on V
C
pin. On start-up, V
C
pin voltage will rise quickly to 0.5V,
then ramp at a rate set by the internal 45碌A(chǔ) pull-up current
and the external capacitor. Battery charging current starts
ramping up when V
C
voltage reaches 0.7V and full current
is achieved with V
C
at 1.1V. With a 0.1碌F capacitor, time to
reach full charge current is about 3ms and it is assumed
that input voltage to the charger will reach full value in less
than 3ms. Capacitance can be increased up to 0.47碌F if
longer input start-up times are needed.
In any switching regulator, conventional timer-based soft
starting can be defeated if the input voltage rises much
slower than the time-out period. This happens because the
switching regulators in the battery charger and the com-
puter power supply are typically supplying a fixed amount
of power to the load. If input voltage comes up slowly
compared to the soft start time, the regulators will try to
U
W
U
U
Figure 3. Undervoltage Lockout
The lockout voltage will be V
IN
= V
Z
+ 1V.
For example, for a 24V adapter to start charging at 22V
IN
,
choose V
Z
= 21V. When V
IN
is less than 22V, D1 keeps V
C
low and charger off.
Charging Current Programming
The basic formula for charging current is (see Block
Diagram):
錚?/div>
2
.
465V
LT1510I相關(guān)型號PDF文件下載
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