9
LT1510/LT1510-5
APPLICATIONS INFORMATION
WUU
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capacity ceramic capacitor (5µF to 10µF) from Tokin or
United Chemi-Con/MARCON, et al., and the old standby,
aluminum electrolytic, which will require more microfarads
to achieve adequate ripple rating. OS-CON can also be used.
The output capacitor C
OUT
is also assumed to absorb
output switching current ripple. The general formula for
capacitor current is:
I
V
V
V
Lf
RMS
BAT
BAT
CC
=
()
−
()()
029 1
1
.
For example, with V
CC
= 16V, V
BAT
= 8.4V, L1 = 30µH 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 0.2Ω and the battery impedance is
raised to 4Ω 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 0.1µF 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 45µA 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 0.1µF 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 0.47µF 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
deliver full power to the load when the input voltage is still
well below its final value. If the adapter is current limited,
it cannot deliver full power at reduced output voltages and
the possibility exists for a quasi “latch” state where the
adapter output stays in a current limited state at reduced
output voltage. For instance, if maximum charger plus
computer load power is 20W, a 24V adapter might be
current limited at 1A. If adapter voltage is less than (20W/1A
= 20V) when full power is drawn, the adapter voltage will be
sucked down by the constant 20W load until it reaches a
lower stable state where the switching regulators can no
longer supply full load. This situation can be prevented by
utilizing
undevoltage lockout
, set higher than the minimum
adapter voltage where full power can be achieved.
A fixed undervoltage lockout of 7V is built into the V
CC
pin.
Internal lockout is performed by clamping the V
C
pin low.
The V
C
pin is released from its clamped state when the V
CC
pin rises above 7V. The charger will start delivering current
about 2ms after V
C
is released, as set by the 0.1µF at V
C
pin. Higher lockout voltage can be implemented with a
Zener diode (see Figure 3 circuit).
Figure 3. Undervoltage Lockout
GND
V
CC
V
C
V
IN
1510 F03
LT1510
2k
D1
1N4001
V
Z
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):
II
V
R
BAT PROG
PROG
=
()()
=
()
2000
2 465
2000
.
