9
LT1511
APPLICATIONS INFORMATION
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Input and Output Capacitors
In the 3A Lithium Battery Charger (Figure 1), 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 capacitors
such as the AVX TPS and Sprague 593D series have high
ripple current rating in a relatively small surface mount
package, but
caution must be used when tantalum capaci-
tors are used for input bypass
. High input surge currents
can be created when the adapter is hot-plugged to the
charger and solid tantalum capacitors have a known
failure mechanism when subjected to very high turn-on
surge currents. Highest possible voltage rating on the
capacitor will minimize problems. Consult with the manu-
facturer before use. Alternatives include new high capacity
ceramic (5µF to 20µF) 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. Sanyo 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
RMS
=
(L1)(f)
V
BAT
V
CC
()
0.29 (V
BAT
) 1 –
For example, V
CC
= 16V, V
BAT
= 8.4V, L1 = 20µH,
and f = 200kHz, I
RMS
= 0.3A.
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 imped-
ance. If the ESR of C
OUT
is 0.2Ω and the battery impedance
is rased to 4Ω with a bead or inductor, only 5% of the
current ripple will flow in the battery.
Soft-Start
The LT1511 is soft started by the 0.33µF capacitor on the
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 0.33µF capacitor, time
to reach full charge current is about 10ms and it is
assumed that input voltage to the charger will reach full
value in less than 10ms. The capacitor can be increased up
to 1µF 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 30W, a 15V adapter might be
current limited at 2.5A. If adapter voltage is less than
(30W/2.5A = 12V) when full power is drawn, the adapter
voltage will be sucked down by the constant 30W load until
it reaches a lower stable state where the switching regu-
lators can no longer supply full load. This situation can be
prevented by utilizing
undervoltage 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,
but an additional adjustable lockout is also available on the
UV pin. Internal lockout is performed by clamping the V
C
pin low. The V
C
pin is released from its clamped state when
the UV pin rises above 6.7V and is pulled low when the UV
pin drops below 6.2V (0.5V hysteresis). At the same time
UV
OUT
goes high with an external pull-up resistor. This
signal can be used to alert the system that charging is
about to start. The charger will start delivering current
about 4ms after V
C
is released, as set by the 0.33µF
