9
LT1769
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APPLICATIONS INFORMATION
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Input and Output Capacitors
In the 2A Lithium-Ion 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 the output charge 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
are possible 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. Selecting a high 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. Sanyo OS-CON can also be used.
The output capacitor (C
OUT
) is also assumed to absorb
output switching ripple current. The general formula for
capacitor ripple 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. Beads or inductors can
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 raised to 4Ω with a bead or inductor, only 5% of the
ripple current will flow into the battery.
Soft-Start and Undervoltage Lockout
The LT1769 is soft-started by the 0.33µF capacitor on the
V
C
pin. On start-up, the V
C
pin voltage will quickly rise to
0.5V, then ramp at a rate set by the internal 45µA pull-up
current and the external capacitor. Charge current starts
ramping up when V
C
pin voltage reaches 0.7V and full
current is achieved with V
C
at 1.1V. With a 0.33µF capaci-
tor, the 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 time-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 the 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 25W, a 15V adapter might be
current limited at 2A. If adapter voltage is less than
(25W/2A = 12.5V) when full power is drawn, the adapter
voltage will be pulled down by the constant 25W 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 LT1769.
This 7V threshold can be increased by adding a resistive
divider to the UV pin as shown in Figure 2. 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 7V and is pulled low when the UV pin drops below
6.5V (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 capacitor. A resistor divider
is used to set the desired V
CC
lockout voltage as shown in
Figure 2. A typical value for R6 is 5k and R5 is found from:
R5 =
R6(V – V )
V
UV
UV
IN
