Datasheet

9
LTC4068-4.2/LTC4068X-4.2
406842fa
APPLICATIO S I FOR ATIO
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Stability Considerations
The constant-voltage mode feedback loop is stable with-
out an output capacitor, provided a battery is connected to
the charger output. With no battery present, an output
capacitor on the BAT pin is recommended to reduce ripple
voltage. When using high value, low ESR ceramic capaci-
tors, it is recommended to add a 1Ω resistor in series with
the capacitor. No series resistor is needed if tantalum
capacitors are used.
In constant-current mode, the PROG pin is in the feedback
loop, not the battery. The constant-current mode stability
is affected by the impedance at the PROG pin. With no
additional capacitance on the PROG pin, the charger is
stable with program resistor values as high as 20k; how-
ever, additional capacitance on this node reduces the
maximum allowed program resistor. The pole frequency
at the PROG pin should be kept above 100kHz. Therefore,
if the PROG pin is loaded with a capacitance, C
PROG
, the
following equation can be used to calculate the maximum
resistance value for R
PROG
:
R
C
PROG
PROG
≤
π
1
210
5
••
Average, rather than instantaneous charge current may be
of interest to the user. For example, if a switching power
supply operating in low current mode is connected in
parallel with the battery, the average current being pulled
out of the BAT pin is typically of more interest than the
instantaneous current pulses. In such a case, a simple RC
LTC4068-4.2
GND
PROG
R
PROG
10k
C
FILTER
406842 F02
CHARGE
CURRENT
MONITOR
CIRCUITRY
Figure 2. Isolating Capacitive Load on PROG Pin and Filtering
Power Dissipation
It is not necessary to design for worst-case power dissi-
pation scenarios because the LTC4068 automatically
reduces the charge current during high power conditions.
The conditions that cause the LTC4068 to reduce charge
current through thermal feedback can be approximated
by considering the power dissipated in the IC. Nearly all
of this power dissipation is generated by the internal
MOSFET—this is calculated to be approximately:
P
D
= (V
CC
– V
BAT
) • I
BAT
where P
D
is the power dissipated, V
CC
is the input supply
voltage, V
BAT
is the battery voltage and I
BAT
is the charge
current. The approximate ambient temperature at which
the thermal feedback begins to protect the IC is:
T
A
= 120°C – P
D
θ
JA
T
A
= 120°C – (V
CC
– V
BAT
) • I
BAT
• θ
JA
Example: An LTC4068 operating from a 5V supply is
programmed to supply 800mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.3V. Assuming
θ
JA
is 50°C/W (see Thermal Considerations), the ambient
temperature at which the LTC4068 will begin to reduce the
charge current is approximately:
T
A
= 120°C – (5V – 3.3V) • (800mA) • 50°C/W
T
A
= 120°C – 1.36W • 50°C/W = 120°C – 68°C
T
A
= 52°C
charged condition and eliminates the need for periodic
charge cycle initiations. The CHRG output enters a pull-
down state during recharge cycles.
If the battery is removed from the charger, a sawtooth
waveform of approximately 100mV appears at the charger
output. This is caused by the repeated cycling between
termination and recharge events. This cycling results in
pulsing at the CHRG output; an LED connected to this pin
will exhibit a blinking pattern, indicating to the user that a
battery is not present. The frequency of the sawtooth is
dependent on the amount of output capacitance.
filter can be used on the PROG pin to measure the average
battery current, as shown in Figure 2. A 10k resistor has
been added between the PROG pin and the filter capacitor
to ensure stability.