Datasheet

10
LTC4054-4.2/LTC4054X-4.2
405442xf
APPLICATIO S I FOR ATIO
WUUU
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 is recommended to reduce ripple voltage. When
using high value, low ESR ceramic capacitors, it is recom-
mended 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
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.
Power Dissipation
The conditions that cause the LTC4054 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 LTC4054 operating from a 5V USB supply is
programmed to supply 400mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.75V. Assum-
ing θ
JA
is 150°C/W (see Board Layout Considerations), the
ambient temperature at which the LTC4054 will begin to
reduce the charge current is approximately:
T
A
= 120°C – (5V – 3.75V) • (400mA) • 150°C/W
T
A
= 120°C – 0.5W • 150°C/W = 120°C – 75°C
T
A
= 45°C
PROG
10k
R
PROG
C
FILTER
405442 F02
CHARGE
CURRENT
MONITOR
CIRCUITRY
LTC4054
GND
Figure 2. Isolating Capacitive Load on PROG Pin and Filtering