LTC4054L-4.2 Datasheet, PDF (11/16 Pages) Linear Technology – 150mA Standalone Linear Li-Ion Battery Charger in ThinSOT

LTC4054L-4.2
APPLICATIO S I FOR ATIO
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_PROG
≤
2π
1
• 10⁵ •
C_PROG
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 LTC4054L 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 from 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_Dθ_JA
T_A = 120°C – (V_CC – V_BAT) • I_BAT • θ_JA
PROG
LTC4054L
GND
10k
R_PROG
CHARGE
CURRENT
MONITOR
CIRCUITRY
C_FILTER
4054L42 F02
Figure 2. Isolating Capacitive Load on PROG Pin and Filtering
4054l42f
11