Datasheet

LTC4054L-4.2
11
4054l42fa
applicaTions inForMaTion
PROG
10k
R
PROG
C
FILTER
4054L42 F02
CHARGE
CURRENT
MONITOR
CIRCUITRY
LTC4054L
GND
Figure 2. Isolating Capacitive Load on PROG Pin and Filtering
Stability Considerations
The constant-voltage mode feedback loop is stable without
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 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 stabil-
ity 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.
However, 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
≤
1
2π • 10
5
• 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
D
θ
JA
T
A
= 120°C – (V
CC
– V
BAT
) • I
BAT
• θ
JA
Example: An LTC4054L operating from a 6V wall adapter
is programmed to supply 150mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.75V. Assum-
ing θ
JA
is 200°C/W, the ambient temperature at which
the LTC4054L will begin to reduce the charge current is
approximately:
T
A
= 120°C – (6V – 3.75V) • (150mA) • 200°C/W
T
A
= 120°C – 0.3375W • 200°C/W = 120°C – 67.5°C
T
A
= 52.5°C
The LTC4054L can be used above 52.5°C, but the charge
current will be reduced from 150mA. The approximate
current at a given ambient temperature can be approxi-
mated by:
I
BAT
=
120°C – T
A
V
CC
– V
BAT
( )
• θ
JA
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to approxi-
mately:
I
BAT
=
120°C – 60°C
6V – 3.75V
( )
• 200°C/W
=
60°C
450°C/A
I
BAT
= 133mA