LTC4061 Datasheet, PDF (15/20 Pages) Linear Technology – Standalone Linear Li-lon Battery Charger with Thermistor Input

LTC4061
APPLICATIO S I FOR ATIO
Programming C/10 Current Detection/Termination
In most cases, an external resistor, R_DET, is needed to set
the charge current detection threshold, I_DETECT. However,
when setting I_DETECT to be 1/10th of I_CHG, the I_DET pin
can be connected directly to the PROG pin. This reduces
the component count, as shown in Figure 6.
V_IN
R_PROG
2k
R_DET
2k
V_CC
BAT
C/5 LTC4061
PROG
I_DET TIMER
GND
500mA
+
V_IN
R_PROG
1k
V_CC
BAT
C/5 LTC4061
PROG
I_DET TIMER
GND
500mA
+
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Figure 6. Two Circuits That Charge at 500mA
Full-Scale Current and Terminate at 50mA
When PROG and I_DET are connected in this way, the full-
scale charge current, I_CHG, is programmed with a different
equation:
R_PROG
=
500V
I_CHG
,
I_CHG
=
500V
R_PROG
Stability Considerations
The battery charger constant voltage mode feedback loop
is stable without any compensation provided a battery is
connected. However, a 1µF capacitor with a 1Ω series
resistor to GND is recommended at the BAT pin to reduce
noise when no battery is present.
When the charger is in constant current mode, the PROG
pin is in the feedback loop, not the battery. The constant
current 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
10kΩ; however, additional capacitance on this node reduces
the maximum allowed program resistor value.
Power Dissipation
When designing the battery charger circuit, it is not neces-
sary to design for worst-case power dissipation scenarios
because the LTC4061 automatically reduces the charge
current during high power conditions. The conditions
that cause the LTC4061 to reduce charge current through
thermal feedback can be approximated by considering the
power dissipated in the IC. Most of the power dissipation
is generated from the internal charger MOSFET. Thus, the
power dissipation is calculated to be approximately:
P_D = (V_CC – V_BAT) • I_BAT
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 = 105°C – P_D • θ_JA
T_A = 105°C – (V_CC – V_BAT) • I_BAT • ^θJA
Example: An LTC4061 operating from a 5V wall adapter
is programmed to supply 800mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.3V. Assuming
θ_JA is 40°C/W (see Thermal Considerations), the ambient
temperature at which the LTC4061 will begin to reduce
the charge current is approximately:
T_A = 105°C – (5V – 3.3V) • (800mA) • 40°C/W
T_A = 105°C – 1.36W • 40°C/W = 105°C – 54.4°C
T_A = 50.6°C
The LTC4061 can be used above 50.6°C ambient, but
the charge current will be reduced from 800mA. The ap-
proximate current at a given ambient temperature can be
approximated by:
I_BAT
=
105°C – T_A
(V_CC – V_BAT )• θ_JA
Using the previous example with an ambient tem-
perature of 60°C, the charge current will be reduced to
approximately:
I_BAT
=
105°C – 60°C
(5V – 3.3V)• 40°C /W
=
45°C
68°C /A
I_BAT = 662mA
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