LTC4057 Datasheet, PDF (7/12 Pages) Linear Technology – Linear Li-Ion Battery Charger with Thermal Regulation in ThinSOT

LTC4057- 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 charge output. When an output capacitor is used,
especially high value low ESR ceramic types, it is recom-
mended that a 1Ω resistor be placed in series with the
capacitor to stabilize the voltage loop. The loop stability is
determined by the bypass capacitor as well as the effective
series resistance of the battery.
When the battery is disconnected and the LTC4057 is still
powered, the voltage regulation loop should be compen-
sated by placing a capacitor greater than 1µF from the BAT
pin to ground with a 1Ω to 2Ω resistor in series with this
capacitor. Alternatively, powering down the LTC4057 or
placing it into shutdown mode when the battery is discon-
nected avoids this problem.
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 value. The pole fre-
quency 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, battery 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 1. A 10k resistor has
been added between the PROG pin and the filter capacitor
to ensure stability.
LTC4057-4.2
PROG
GND
10k
R_PROG
CHARGE CURRENT
MONITOR CIRCUITRY
C_FILTER
4057 F01
Figure 1. Isolating Capacitive Load on PROG Pin and Filtering
Power Dissipation
The conditions that cause the LTC4057 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_Dθ_JA
T_A = 120°C – (V_CC – V_BAT) • I_BAT • θ_JA
Example: An LTC4057 operating from a 4.5V USB supply
is programmed to supply 600mA full-scale current to a
discharged Li-Ion battery with a voltage of 3.7V. Assuming
θ_JA is 150°C/W (see Board Layout Considerations), the
ambient temperature at which the LTC4057 will begin to
reduce the charge current is approximately:
T_A = 120°C – (4.5V – 3.7V) • (600mA) • 150°C/W
T_A = 120°C – 0.48W • 150°C/W = 120°C – 72°C
T_A = 48°C
The LTC4057 can be used above 48°C ambient, but the
charge current will be reduced from 600mA. The approxi-
mate current at a given ambient temperature can be
approximated by:
I_BAT
=
120°C – T_A
(V_CC − V_BAT )• θ_JA
4057f
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