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KB4540
APPLICATIONS INFORMATION
CHARGE
Stability Considerations
10k
CURRENT
MONITOR
CIRCUITRY
PROG
KB4540
GND
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.
R
C
FILTER
PROG
Figure 2. Isolating Capacitive Load on PROG Pin and Filtering
Power Dissipation
The conditions that cause the KB4540 to reduce charge
current through thermalfeedback can be approximatedby
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:
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, CPROG, the
following equation can be used to calculate the maximum
PD = (VCC – VBAT) • IBAT
where PD is the power dissipated, VCC is the input supply
voltage, VBAT is the battery voltage and IBAT is the charge
current. The approximate ambient temperature at which
the thermal feedback begins to protect the IC is:
resistance value for RPROG
:
TA = 120°C – PDθJA
1
RPROG
≤
TA = 120°C – (VCC – VBAT) • IBAT • θJA
2π 105 CPROG
Example: An KB4540 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-
ingθJA is150°C/W(seeBoardLayoutConsiderations),the
ambient temperature at which the KB4540 will begin to
reduce the charge current is approximately:
Average, rather than instantaneous, charge current may
beofinteresttotheuser.Forexample,ifaswitchingpower
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.
TA = 120°C – (5V – 3.75V) • (400mA) • 150°C/W
TA = 120°C – 0.5W • 150°C/W = 120°C – 75°C
TA = 45°C
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KINGBOR [ KINGBOR TECHNOLOGY CO ]
