KB4540 datasheet(10/15 Pages) KINGBOR

10 / 15 page

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 1

Ω 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, CPROG, the

following equation can be used to calculate the maximum

resistance value for RPROG:

PROG

≤

2105

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 KB4540 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:

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:

TA = 120°C – PDθJA

TA = 120°C – (VCC – VBAT) • IBAT • θJA

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-

ing

θJAis150°C/W(seeBoardLayoutConsiderations),the

ambient temperature at which the KB4540 will begin to

reduce the charge current is approximately:

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

PROG

10k

RPROG

CFILTER

CHARGE

CURRENT

MONITOR

CIRCUITRY

KB4540

GND

Figure 2. Isolating Capacitive Load on PROG Pin and Filtering

KB4540

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