Datasheet

R1(2010)

2Ω

2.2μF

0.47-1μF

R2(1206)

10-20 Ω

Adapter

connector

VCC pin

Not Recommended for New Designs

bq24725

www.ti.com

SLUS702A –JULY 2010–REVISED NOVEMBER 2010

The maximum charging current in non-synchronous mode can be up to 0.25A for a 10mΩ charging current

sensing resistor or 0.5A if battery voltage is below 2.5V. The minimum duty cycle happens at lowest battery

voltage. Choose the bottom-side MOSFET with either an internal Schottky or body diode capable of carrying the

maximum non-synchronous mode charging current.

Input Filter Design

During adapter hot plug-in, the parasitic inductance and input capacitor from the adapter cable form a second

order system. The voltage spike at VCC pin maybe beyond IC maximum voltage rating and damage IC. The

input filter must be carefully designed and tested to prevent over voltage event on VCC pin.

There are several methods to damping or limit the over voltage spike during adapter hot plug-in. An electrolytic

capacitor with high ESR as an input capacitor can damp the over voltage spike well below the IC maximum pin

voltage rating. A high current capability TVS Zener diode can also limit the over voltage level to an IC safe level.

However these two solutions may not have low cost or small size.

A cost effective and small size solution is shown in Figure 20. The R1 and C1 are composed of a damping RC

network to damp the hot plug-in oscillation. As a result the over voltage spike is limited to a safe level. D1 is used

for reverse voltage protection for VCC pin. C2 is VCC pin decoupling capacitor and it should be place to VCC pin

as close as possible. C2 value should be less than C1 value so R1 can dominant the equivalent ESR value to

get enough damping effect. R2 is used to limit inrush current of D1 to prevent D1 getting damage when adapter

hot plug-in. R2 and C2 should have 10us time constant to limit the dv/dt on VCC pin to reduce inrush current

when adapter hot plug in. R1 has high inrush current. R1 package must be sized enough to handle inrush current

power loss according to resistor manufacturer’s datasheet. The filter components value always need to be

verified with real application and minor adjustments may need to fit in the real application circuit.

Figure 20. Input Filter

bq24725 Design Guideline

The bq24725 has a unique short circuit protection feature. Its cycle-by-cycle current monitoring feature is

achieved through monitoring the voltage drop across R

DS(on)

of the MOSFETs after a certain amount of blanking

time. For a MOSFET short or inductor short circuit, the over current condition is sensed by two comparators, and

two counters are triggered. After seven occurrences of a short circuit event, the charger will be latched off. To

reset the charger from latch-off status, reconnect the adapter. Figure 21 shows the bq24725 short circuit

protection block diagram.

Product Folder Links :bq24725