Datasheet

RIPPLE
COUT RIPPLE
I
I = 0.29 I
2 3
» ´
´
CIN CHG
I = I D × (1 D)´ -
IN
RIPPLE
S
V D (1 D)
I =
Lf
´ ´ -
´
SAT CHG RIPPLE
I I + (1/2) I³
Not Recommended for New Designs
bq24725
www.ti.com SLUS702A –JULY 2010–REVISED NOVEMBER 2010
working life. When the system is totally shutdown, it is not necessary to let the internal BATFET charge pump
work. The host controller can use a digital signal EN to disconnect the battery power path to the VCC pin by U2
in Figure 1. As a result, battery quiescent current can be minimized. The host controller still can get power from
BATFET body diode because the total system current is the lowest when the system is shutdown, so there is no
high conduction loss of the body diode.
Inductor Selection
The bq24725 has three selectable fixed switching frequency. Higher switching frequency allows the use of
smaller inductor and capacitor values. Inductor saturation current should be higher than the charging current
(I
CHG
) plus half the ripple current (I
RIPPLE
):
(4)
The inductor ripple current depends on input voltage (V
IN
), duty cycle (D = V
OUT
/V
IN
), switching frequency (f
S
) and
inductance (L):
(5)
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. For example, the battery charging
voltage range is from 9V to 12.6V for 3-cell battery pack. For 20V adapter voltage, 10V battery voltage gives the
maximum inductor ripple current. Another example is 4-cell battery, the battery voltage range is from 12V to
16.8V, and 12V battery voltage gives the maximum inductor ripple current.
Usually inductor ripple is designed in the range of (20-40%) maximum charging current as a trade-off between
inductor size and efficiency for a practical design.
The bq24725 has charge under current protection (UCP) by monitoring charging current sensing resistor cycle-
by-cycle. The typical cycle-by-cycle UCP threshold is 5mV falling edge corresponding to 0.5A falling edge for a
10mΩ charging current sensing resistor. When the average charging current is less than 125mA for a 10mΩ
charging current sensing resistor, the low side MOSFET is off until BTST capacitor voltage needs to refresh the
charge. As a result, the converter relies on low side MOSFET body diode for the inductor freewheeling current.
Input Capacitor
Input capacitor should have enough ripple current rating to absorb input switching ripple current. The worst case
RMS ripple current is half of the charging current when duty cycle is 0.5. If the converter does not operate at
50% duty cycle, then the worst case capacitor RMS current occurs where the duty cycle is closest to 50% and
can be estimated by Equation 6:
(6)
Low ESR ceramic capacitor such as X7R or X5R is preferred for input decoupling capacitor and should be
placed to the drain of the high side MOSFET and source of the low side MOSFET as close as possible. Voltage
rating of the capacitor must be higher than normal input voltage level. 25V rating or higher capacitor is preferred
for 19-20V input voltage. 10-20μF capacitance is suggested for typical of 3-4A charging current.
Ceramic capacitors show a dc-bias effect. This effect reduces the effective capacitance when a dc-bias voltage is
applied across a ceramic capacitor, as on the input capacitor of a charger. The effect may lead to a significant
capacitance drop, especially for high input voltages and small capacitor packages. See the manufacturer's data
sheet about the performance with a dc bias voltage applied. It may be necessary to choose a higher voltage
rating or nominal capacitance value in order to get the required value at the operating point.
Output Capacitor
Output capacitor also should have enough ripple current rating to absorb output switching ripple current. The
output capacitor RMS current is given:
(7)
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