BQ25703A Datasheet, PDF (68/81 Pages) Texas Instruments – I2C Multi-Chemistry Battery Buck-Boost Charge Controller With System Power Monitor and Processor Hot Monitor

bq25703A
SLUSCU1 – MAY 2017
4~6x10uF
(0805)
RACP
10ohm
CACP
15nF
RAC
CDIFF
Open
Q1
RACN
10ohm
1nF+10nF
(0402)
CACN
15nF
www.ti.com
ACP
ACN
HIDRV1
bq2570x
Copyright © 2017, Texas Instruments Incorporated
Figure 48. ACN-ACP Input Filter
9.2.2.3 Inductor Selection
The bq25703A has two 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):
I_SAT ³ I_CHG + (1/2) I_RIPPLE
(3)
The inductor ripple current in buck operation depends on input voltage (V_IN), duty cycle (D_BUCK = V_OUT/V_IN),
switching frequency (f_S) and inductance (L):
I_{RIPPLE_BUCK}
=
V_IN
´
D ´ (1
f_S ´ L
-
D)
(4)
During boost operation, the duty cycle is:
D_BOOST = 1 – (V_IN/V_BAT)
and the ripple current is:
I_{RIPPLE_BOOST} = (VIN × D_BOOST) / (f_S × L)
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. For example, the battery charging
voltage range is from 9 V to 12.6 V for 3-cell battery pack. For 20-V adapter voltage, 10-V battery voltage gives
the maximum inductor ripple current. Another example is 4-cell battery, the battery voltage range is from 12 V to
16.8 V, and 12-V 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.
9.2.2.4 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 in buck mode. 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 5:
I_CIN = I_CHG ´ D × (1 - D)
(5)
68
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