BQ24171_15 Datasheet, PDF (27/39 Pages) Texas Instruments – bq24171 JEITA-Compliant Stand-Alone, Switched-Mode Li-Ion and Li-Polymer Battery Charger With Integrated MOSFETs and Power Path Selector

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bq24171
SLUSAF2C – FEBRUARY 2011 – REVISED APRIL 2015
The bq24171 has an internal loop compensator. To achieve good loop stability, the resonant frequency of the
output inductor and output capacitor should be designed from 15 kHz to 25 kHz. The preferred ceramic capacitor
has a 25-V or higher rating, X7R or X5R.
10.2.2.4 Input Filter Design
During adapter hot plug-in, the parasitic inductance and the input capacitor from the adapter cable form a
second-order system. The voltage spike at the AVCC pin may be beyond the IC maximum voltage rating and
damage the IC. The input filter must be carefully designed and tested to prevent an overvoltage event on the
AVCC pin.
There are several methods to damping or limiting the overvoltage spike during adapter hot plug-in. An electrolytic
capacitor with high ESR as an input capacitor can damp the overvoltage spike well below the IC maximum pin
voltage rating. A high current capability TVS Zener diode can also limit the overvoltage level to an IC safe level.
However, these two solutions may not be lowest cost or smallest size.
A cost-effective and small-size solution is shown in Figure 20. R1 and C1 are composed of a damping RC
network to damp the hot plug-in oscillation. As a result, the overvoltage spike is limited to a safe level. D1 is used
for reverse voltage protection for the AVCC pin. C2 is the AVCC pin decoupling capacitor and it should be placed
as close as possible to the AVCC pin. R2 and C2 form a damping RC network to further protect the IC from high
dv/dt and high voltage spikes. The C2 value should be less than the C1 value so R1 can dominant the equivalent
ESR value to provide enough damping effect for hot plug-in. R1 and R2 must be sized to handle in-rush current
power loss according to the resistor manufacturer’s data sheet. Verify the filter component values with a real
application and make any minor adjustments needed to fit in the real application circuit.
If the input is 5 V (USB host or USB adapter), diode D1 can be omitted. R2 must be 5 Ω or higher to limit the
current if the input is reversely inserted.
D1
Adapter
Connector
R1(2010)
2W
C1
2.2 mF
R2(1206)
4.7 - 30 W
AVCC pin
C2
0.1 - 1 mF
Figure 20. Input Filter
10.2.2.5 Input ACFET and RBFET Selection
N-type MOSFETs are used as input ACFET(Q1) and RBFET(Q2) for better cost-effective and small-size solution,
as shown in Figure 23. Normally, there is a total capacitance of 50 µF connected at PVCC node; 10-µF capacitor
for buck converter of bq24171 and 40-µF capacitor for system side. There is a surge current during Q1 turnon
period when a valid adapter is inserted. Decreasing the turnon speed of Q1 can limit this surge current in
desirable range by selecting a MOSFET with relative bigger C_GD and/or C_GS. If Q1 turns on too fast, we must add
external C_GD and/or C_GS. For example, 4.7-nF C_GD and 47-nF C_GS are adopted on EVM while using NexFET
CSD17313 as Q1.
ADAPTER ^Q1
Q2
R_AC
SYS
R_IN
2
C_IN
2.2m
CGS
C_GD
RGS
499k
R11
4.02k
R12
4.02k
C4 1m
CSYS
40m
PVCC
CMSRC
ACDRV
Figure 21. Input ACFET and RBFET
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