BQ25703A Datasheet, PDF (67/81 Pages) Texas Instruments – I2C Multi-Chemistry Battery Buck-Boost Charge Controller With System Power Monitor and Processor Hot Monitor

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Typical Application (continued)
DESIGN PARAMETER
Battery Charge Current⁽²⁾
Minimum System Voltage⁽²⁾
bq25703A
SLUSCU1 – MAY 2017
EXAMPLE VALUE
3072 mA for 2s battery
614 mV for 2s battery
9.2.2 Detailed Design Procedure
The parameters are configurable using the evaluation software. The simplified application circuit (see Figure 46,
as the application diagram) shows the minimum component requirements. Inductor, capacitor, and MOSFET
selection are explained in the rest of this section. Refer to the EVM user's guide (SLUUBG6) for the complete
application schematic.
9.2.2.1 Input Snubber and Filter for Voltage Spike Damping
During adapter hot plug-in, the parasitic inductance and input capacitor from the adapter cable form a second
order system. The voltage spike at VBUS 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 VBUS pin.
There are several methods to damp 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 47. 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 VBUS pin. C2 is VBUS pin decoupling capacitor and it should be placed as
close as possible to VBUS pin. C2 value should be less than C1 value so R1 can dominate 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 10 µs time constant to limit the dv/dt on VBUS 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 data sheet. The filter components' value always need to
be verified with real application and minor adjustments may need to fit in the real application circuit.
D1
R1(2010)
Adapter ^2W
connector _C1
2.2mF
R2(0805)
1W
VBUS pin
C2
0.47-1mF
Figure 47. Input Filter
9.2.2.2 ACP-ACN Input Filter
The bq25703A has average current mode control. The input current sensing through ACP/ACN is critical to
recover inductor current ripple. Parasitic inductance on board will generate high frequency ringing on ACP-ACN
which overwhelms converter sensed inductor current information, so it is difficult to manage parasitic inductance
created based on different PCB layout. Bigger parasitic inductance will generate bigger sense current ringing
which will cause the average current control loop to go into oscillation.
For real system board condition, we suggest to use below circuit design to get best result and filter noise induced
from different PCB parasitic factor. With time constant of filter from 47 nsec to 200 nsec, the filtering on ringing is
effective and in the meantime, the delay of on the sensed signal is small and therefore poses no concern for
average current mode control.
Copyright © 2017, Texas Instruments Incorporated
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