Table 4. Design Parameters

10.2.2.1 Inductor Selection

The bq24171 has a 1600-kHz switching frequency to allow the use of small inductor and capacitor values. Inductor saturation current should be higher than the charging current (I_CHG) plus half the ripple current (I_RIPPLE):

Equation 12.

Inductor ripple current depends on input voltage (V_IN), duty cycle (D = V_OUT/V_IN), switching frequency (fs), and inductance (L):

Equation 13.

The maximum inductor ripple current happens with D = 0.5 or close to 0.5. Usually inductor ripple is designed in the range of 20% to 40% of the maximum charging current as a trade-off between inductor size and efficiency for a practical design.

10.2.2.2 Input Capacitor

The 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 I_CIN occurs where the duty cycle is closest to 50% and can be estimated by the following equation:

Equation 14.

A low ESR ceramic capacitor such as X7R or X5R is preferred for the input decoupling capacitor and should be placed as close as possible to the drain of the high-side MOSFET and source of the low-side MOSFET. The voltage rating of the capacitor must be higher than the normal input voltage level. A 25-V rating or higher capacitor is preferred for a 15-V input voltage. A 20-μF capacitance is suggested for a typical 3-A to 4-A charging current.

10.2.2.3 Output Capacitor

The output capacitor also should have enough ripple current rating to absorb output switching ripple current. The output capacitor RMS current I_COUT is given as:

Equation 15.

The output capacitor voltage ripple can be calculated as follows:

Equation 16.

At certain input/output voltages and switching frequencies, the voltage ripple can be reduced by increasing the output filter LC.

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.

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.

Figure 21. Input ACFET and RBFET