Table 10. Design Parameters

10.2.2.1 Systems Design Specifications

• VBUS = 5 V
• V_BAT = 4.2 V (1-Cell)
• I_(charge) = 1.25 A
• Inductor ripple current = 30% of fast charge current

1. Determine the inductor value (L_OUT) for the specified charge current ripple:

, the worst case is when battery voltage is as close as to half of the input voltage.

Equation 1.

L_OUT = 1.11 μH

Select the output inductor to standard 1 μH. Calculate the total ripple current with using the 1-μH inductor:

Equation 2.
Equation 3.

ΔI_L = 0.42 A

Calculate the maximum output current:

Equation 4.
Equation 5.

I_LPK = 1.46 A

Select 2.5mm by 2mm 1-μH 1.5-A surface mount multi-layer inductor. The suggested inductor part numbers are shown as following.

Table 11. Inductor Part Numbers⁽¹⁾

PART NUMBER	INDUCTANCE	SIZE	MANUFACTURER
LQM2HPN1R0MJ0	1 μH	2.5 x 2.0 mm	Murata
MIPS2520D1R0	1 μH	2.5 x 2.0 mm	FDK
MDT2520-CN1R0M	1 μH	2.5 x 2.0 mm	TOKO
CP1008	1 μH	2.5 x 2.0 mm	Inter-Technical

(1) See Third-Party Products discalimer
2. Determine the output capacitor value (C_OUT) using 40 kHz as the resonant frequency:
Equation 6.
Equation 7.
Equation 8.

C_OUT = 15.8 μF

Select two 0603 X5R 6.3V 10-μF ceramic capacitors in parallel i.e., Murata GRM188R60J106M.

3. Determine the sense resistor using the following equation:
Equation 9.

The maximum sense voltage across the sense resistor is 85 mV. In order to get a better current regulation accuracy, V_(RSNS) should equal 85mV, and calculate the value for the sense resistor.

Equation 10.

R_(SNS) = 68 mΩ

This is a standard value. If it is not a standard value, then choose the next close value and calculate the real charge current. Calculate the power dissipation on the sense resistor:

P_(RSNS) = I_(CHARGE) ² × R_(SNS)

P_(RSNS) = 1.25² × 0.068

P_(RSNS) = 0.106 W

Select 0402 0.125-W 68-mΩ 2% sense resistor, i.e. Panasonic ERJ2BWGR068.

4. Measured efficiency and total power loss with different inductors are shown in Figure 26 and Figure 27. SW node and inductor current waveform are shown in Figure 2.

TA = 25°C

VBUS = 5 V

VBAT = 3 V

Figure 26. Measured Battery Charge Efficiency

TA = 25°C

VBUS = 5 V

VBAT = 3 V

Figure 27. Measured Battery Charge Loss

10.2.2.2 Charge Current Sensing Resistor Selection Guidelines

Both the termination current range and charge current range depend on the sensing resistor (R_SNS). The termination current step (I_{OTERM_STEP}) can be calculated using Equation 11:

Equation 11.

Table 12 shows the termination current settings for three sensing resistors.

For example, with a 68-mΩ sense resistor, V_(ITERM2)=1, V_(ITERM1)=0, and V_(ITERM0)=1, I_TERM = [ (13.6mV x 1) + (6.8mV x 0) + (3.4mV x 1) + 3.4mV ] / 68mΩ = 200mA + 0 + 50mA + 50mA = 300mA.

The charge current step (I_{O(CHARGE_STEP)}) is calculated using Equation 12:

Equation 12.

Table 13 shows the charge current settings for three sensing resistors.

Using bq24156A as an example, with a 68-mΩ sense resistor, V_(CHRG3)=1, V_(CHRG2)=0, V_(ICHRG1)=0, and V_(ICHRG0)=1, I_TERM = [ (54.4mV x 1) + (27.2mV x 0) + (13.6mV x 0) + (6.8mV x 1) + 37.4mV ] / 68mΩ = 800mA + 0 + 0 + 100mA = 900mA.

10.2.2.3 Output Inductor and Capacitance Selection Guidelines

The IC provides internal loop compensation. With the internal loop compensation, the highest stability occurs when the LC resonant frequency, f_o, is approximately 40 kHz (20 kHz to 80 kHz). Equation 13 can be used to calculate the value of the output inductor, L_OUT, and output capacitor, C_OUT.

Equation 13.

To reduce the output voltage ripple, a ceramic capacitor with the capacitance between 4.7 μF and 47 μF is recommended for C_OUT, see the application section for components selection.