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 2.5-mm by 2-mm, 1-μH, 1.5-A surface mount multi-layer inductor. The suggested inductor part numbers are shown in Table 11.

Table 11. Inductor Part Numbers

Part Number	Inductance	Size	Manufacturer
LQM2HPN1R0MJ0	1 μH	2.5 × 2.0 mm	Murata
MIPS2520D1R0	1 μH	2.5 × 2.0 mm	FDK
MDT2520-CN1R0M	1 μH	2.5 × 2.0 mm	TOKO
CP1008	1 μH	2.5 × 2.0 mm	Inter-Technical

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 6.3-V 10-μF ceramic capacitors in parallel, that is, Murata GRM188R60J106M.

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

The maximum sense voltage across the sense resistor is 85 mV. To get a better current regulation accuracy, V_(RSNS) should equal 85 mV, 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, that is, Panasonic ERJ2BWGR068.

For 1.5A application, R_(SNS)= 85mV/1.55A = 55 mΩ

4. Measured efficiency and total power loss with different inductors are shown in Figure 29. SW node and inductor current waveform are shown in Figure 37.
Figure 29. Measured Efficiency and Power Loss

9.2.2.1 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 = [(13.6 mV × 1) + (6.8 mV × 0) + (3.4 mV × 1) + 3.4 mV] / 68 mΩ = 200 mA + 0 + 50 mA + 50 mA = 300 mA.

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.

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

9.2.2.2 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, ƒ_o, is approximately 40 kHz (20 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, TI recommends a ceramic capacitor with the capacitance between 4.7 to 47 μF for C_OUT. See previous sections in the Detailed Design Procedure for components selection.