BQ25731: feasability very low charging current <300mA

Values Based on 5x Panasonic BK3 10 CH for the BQ25731

Inductor Selection

I_CHG

Charge current

333

mA/Pack @ 5 Cells

I_RIPPLE

Inductor current ripple peak-to-peak value

2930

mA

I_SAT ≥ I_CHG + (0.5)I_RIPPLE

Minimum Saturation Current of the Inductor

1798

mA

V_IN

Input Voltage to the Charger

24

V

V_BAT

Maximum Battery Charge Voltage

7,5

V

V_OUT

USB OTG Output Voltage

24

V

P_OUT

USB OTG Maximum Power Output

20

W

f_S

Switching Frequency

800

kHz

L

Inductance

2,20

µH

D_BUCK = V_OUT/V_IN

Duty Cycle Buck

0,31

I_{RIPPLE_BUCK} = V_IN × D_BUCK× (1-D_BUCK) / (f_S × L)

Inductor Ripple Current in Buck Mode

2930

mA

D_BOOST = 1 – (V_OUT/V_BAT)

Duty Cycle Boost (USB OTG Boost Converter)

0,69

I_{RIPPLE_BOOST} = (V_OUT × D_BOOST) / (f_S × L)

Inductor Ripple Current in Boost Mode

2930

mA

η

efficiency of the Buck Converter

80

%

I_{L(DC_MAX)} = (V_BAT × I_CHG) / (V_IN ×  η)

Max DC current

130,21

mA

η

efficiency of the Boost Converter

90

%

I_OUT(MAX)

Maximum Supply Current from BMS

833

mA

I_{L(DC_MAX)} = (V_OUT × I_OUT(MAX)) / (V_IN ×  η)

Max DC current

2963

mA

Safety Factor

Aditional margin for component ratings

50

%

I_{CIN_BOOST} = I_CHG × √(D_BOOST × (1 - D_BOOST))

154,50

mA

Inductor Values

L

2,2µH

I_SAT

> 2700mA

I_{L(DC_MAX)}

> 4440mA