BQ24250C_15 Datasheet, PDF (35/46 Pages) Texas Instruments – bq24250C 2A Single Input I2C, Standalone Switch-Mode Li-Ion Battery Charger with Power-Path Management

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bq24250C
SLUSBY7 – JULY 2014
Typical Application (continued)
9.2.1 Design Requirements
Use the following typical application design procedure to select external components values for the bq24250C
device.
SPECIFICATION
Input DC voltage, VIN
Input current
Charge current
Output regulation voltage
Output regulation voltage
LDO
Table 4. Design Parameters
TEST CONDITION
Recommended input voltage range
Recommended input current range
Fast charge current range
Standalone mode or I2C default mode
I2C host mode: operating in voltage regulation,
programmable range
LDO output voltage
MIN TYP MAX UNIT
4.35
10.5
V
2
A
0.5
2
A
4.2
V
3.5
4.44
V
4.9
V
9.2.2 Detailed Design Procedure
9.2.2.1 Inductor Selection
The inductor selection depends on the application requirements. The bq24250C is designed to operate at around
1 µH. The value will have an effect on efficiency, and the ripple requirements, stability of the charger, package
size, and DCR of the inductor. The 1μH inductor provides a good tradeoff between size and efficiency and ripple.
Once the inductance has been selected, the peak current is needed in order to choose the saturation current
rating of the inductor. Make sure that the saturation current is always greater than or equal to the calculated
IPEAK. The following equation can be used to calculate the current ripple:
ΔI_L = {VBAT (VIN – VBAT)}/(VIN x ƒs x L)
(6)
Then use current ripple to calculate the peak current as follows:
I_PEAK = Load x (1 + ΔI_L/2)
(7)
In this design example, the regulation voltage is set to 4.2V, the input voltage is 5V and the inductance is
selected to be 1µH. The maximum charge current that can be used in this application is 1A and can be set by
I2C command. The peak current is needed in order to choose the saturation current rating of the inductor. Using
equation 6 and 7, ΔI_L is calculated to be 0.224A and the inductor peak current is 1.112A. A 1µF BAT cap is
needed and 22µF SYS cap is needed on the system trace.
The default settings for external fast charge current and external setting of current limit are chosen to be
IFC=500mA and ILIM=1A. RISET and RILIM need to be calculated using equation 1 and 2 in the data sheet.
The fast charge current resistor (RISET) can be set as follows:
RISET=250/0.5A=500Ω
The input current limit resistor (RILIM) can be set as follows:
RILIM= 270/1A=270Ω
The external settings of VIN_DPM can be designed by calculating R1 and R2 according to equation 3 in this data
sheet and the typical application circuit. VIN_DPM should be chosen first along with R1. VIN_DPM is chosen to
be 4.48V and R1 is set to 274KΩ in this design example. Using equation 3, the value of R2 is calculated to be
100KΩ.
In this design example, the application needs to be JEITA compliant. Thus, T_COLD must be 0°C and T_HOT must be
60°C. If an NTC resistor is chosen such that the beta is 4500K and the nominal resistance is 13KΩ, the
calculated R3 and R4 values are 5KΩ and 8.8KΩ respectively. These results are obtained from equation 4 and 5
in this data sheet.
Copyright © 2014, Texas Instruments Incorporated
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