BQ24257 Datasheet, PDF (34/45 Pages) Texas Instruments – 2A Single Input I2C, Standalone Switch-Mode Li-Ion Battery Charger with Integrated Current Sense Resistor

bq24257
bq24258
SLUSBG0B – FEBRUARY 2013 – REVISED JULY 2013
APPLICATION INFORMATION
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Inductor Selection
The inductor selection depends on the application requirements. The bq2425x 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 1 μH 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
I_PEAK. 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 = I_LOAD x (1 + ΔI_L/2)
(7)
In this design example, the regulation voltage is set to 4.2 V, the input voltage is 5 V and the inductance is
selected to be 1 µH. The maximum charge current that can be used in this application is 1 A 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, Δ_IL is calculated to be 0.224 A and the inductor peak current is 1.112 A. A 22 µF BAT cap is
needed and 1 µF 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
I_FC = 500 mA and I_LIM = 1 A. R_ISET and R_ILIM need to be calculated using Equation 1 and Equation 2.
The fast charge current resistor (R_ISET) can be set as follows:
R_ISET = 250/0.5A = 500 Ω
(8)
The input current limit resistor (R_ILIM) can be set as follows:
R_ILIM = 270/1A = 270 Ω
(9)
The external settings of V_{IN_DPM} can be designed by calculating R1 and R2 according to equation 3 in this data
sheet and the typical application circuit. V_{IN_DPM} should be chosen first along with R1. V_{IN_DPM} is chosen to be 4.6
V and R1 is set to 274KΩ in this design example. Using Equation 3, the value of R2 is calculated to be 100 KΩ.
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 4500 K and the nominal resistance is 13 KΩ, the
calculated R2 and R3 values are 5 KΩ and 8.8 KΩ respectively. These results are obtained from Equation 4 and
Equation 5.
Layout Guidelines
1. Place the BOOT, PMID, IN, BAT, and LDO capacitors as close as possible to the IC for optimal performance.
2. Connect the inductor as close as possible to the SW pin, and the SYS/CSIN cap as close as possible to the
inductor minimizing noise in the path.
3. Place a 1-μF PMID capacitor as close as possible to the PMID and PGND pins, making the high frequency
current loop area as small as possible.
4. The local bypass capacitor from SYS/CSIN to GND must be connected between the SYS/CSIN pin and
PGND of the IC. This minimizes the current path loop area from the SW pin through the LC filter and back to
the PGND pin.
5. Place all decoupling capacitors close to their respective IC pins and as close as possible to PGND (do not
place components such that routing interrupts power-stage currents). All small control signals must be routed
away from the high-current paths.
6. To reduce noise coupling, use a ground plane if possible, to isolate the noisy traces from spreading its noise
all over the board. Put vias inside the PGND pads for the IC.
7. The high-current charge paths into IN, Micro-USB, BAT, SYS/CSIN, and from the SW pins must be sized
appropriately for the maximum charge current to avoid voltage drops in these traces.
8. For high-current applications, the balls for the power paths must be connected to as much copper in the
board as possible. This allows better thermal performance because the board conducts heat away from the
IC.
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