BQ24188_15 Datasheet, PDF (38/52 Pages) Texas Instruments – bq24188 2A, 30V, Host-Controlled Single-Input, Single Cell Switchmode Li-Ion Battery Charger with Power Path Management and USB-OTG Support

bq24188
SLUSC44A – DECEMBER 2014 – REVISED MAY 2015
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Once the inductance has been selected, the peak current must be calculated in order to choose the current
rating of the inductor. Use Equation 5 to calculate the peak current.
I_PEAK
=
I_LOAD(MAX)
´
æ
çè¹
+
%_RIPPPLE
2
ö
÷ø
(5)
The inductor selected must have a saturation current rating greater than or equal to the calculated I_PEAK. Due to
the high currents possible with the bq24188, a thermal analysis must also be done for the inductor. Many
inductors have 40°C temperature rise rating. This is the DC current that will cause a 40°C temperature rise
above the ambient temperature in the inductor. For this analysis, the typical load current may be used adjusted
for the duty cycle of the load transients. For example, if the application requires a 1.5A DC load with peaks at
2.5A 20% of the time, a Δ40°C temperature rise current must be greater than 1.7A:
I_TEMPRISE = I_LOAD + D × (I_PEAK – I_LOAD) = 1.5 A + 0.2 × (2.5 A – 1.5 A) = 1.7 A
The internal loop compensation of the bq24188 is designed to be stable with 10 µF to 150 µF of local
capacitance but requires at least 20 µF total capacitance on the SYS rail (10 µF local + ≥ 10 µF distributed). The
capacitance on the SYS rail can be higher than 150 µF if distributed amongst the rail. To reduce the output
voltage ripple, a ceramic capacitor with the capacitance between 10 µF and 47 µF is recommended for local
bypass to SYS. If greater than 100 µF effective capacitance is on the SYS rail, place at least 10 µF bypass on
the BAT terminal. Pay special attention to the DC bias characteristics of ceramic capacitors. For small case
sizes, the capacitance can be derated as high as 70% at workable voltages. All capacitances specified in this
datasheet are effective capacitance, not capacitor value.
10.2.4 Application Curves
Figure 28. Startup With No Battery
VBAT = 3.8V
VREG = 4.2V
VIN = 5V
ICHG = 0.5V
Figure 29. Battery Detection
38
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