BQ24187 Datasheet, PDF (32/44 Pages) Texas Instruments – 1A USB-OTG Support

bq24187
SLUSBM0 – APRIL 2014
Not Recommended for New Designs
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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 and Equation 6 to calculate the peak current.
%_RIPPLE
=
V_IN
- V_BAT
L
´
V_BAT
V_IN ´ ¦_SW
(5)
For the 5V adapter case, a good rule of thumb is to use 3.5V as V_BAT. This provides a reasonable worst case
ripple. For higher adapters, the closer to 50% duty cycle, the worse the ripple.
I_PEAK
=
I_CHARGE
´
çæ¹⁺
è
%_RIPPPLE
2
ö
÷
ø
(6)
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 bq24187, 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 ) = 1.5A + 0.2 ´ (2.5A - 1.5A) = 1.7A
(7)
The internal loop compensation of the bq24187 is designed to be stable with 10µF to 150µF of local capacitance
but requires at least 20µF total capacitance on CS+. To reduce the output voltage ripple, a ceramic capacitor
with the capacitance between 20µF and 47µF is recommended for local bypass to CS+. If more than 100µF is
placed on CS+, place at least 10µF from BAT to GND.
9.2.3 Application Curves
Figure 17. Boost Startup No Load
Figure 18. Boost Startup 1A Load
32
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