BQ25570_14 Datasheet, PDF (29/41 Pages) Texas Instruments – Ultra Low Power Harvester Power Management IC with Boost Charger, and Nano-Powered Buck Converter

bq25570
www.ti.com
SLUSBH2C – MARCH 2013 – REVISED JANUARY 2014
Boost Charger Inductor Selection
For the boost charger to operate properly, an inductor of appropriate value must be connected between
LBOOST, pin 20, and VIN_DC, pin 2. The boost charger internal control circuitry is designed to control the
switching behavior with a nominal inductance of 22 µH ± 20%. The inductor must have a peak current capability
of > 300 mA with a low series resistance (DCR) to maintain high efficiency.
A list of inductors recommended for this device is shown in Table 2.
INDUCTANCE (µH)
22
22
Table 2.
DIMENSIONS (mm)
4.0x4.0x1.7
3.8x3.8x1.65
PART NUMBER
LPS4018-223M
744031220
MANUFACTURER
Coilcraft
Wuerth
Buck Converter Inductor Selection
For buck converter to operate properly, an inductor of appropriate value must be connected between LBUCK, pin
16, and VOUT, pin 14. The buck converter internal control circuitry is designed to control the switching behavior
with a nominal inductance of 10 µH ± 20%. The inductor must have a peak current capability of > 200 mA with a
low series resistance (DCR) to maintain high efficiency. The speed of the peak current detect circuit sets the
inductor's lower bound to 4.7 µH. When using a 4.7 uH, the peak inductor current will increase when compared
to that of a 10 µH inductor, resulting in slightly higher major frequency.
A list of inductors recommended for this device is shown in Table 3.
INDUCTANCE (µH)
10
10
10
10
10
4.7
Table 3.
DIMENSIONS (mm)
2.0 x 2.5 x 1.2
4.0x4.0x1.7
2.8x2.8x1.35
3.0x3.0x1.5
2.5x2.0x1.2
2.0 x 2.5 x 1.2
PART NUMBER
DFE252012C-H-100M
LPS4018-103M
744029100
74438335100
74479889310
DFE252012R-H-4R7M
MANUFACTURER
Toko
Coilcraft
Wuerth
Wuerth
Wuerth
Toko
Capacitor Selection
In general, all the capacitors need to be low leakage. Any leakage the capacitors have will reduce efficiency,
increase the quiescent current and diminish the effectiveness of the IC for energy harvesting.
VREF_SAMP Capacitance
The MPPT operation depends on the sampled value of the open circuit voltage and the input regulation follows
the voltage stored on the CREF capacitor. This capacitor is sensitive to leakage since the holding period is
around 16 seconds. As the capacitor voltage drops due to any leakage, the input regulation voltage also drops
preventing proper operation from extraction the maximum power from the input source. Therefore, it is
recommended that the capacitor be an X7R or COG low leakage capacitor.
VIN_DC Capacitance
Energy from the energy harvester input source is initially stored on a capacitor, CIN, connected to VIN_DC, pin
2, and VSS, pin 1. For energy harvesters which have a source impedance which is dominated by a capacitive
behavior, the value of the harvester capacitor should scaled according to the value of the output capacitance of
the energy source, but a minimum value of 4.7 µF is recommended.
VSTOR Capacitance
Operation of the bq25570 requires two capacitors to be connected between VSTOR, pin 19, and VSS, pin 1. A
high frequency bypass capacitor of at 0.01 µF should be placed as close as possible between VSTOR and VSS.
In addition, a low ESR capacitor of at least 4.7 µF should be connected in parallel.
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