Datasheet

bq25504

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SLUSAH0A –OCTOBER 2011–REVISED SEPTEMBER 2012

APPLICATION INFORMATION

INDUCTOR SELECTION

For the bq25504 to operate properly, an inductor of appropriate value must be connected between Pin 16 (LBST)

and Pin 2 (VIN_DC) for the boost converter.

For the boost converter and or charger, the inductor must have an inductance = 22 µH and have a peak current

capability of ≥250 mA with the minimum series resistance to keep high efficiency.

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 leakage be less than 2 nA at 3 V bias.

VIN_DC Capacitance:

Energy from the energy harvester input source is initially stored on a capacitor CHVR tied to Pin 2 (VIN_DC) and

ground (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 an initial value of 4.7 µF is recommended.

:VSTOR Capacitance

Operation of the BQ25504 requires a two capacitors to be connected between Pin 15 (VSTOR) and ground. A

high frequency bypass capacitor of at 0.01 µF should be placed as close as possible between VSTOR and GND.

In addition, a bulk capacitor of at least 4.7 µF should be connected between Pin 15 and ground to assure stability

of the boost converter, especially when the battery is fully charged and the converter in output voltage limiting

mode.

Additional Capacitance on VSTOR or VBAT:

If there are large, fast system load transients and, or the storage element has high resistance, then the CSTOR

capacitors may momentarily discharge below the VBAT_UV threshold in response to the transient. This causes

the bq25504 to turn off the PFET switch between VSTOR and VBAT and turn on the boost converter. Of, the

CSTOR capacitors may further discharge below the VSTOR_CHGEN threshold and cause the bq25504 to enter

Cold Start. For instance, some Li-ion batteries or thin-film batteries may not have the current capacity to meet the

surge current requirements of an attached low power radio. To prevent VSTOR from drooping, either increase

the CSTOR capacitance or add additional capacitance in parallel with the storage element is recommended. For

example, if the bq25504 is configured to charge the storage element to 4.2 V and a 500 mA load transient of 50

µs duration infrequently occurs, then, solving I = C x dv/dt for CSTOR gives :

CSTOR ≥ 500 mA x 50 µs/(4.2 V – 1.8 V) = 10.5 µF (6)

Note that increasing CSTOR is the recommended solution but will cause the bq25504 to operate in the less

efficient Cold Start mode for a longer period at startup compared to using CSTOR = 4.7 µF. If longer Cold Start

run times are not desired, then place the additional capacitance in parallel with the storage element.

For a recommended list of standard components, see the EVM User’s guide (SLUU654).

Product Folder Links :bq25504