The datasheet for the BQ24725A specifies 10-20uF of input and output capacitance, and further advises care when choosing ceramic capacitors due to capacitance derating at high voltages:
Low ESR ceramic capacitor such as X7R or X5R is preferred for input decoupling capacitor and should be placed to the drain of the high side MOSFET and source of the low side MOSFET as close as possible. Voltage rating of the capacitor must be higher than normal input voltage level. 25V rating or higher capacitor is preferred for 19-20V input voltage. 10-20μF capacitance is suggested for typical of 3-4A charging current.
Ceramic capacitors show a dc-bias effect. This effect reduces the effective capacitance when a dc-bias voltage is applied across a ceramic capacitor, as on the input capacitor of a charger. The effect may lead to a significant capacitance drop, especially for high input voltages and small capacitor packages. See the manufacturer's data sheet about the performance with a dc bias voltage applied. It may be necessary to choose a higher voltage rating or nominal capacitance value in order to get the required value at the operating point.
I'm designing a project with a BQ24725A to charge a 4S Li-Ion battery, with the input power coming from a 24V adapter, but I'm having a hard time finding ceramic capacitors that meet the required 20uF when taking into account voltage derating.
The reference design and the EVM both specify "Capacitor, Ceramic, 10µF, 25V, 10%, X7R, 1206" , but all of the creamic capacitors that I've found derate by 80% or more at 24V, so they'd only provide about 4uF of capacitance.
So far the best solutions I've found are to use 6x or 8x of these: psearch.en.murata.com/.../GRM31CR71H475KA12#.html 4.7uF capacitors, since they only derate to 2.5uF at 24V
I've also been considering low-ESR (~40 mOhm) aluminum polymer capacitors, but they seem to be more expensive and can generally handle less ripple current, or Tantalum, but a tantalum rated to 2x my working voltage (so there's less chance of setting it on fire) is even more expensive, and has similar limitations on the ripple current.
I'd love to hear is anyone:
- Knows the specific part number for the capacitors used on the BQ24725A EVM
- Knows if I really need 20uF of input and output capacitance
- Has guidance or capacitor recommendations that I may have overlooked
