JAJSL52A april 2020 – february 2021 BQ25968
PRODUCTION DATA
The first step is the determine if an external OVP FET is required in the application. Choosing to include the external OVP FET allows protection of the device if an over-voltage event occurs. If not using the external OVP FET capable part (BQ25970 or BQ25968), it is recommended to have some other TVS mechanism to protect the device.
The next step is to determine the number of CFLY caps to put on each phase of the design. It is important to consider the current rating of the caps, their ESR, and the capacitance rating. Be sure to consider the bias voltage derating for the caps, as the CFLY caps are biased to half of the input voltage, and this will affect their effective capacitance. An optimal system will have four 22-µF caps per phase, for a total of 8 caps per device. The recommended parts for this configuration are shown below, and result in the lowest cost, acceptable efficiency, and acceptable voltage and current ripple. It is possible to use fewer caps, with a minimum recommendation of 3. Using fewer caps will result in higher voltage and current ripple on the output, as well as lower efficiency. Using more than 4 caps per phase will not significantly improve the output voltage or current ripple, or efficiency.
The default switching frequency, fSW, for the power stage is 500 kHz. The switching frequency can be adjusted in register 0x0Bh using the FSW_SET bits. Using a lower switching frequency will increase the efficiency, but also increase the voltage and current ripple. If using 3 22-µF caps per phase, it is recommended to use the default fSW of 500 kHz. If using 4 22-µF caps per phase, either 500 kHz or 300 kHz is recommended.
CAPACITANCE (µF) | SIZE, VOLTAGE RATING, TEMP CHAR | CAPACITOR TYPE | SUPPLIER(1) | COMMENT |
---|---|---|---|---|
20 | 0704, 16 V, X5R | GRMJN7R61C206ME05 | Murata | Lowest ESR (high efficiency) when using four caps |
22 | 0603, 10 V, X5R | GRM188R61A226ME15 | Murata | Lowest Cost and Smallest Size (Recommended) when using four caps |