TRS3122E: Charge Pump Selection

Dylan,

An engineer has been notified of this post and will respond as soon as possible.

Regards,

Eric Hackett 

Dylan,

Here is a blog explaining the circuit design and how different capacitor value affects the waveforms.

How the RS-232 transceiver’s regulated charge-pump circuitry works - Analog - Technical articles - TI E2E support forums

Roughly speaking, it's OK to use different values as long as the ratio is contained. 

Regards,

Hao

Hi Hao,

My customer have two additional questions about the capacitor selection. Could you help give some suggestions?

1. In the blog, it only states the relationship between C1 and C3 capacitors, but not how to select the capacitor value of C1. Could you help give some insights of how to calculate C1?

2. In the blog, it shows that frequency will go lower if ripple becomes larger, and frequency will become larger if ripple becomes smaller. In the real application, such as bike, whether lower ripple range will be higher priority or the lower the ripple frequency?

Thanks!

Dylan

Dylan,

C1 is also called the flying capacitor. Reducing it increases the charge pump's output impedance. The datasheet values were chosen to support the worst load in applications.  

The ripple and frequency tradeoff depends on the system design, like some specific EMI requirement. As long as the peak voltage of ripples is less than the abs max, there are no reliability concerns.