TRS3221E: TRS3221E charge pump pin voltage

Hello,

V+ and V- are the charge pump output pins. These are the supplies that are used to provide the drive strength to the bus pins. An internal oscillator generates a clock signal that controls internal switches. C1 is oftentimes consider the “flying capacitor” and C3 the “storage capacitor. In cycle 1 - The voltage will charge to VCC on C1, so in this cycle, the voltage across C1 will be VCC. The voltage across C3 (V+ pin) will depend on if the cap is connected to VCC or GND. The typical application block (section 9.2) recommends connecting to GND, but notice in footnote (A) this can also be tied to VCC. The next cycle, the switch will connect C1- to VCC and C1+ to V+. Since the voltage difference on C1 can’t change instantaneously, V+ = 2VCC. The same concept of these switching cycle is true for V-. In cycle 1, C2 charges to V+ and in the next cycle the charge is dumped to V- with polarity reversed.

The functionality of the RS-232 transceiver charge pump is described here: How the RS-232 transceiver’s regulated charge-pump circuitry works (ti.com). I would recommend looking at the figure diagrams broken down by cycle. Additionally, the section on “how regulation works” walks through an example using Vcc = 3.3V and the impact of changing the capacitor values.

Regards, Amy