TUSB1002: AC coupling

Ishiwata-san

1. Please use TUSB1002A instead TUSB1002

2. Make sure HD3SS3220 RX1 and RX2 are AC coupled, recommend value is 0.22uF

3. HD3SS3220 and TUSB1002A DC coupled

4. TUSB1002A RX2 AC coupling is optional. If used, the value is 0.33uF.

Thanks

David

Ishiwata-san

Please see below block diagram.

Thanks

David

Hi Ishiwata-san,

David is out of the office so I will answer your questions until he returns.

1. You want to keep the total capacitance on the USB line within the spec for USB. Having capacitors on both sides of the HD3SS3220 could drop the series capacitance below 75nF (the lowest allowable value for USB3 signals). It is ok to DC couple the MUX to the 1002A as the MUX simply connects TX to TX1/2 and RX to RX1/2. Since you have capacitors on TX1/2 and RX1/2, DC coupling to the 1002A is ok.

2. TX and RX capacitors have different recommended values based on the allowable capacitance of a USB signal line. The USB spec requires that all transmitters (TX) be AC coupled while receiver (RX) capacitors are optional.

The total capacitance must stay between 75 - 265nF. This includes the capacitor on the TX side and the capacitor on the RX side. Since a transmitter will usually have AC capacitors on the lower end (between 100nF - 220nF), the RX should have a capacitance on the higher end (closer to 330nF). This is so the total capacitance stays above 75nF, as when you add capacitors in series the total capacitance drops. 

3. Depending on the device plugged in, 220nF or higher RX capacitors could work. For example, if the AC capacitor on the device TX is 220nF, then a 220nF RX capacitor is ok because the total capacitance is still above 75nF. If the device has a lower value capacitor on the transmitter, a higher value RX capacitor would be better (usually 330nF).

Best,

Shane

Ishiwata-san

For question 1, the HD3SS3220 requires a bias voltage, the designer must place the capacitors on one side of the switch. We choose to place the capacitors between the HD3SS3220 and the connector because we can't always guarantee the common mode voltage of the external device connected to the connector meets the bias voltage requirement of HD3SS3220. 

With the capacitors between HD3SS3220 and the connector, we then choose to DC couple between TUSB1002A and HD3SS3220 so the TUSB1002A common mode voltage would bias the HD3SS3220. The TUSB1002A common mode voltage also meets the HD3SS3220 bias voltage requirement.

If you choose to place bias resistors on both sides of the switch, an external biasing voltage should be provided. The external biasing voltage must meet HD3SS3220 bias voltage requirement.

Thanks

David