D+ PullUp Voltage Less

bsuthar · ‎2025-09-23

Hello Team,

Good Day !

Considering USB Specification , D+ line Pull-Up Should be high Once USB Connectors Plugs in , I Tried to Implement Following SCH to accomplish it.

3.3V Enabled through Transistor Q4 (MMBT2222ALT1G).

VBUS Signal Comes from Voltage Divider Network when USB Plugs in , See Below :

J14 is USB 2.0 Type B Connector.

Issue is When I plugged USB , Voltage Down to 2.5VDC from 2.8V @ L2 pin 4 ( i.e. D+ line). Ideally , It should be (3.3V- Vce ).

As per USB specs. It should be Minimum 2.8V on D+ pull up.

Device is : STM32F303

Regards,

Bhupendra

FBL · ‎2025-09-23

Hi @bsuthar

Use a PNP transistor as a high-side switch for the D+ pull-up resistor
Make sure that Pull-up resistor on D+ line must be connected to 3.3 V rail when USB is plugged in.
Here the implementation provided for eval board F303.

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Best regards,
FBL

bsuthar · ‎2025-09-23

Thanks for Suggestion , FBL

I believe U5V = 5VDC from USB and VDD = 3.3V Correct ?

Is Circuit snap From Nucleo-F303RE board ?

Is It good Idea to use MOSFET ( P-Channel ) instead of Transistor ?

FBL · ‎2025-09-24

Hi @bsuthar

In such case, the existing NPN transistor solution is acceptable and simpler to implement.

It's snap from STM32303E-EVAL | Product

Here is the schematics

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Best regards,
FBL

bsuthar · ‎2025-09-25

Hello FBL,

Sorry , I could not get your comment " In such case, the existing NPN transistor solution is acceptable and simpler to implement."

Can you pls elaborate ?

FBL · ‎2025-09-25

Hi @bsuthar

NPN BJT is simpler drive (base resistor), since the current is low, the existing NPN as provided are sufficient. If heat dissipation are critical, you can provide proper gate drive voltage, a P-Channel MOSFET is often a better choice for high-side switching compared to BJTs.

According to your implementation, you didn't precisely mention R45 value.

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Best regards,
FBL

bsuthar · ‎2025-09-25

Hello FBL,

R45 = 1.5K as per USB FS Specification on D+ line.

With Given Schematic in First Post , I tried Both , with NPN Transistor and N-Channel Mosfet.

I received Lower Voltage in case of MOSFET at Output of R45 ( i.e 2.4V ) as compared to Transistor.

In Both the case , Volatge on D+ line was Less than Required.

Moreoevr, your suggested eval board F303 schematic uses NPN as High side switch.

Regards,

Bhupendra