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LH1
Associate
March 25, 2026
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DDR VTT Power Supply Issues

My device is quite power-sensitive. If I use a single DDR3L module with an STM32MP157 or 135, can I eliminate the need for a VTT power supply?

ZAKARIA_C1Best answer
ST Employee
March 25, 2026

Hello @LH1,

Based on ST's AN5692, yes you can eliminate the VTT power supply when using a single DDR3/3L module with the STM32MP13x (and similarly with STM32MP157) by adopting the cost-optimized point-to-point topology.

How It Works

In a standard DDR3L design, VTT is required to terminate the address/command (A/C) bus and reduce signal reflections. However, in a point-to-point topology with a single DRAM, the transmission line stubs are eliminated, making on-board termination unnecessary. Instead, the reflections are controlled via 33 Ω series resistors on every A/C signal, which act as source termination.

Key Design Rules to Follow

  • A/C bus — point-to-point connection, no on-board termination, 33 Ω series resistors on each signal (can be removed if the DDR is placed very close to the SoC)

  • CLK_N / CLK_P — terminated differentially with a single 100 Ω resistor (no VTT needed here either)

  • Data bus (DQ, DQM, DQS) — standard point-to-point, no termination required

Power Sensitivity Benefit

Eliminating VTT is a significant win for power-sensitive designs, since VTT typically mirrors half the VDDQ voltage and can draw tens to hundreds of mA dynamically during bus activity. Removing it reduces both BOM cost and standby/active power consumption.

Important Caveats

  • This topology is only valid for a single DDR module as soon as you add a second chip (x32 or dual-rank), you reintroduce stubs and will likely need termination again

  • Placement still matters: keep the DDR3L as physically close as possible to the STM32MP1x to minimize trace lengths and benefit from removing the 33 Ω resistors entirely

  • Always validate signal integrity with your specific PCB stackup, especially if traces exceed a few centimeters

For more details on DDR topology and cost-optimized designs, please refer to AN5692 available on www.st.com.

Best Regards,
Zakaria

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ZAKARIA_C1Best answer
ST Employee
March 25, 2026

Hello @LH1,

Based on ST's AN5692, yes you can eliminate the VTT power supply when using a single DDR3/3L module with the STM32MP13x (and similarly with STM32MP157) by adopting the cost-optimized point-to-point topology.

How It Works

In a standard DDR3L design, VTT is required to terminate the address/command (A/C) bus and reduce signal reflections. However, in a point-to-point topology with a single DRAM, the transmission line stubs are eliminated, making on-board termination unnecessary. Instead, the reflections are controlled via 33 Ω series resistors on every A/C signal, which act as source termination.

Key Design Rules to Follow

  • A/C bus — point-to-point connection, no on-board termination, 33 Ω series resistors on each signal (can be removed if the DDR is placed very close to the SoC)

  • CLK_N / CLK_P — terminated differentially with a single 100 Ω resistor (no VTT needed here either)

  • Data bus (DQ, DQM, DQS) — standard point-to-point, no termination required

Power Sensitivity Benefit

Eliminating VTT is a significant win for power-sensitive designs, since VTT typically mirrors half the VDDQ voltage and can draw tens to hundreds of mA dynamically during bus activity. Removing it reduces both BOM cost and standby/active power consumption.

Important Caveats

  • This topology is only valid for a single DDR module as soon as you add a second chip (x32 or dual-rank), you reintroduce stubs and will likely need termination again

  • Placement still matters: keep the DDR3L as physically close as possible to the STM32MP1x to minimize trace lengths and benefit from removing the 33 Ω resistors entirely

  • Always validate signal integrity with your specific PCB stackup, especially if traces exceed a few centimeters

For more details on DDR topology and cost-optimized designs, please refer to AN5692 available on www.st.com.

Best Regards,
Zakaria

In order to give better visibility on the answered topics, please click on 'Best answer' on the reply which solved your issue or answered your question.
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