Memory needs its own set of supplies, and they behave differently from most rails: some must stay on while the laptop sleeps, one sits at exactly half of another, and the voltages change from one memory generation to the next. When a laptop gives a memory error, or stays on a black screen with the fan running, the memory rails are among the first things to check. This guide explains each one.
The main memory rails
| Rail | Role | Typical value by generation |
|---|---|---|
| VDDQ (VDD) | Main supply for memory chips and the processor's memory interface | DDR3L 1.35 V, DDR4 1.2 V |
| VTT | Termination supply for address and command lines | Half of VDDQ: about 0.675 V (DDR3L), 0.6 V (DDR4) |
| VPP | Higher activation voltage | DDR4 2.5 V |
| VREF | Reference for the input receivers | Half of VDDQ, very low current |
Low-power memory soldered on thin laptops uses different rails: LPDDR4X, for example, typically uses about 1.8 V, 1.1 V and 0.6 V supplies; LPDDR5 uses a similar set with lower values. DDR5 modules carry their own power-management chip and take a higher input from the board, which then makes the module's rails on the module itself. Always check the values for your board in its schematic — the glossary entries for vddq, vtt and vpp describe the roles in general.
How they are made
VDDQ
VDDQ carries the largest current of the memory rails, so it comes from a buck converter. On many boards a single controller makes both VDDQ and VTT: a switching converter for VDDQ and a small linear regulator for VTT that tracks half of VDDQ.
VTT
VTT has an unusual job: it must both supply and absorb current, because the termination resistors pull lines towards the middle voltage from both sides. That is why it comes from a dedicated sink-and-source regulator rather than an ordinary LDO. VTT is needed only while memory is actively running; it is switched off in sleep.
VREF
VREF is usually made by a resistor divider from VDDQ, sometimes buffered, or by the VTT regulator's reference output. It carries almost no current. On newer memory types parts of the reference are generated inside the chips.
VPP
VPP is a small, higher-voltage rail, typically from a small buck or an LDO. It carries little current but must be present for DDR4 memory to work.
When they turn on
| State | VDDQ | VTT | VPP |
|---|---|---|---|
| Off (S5) | Off | Off | Off |
| Sleep (S3) | On — memory keeps its contents | Off | Usually on |
| Running (S0) | On | On | On |
Memory rails are therefore enabled by the sleep-state signals: VDDQ by the signal that indicates "at least sleep" and VTT by the one that indicates "running". The names vary — SLP_S4# and SLP_S3# on many Intel-based boards, with board-specific names for the enables derived from them. Following the enables in the schematic is covered in Enable Signals in Laptop Power Sequencing.
Diagnosing memory rails
With the laptop powered and the power button pressed:
- Measure VDDQ at the capacitors near the memory or the slot.
- Measure VTT — it should be close to half of VDDQ.
- Measure VPP if the memory type uses it.
- Check VREF — about half of VDDQ.
| Finding | Likely cause |
|---|---|
| VDDQ missing, regulator has input | Enable from sleep-state signal, controller, short on VDDQ |
| VDDQ present, VTT missing | VTT enable, VTT regulator, short on VTT |
| VTT not half of VDDQ | VTT regulator fault or a load pulling it |
| All present, still memory error | Module, slot, SPD bus, data lines — see the memory errors guide |
With power removed, measure each rail to ground. VDDQ shorts are often capacitors or memory chips; VTT shorts are often the termination resistor arrays or capacitors near them. A shorted module in a slot shorts the rail too — remove it and measure again.
The memory error routine continues in Laptop Memory Errors and Beep Codes: Board-Level Causes, and reading the memory pages of a schematic in How to Read the Memory (DDR) Pages of a Laptop Schematic.
Where to measure
Good probe points for memory rails are:
- the bulk capacitors at the output of the VDDQ converter, next to its inductor;
- the small capacitors along the memory slot or around soldered memory chips;
- the termination resistor arrays near the slot, which sit on VTT;
- the VTT regulator's output capacitor, usually a few ceramics next to the controller.
Measuring VDDQ both at the converter and at the memory tells you whether the rail is lost on the way, for example through a damaged via under the slot. For VTT, measure with the laptop fully running: in sleep or soft-off it is supposed to be at zero, and a zero reading at that moment proves nothing. Write the values down; when two boards of the same model are on the bench, comparing the memory rails side by side is the fastest way to spot a small deviation.
Soldered low-power memory
Thin laptops with soldered LPDDR memory have no slot and no VTT in the classic sense, but they have more supply rails per chip, some of them very low voltages. They are often made by a dedicated multi-output regulator or by the processor platform's power chip. The diagnosis is the same — supply, enable, short to ground — but the rail names and values come entirely from the schematic, and a fault in a soldered memory chip usually needs BGA rework.
Sleep problems linked to memory rails
Because VDDQ must stay up during sleep, faults on it show up in a distinctive way: the laptop works but loses everything when it sleeps, or wakes up into a fresh boot. If VDDQ drops in sleep, check its enable path from the "at least sleep" signal and the regulator's behaviour at very light load. A laptop that drains its battery quickly in sleep can also point at a memory rail that stays fully loaded because VTT or another run rail fails to switch off.
Example: black screen, fan running
A laptop powers on with the fan running but shows nothing and never beeps. VDDQ measures 1.2 V and VPP 2.5 V, but VTT reads zero. The VTT regulator's enable — derived from the running-state signal — is active, and its input is present. With power removed, VTT reads under one ohm to ground. Lifting a zero-ohm link between the regulator and the termination network shows the short on the network side: a ceramic capacitor next to the termination resistor arrays is cracked. With it replaced, VTT comes up at 0.6 V and the laptop boots.
FAQ
Why is VTT half of VDDQ?
Termination pulls lines towards the middle of the signal swing, which is half of VDDQ.
Should VTT be present in sleep?
No. In sleep, memory keeps its data on VDDQ alone; VTT is off.
Can a bad memory module short VDDQ?
Yes. Remove the module and measure the rail again to see whether the short goes with it.
Why is my VDDQ slightly different from the nominal value?
Small differences are normal; tolerances and load affect it. Large differences point at a feedback or regulator fault.
Do soldered-memory laptops use the same rails?
The roles are similar, but low-power memory uses different voltages. Check the schematic.
Bottom line
Memory needs VDDQ, VTT, VPP and VREF in the right state at the right time. VDDQ stays on in sleep, VTT only while running and at half of VDDQ. Measure them in that order, check their enables against the sleep-state signals, and look for shorts on termination networks and capacitors as well as on memory chips.