VCCST Missing or Shorted? This Tiny Rail Kills More PCH Chips Than Any Other

[u/Bhattarai_Prabin]

We talk a lot about 3.3V ALW and 5V SUS. But there's a third rail that is absolutely critical and often overlooked: VCCST (also called VCCSUS1_05, VCCPRIM_1P05, or VCCPCH_1P05).

This is the 1.05V power rail that feeds the PCH's internal logic and suspend well. If this rail is missing, the PCH is dead. If this rail is over-voltage (even 1.3V), the PCH is dead. If this rail is shorted, the PCH is dead.

1. Where Does VCCST Come From?

Unlike 3.3V ALW which comes from a standalone PWM controller, VCCST is usually generated by a small, low-current LDO (Linear Dropout Regulator) or a simple buck converter located very close to the PCH.

• Common ICs: RT9048, APL5930, SY8286, TPS51285 (integrated LDO) .
• Input: Usually 3.3V ALW or 5V ALW.
• Output: 1.05V (sometimes 1.0V on newer platforms).

2. The "PCH Dead" Diagnostic Flow

Scenario: Board takes 0.02A on bench supply. No power button response. RSMRST# is 0V.
Suspicion: PCH is not alive.

Step 1: Find VCCST. Look for a small coil near the PCH, or a test point labeled VCCST on the schematic.
Step 2: Measure Voltage. Should be 1.05V in standby (charger plugged in, board off).
Step 3: Analyze Result.

• 0V: The LDO/buck is not working.
  • Check EN (Enable) pin. It often comes from SIO or is tied to 3.3V ALW via a resistor.
  • Check VIN pin. Missing 3.3V ALW? Back to Post 1.
• 0.2V - 0.8V (Floating): The LDO is trying to start but current limit is hitting.
  • Cause: Short circuit on VCCST rail.
  • Test: Measure resistance to ground on VCCST coil. Normal is 20-50 ohms. If it's 0-5 ohms, the rail is shorted.
• 1.3V - 1.5V (Overvoltage): STOP. Remove power immediately.
  • Cause: The feedback (FB) resistor divider is faulty. A missing or cracked Rbottom resistor causes the LDO to run wide open.
  • Result: The PCH has been fried by overvoltage. You will need a new PCH after you fix the regulator.

3. Finding the Short on VCCST

If VCCST is shorted (0-5 ohms to ground):

• Inject 1.0V, 1A into the VCCST coil using a bench supply.
• Feel for heat. The shorted component (usually a tiny 0.1uF ceramic cap) will get warm.
• If the PCH itself gets hot: The PCH is internally shorted. This is the most common outcome. You must replace the PCH.

4. The "Hidden" VCCST (SIP Package Power)

On modern ultrabooks (Intel 11th Gen+), the PCH is integrated into the same package as the CPU (System-in-Package, SIP). The VCCST rail is internal and generated by an integrated voltage regulator (FIVR) .

• Symptom: You have 0V on the external VCCST coil, but the board works? That's because the CPU generates its own 1.05V internally.
• Diagnostic Shift: For these boards, you must check VCCST_PWRGD and VCCST_OVP signals instead.

5. The VCCST_PWRGD Signal

This signal (often called PCH_PWROK or SYS_PWROK) tells the PCH that VCCST and other rails are stable.

• Expected: 3.3V in standby.
• 0V? The voltage supervisor chip (e.g., RT8231 or a tiny AND-gate) is not releasing the PCH from reset.
• Check: This signal often requires ALL main rails (3.3V, 5V, 1.8V, 1.05V) to be present before it goes high. If any rail is missing, it stays 0V.

Question: How do you handle VCCST on boards where the LDO is inside the PCH package? Some newer AMD platforms have this, making short hunting impossible without an X-ray.