MacBook SMC Circuit and PPBUS_G3H voltage adjustment
This article is for MacBook logic board component-level repair professionals, written by IT-Tech Online, the Mac repair specialist in Melbourne, Australia.
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System Management Controller (SMC) is a programmable chip containing Apple computer management codes. We will use the MacBook Air 13” 2015-2017 logic board schematics 820-00165 as references.
Every Apple computer model has its own unique codes. In other words, an SMC chip from one model can not be used in other models. The markings on the chip may be the same, but the codes in the chip matters.
SMC is a very complex chip. It co-operates with Intel CPU/PCH to manage and monitor MacBook power from start to finish including screen brightness control, MacBook battery charge control and so on. We need to provide SMC power supply, SMC clock and SMC reset signals to kick-start the SMC chip shown on the below diagram:
The PP3V42_G3H power rail created by U7090 will provide main power to the SMC chip U5000 via the BGA ball called VBAT. The PP3V42_G3H also goes through L5001 to power the SMC internal logic model VDDA. The SMC will then create PP1V2_S5_SMC_VDDC for its own sensors management module to monitor system temperatures, voltages and currents. Once the SMC has received power, the SMC crystal circuit will start to output 12 MHZ square clocks for the SMC.
U5110 monitors the voltage of PP3V42_G3H. If PP3V42_G3H is stable and not less than 3.0V, U5100 will assume this power rail is safe to use and output a delayed SMC_RESERT_L signal to reset the SMC chip. The SMC_RESET_L voltage is 3.3V. The delay time is determined by the values of R5101and C5101. U5110 also outputs PP3V3_S5_AVREF_SMC references voltage for the analog-digital conversion function of SMC sensors circuits including current sensors, voltage sensors and temperature sensors. The PP3V3_S5_AVREF_SMC voltage is 3.3V.
On receiving SMC_BC_ACOK signal from charging management chip U7100, SMC will communicate with the MagSafe charger via the one-wired circuit and turn no the green light on the charger. Meanwhile, SMC uses SMBus to communicate with U7100 to adjust the PPBUS_G3H power rail from base voltage 8.1V to the standard 8.45V. You can measure PPBUS_G3H on coil L7130. If the PPUBUS_G3H voltage is around 8.45V, the SMC is functioning now. At least the SMC power, clock, reset and one-wire circuits are all good. Otherwise, you have to solve the SMC issue first.
SMC uses the same SMBus to read battery status to determine the battery charging current. SMS monitors system input current (AMON) and battery charging current (BMON) all the time.
When SMC receives SMC_BC_ACOK signal from the charging circuit PMIC U7100 indicating that the power charger is ready for use, SMC will send out the SMC_PM_G2_EN signal to standby circuit PMIC U7501. This signal will transform the MacBook from G2 state (Off state) to S5 state (deep sleep state).
Checkpoint: SMC_PM_G2_EN is a control signal, the voltage is 3.3V. You can measure it on pin 1 of resistor R8140.
SMC is a very important and complex chip designed by Apple. There is no “new” SMC chip you can buy. It has to be from a donor board with the same model and year. We will discuss more SMC functions later.
Read More:
- MacBook standby circuit and PP3V3_S5, PP3V3_SUS and PM_RSMRST_L signals.
- When will the CPU or PCH shoot out the PM_SLP_S4_L signal?
