Next article: MacBook SMC Circuits and PPBUS_G3H voltage adjustment.
We will discuss the early stage of MacBook power-on sequences here. It covers the creation of PP3V42_G3H and PPBUS_G3H power rails and their normal voltages. At this level, in order to discuss in greater detail, we need to make use of Apple schematic diagrams, here we will be taking the 820-3437 MacBook Air 2013 A1466 Intel Haswell CPU logic board as an example.
1. PP3V42_G3H power rail creation.
The always-on 3.42V power rail PP3V42_G3H can be created directly from an internal battery or Magsafe charger shown as below:
The MacBook Battery power goes through R7006, D7005 to U7090 the power management chip. Charger power also can go through R7005, D7005 to U7090. D7005 is used to prevent the 14.5V charger voltage to be delivered directly to the battery. U7090 is a low power 300mA buck converter chip. It converts 8.4V from the battery or 14.5V from the charger to 3.42V PP3V42_G3H power rail. This power rail powers the System Management Controller (SMC). The Mac is in off mode (G3) but the SMC keeps scanning the keyboard power-on button and MagSafe connector for actions.
Checkpoint 1: PP3V42_G3H power rail can be measured at L7095 inductor (coil). PP3V42_G3H voltage is 3.42V.
If you have a PP3V42_G3H low voltage or PP3V42_G3H shorted to ground, you need to make sure the short is not from the load side first and isolate the problem. Then check these components: U7090, R7006, R7005, C7094, R7095 and R7096.
2. PPBUB_G3H power rail creation.
When we connect Magsafe2 to the MacBook, the Magsafe2 will output 3V with a limited current to test the load and detect shorted circuit. If the load is 170K then 14.5V will be output to Magsafe connector. The power management chip U7000 is responsible for creating the 8.45V power rail PPBUS_G3H shown below:
U7000 receives power from pin 2 CHGR_DCIN 14.5V then outputs 5.1V VDD on pin 19. VDD goes through R7101 and comes back as VDDP to power the PWM module of U7000. Charger 14.5V is divided by R7110 and R7111 as 3.81V feeding to pin 3 ACIN for voltage detection. 3.81V is above ACIN threshold 3.2V and SMC_BC_ACOK will be sent out from pin 14 to one-wire circuits. SMC will use the one-wire circuit to read code from the chip on the Magsafe connector to identify the type of charger. A green light indicates that the one-wire circuit and charger are both fine.
Checkpoint 2: The SMC_BC_ACOK is a control signal, the voltage is 3.3V.
When the pin 2 DCIN voltage is higher than pin 17 CSON battery voltage, pin 1 output AGATE 0V and R7186 is effectively shorted to ground. PPDCIN_G3H is the MagSafe charger output, the voltage is 14.5V. This 14.5V will be divided by R7185 and R7186 and 6V will be applied to the gate of Q7180 P-channel dual MOSFET pin 3. Q7180 pin 1 and 2 source voltage is 14.5V and the gate pin 3 is 6V so the MOSFET on the left will be open. 14.5V power will go through the left MOSFET and then the internal diode of the right MOSFET to reach pin 4 and 5. 0.4V drops on the internal diode and the 14.1V power will go through R7120 for current sensing.
When the current reaches to 0.4A, the 8mV voltage drop on R7120 will be sent to U7000 pin 28 CSIP and pin 27 CSIN charger current sensing circuit. U7000 will output 0V CHGR_SGATE and R7181 is effectively shorted to ground. 14.1V will be divided by R7180 and R7181 and 5.4V will be applied to Q7180 gate pin 6. The MOSFET on the right will be fully open to provide high current for more circuit power-up. And the 0.4V voltage drop will disappear. If over-current is detected, U7000 will pull up AGATE and SGATE to close Q7180, cutting off power from the charger.
The PWM module of U7000 then starts to work. Pin 23 PHASE will check the PPBUS_G3H to make sure no short on this power rail and pin 24/21 output UGATE and LGATE to control Q7130 and create the PPBUS_G3H power rail. Pin 6 CELL is used to set the PPBUS_G3H “base” voltage. When CELL pulls up to 3.42V, the PPBUS_G3H “base” voltage is 12.22V (equal to 3-cell battery voltage). When CELL pulls down to 0V, the PPBUS_G3H “base” voltage is 8.1V (equals to 2-cell battery voltage). When CELL opens to float, the PPBUS_G3H “base” voltage is 4.05V (equals to 1-cell battery voltage). PPBUS_G3H “base” voltage is 8.1V for this logic board.
SMC will use SMBus to communicate with U7100 to adjust the PPBUS_G3H power rail from base voltage 8.1V to the standard voltage 8.45V. You can measure PPBUS_G3H on coil L7130.
Checkpoint 3: The standard PPBUS_G3H voltage is 8.45V after SMC adjustment.
If you have a PPUBUS_G3H low issue, check the SMC first. If you have a very low PPBUS_G3H voltage or PPBUS_G3H missing issue, make sure the load side is not shorted to ground then check these components: U 7100, D7105, Q7180, Q7130.
5. iMac Repairs