MacBook Power Up Circuits
At this level, in order to discuss in greater detail, we need to make use of schematic diagrams, here we will be taking the 820-3437 MacBook Air 2013 A1466 Intel Haswell CPU logic board as an example.
1. The always-on 3.42V power rail PP3V42_G3H can be created directly from an internal battery or Magsafe charger shown as below:
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. 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 can be measured at L7095 inductor (coil).
2. If you connect Magsafe2 to the Mac, the Magsafe2 will output 3V with a limited current to test the load and detect shorted circuits. If the load is 170K then 14.5V will be output to Magsafe connector. The power management chip U7000 is responsible for creating the 8.4V 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.
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. Charger power PPDCIN_G3H 14.5V is 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.