Tesla Model 3 front control board control and monitoring of multiple vehicle-mounted low-voltage electrical components, such as the body sensor, external lighting system, and door and window control modules. The 5V low-voltage power supply is the core of its internal chip and sensor work, and this stable 5V voltage is output by the LMR14020 DC-DC buck converter driver chip, which converts the vehicle’s 12V low-voltage battery power into the precise 5V required by the internal components.
What are the functions of the LMR14020 IC?
LMR14020 is a high-performance, 8pins DC-DC buck (step-down) power management IC tailored for automotive applications. Its internal MOSFET integration simplifies circuit design while ensuring reliable voltage conversion.
1. BOOT (Pin 1) - Bootstrap Boost Pin: This pin works with an external 0.1 μF capacitor connected between BOOT and SW pins. The capacitor’s core role is to boost the voltage, enabling proper operation of the internal MOSFET.
2. VIN (Pin 2) - Power Input Pin: Directly receives 12V power supply, serving as the primary energy source for the IC.
3. EN (Pin 3) - Enable Pin: The IC activates and starts working only when this pin is at a high level. Under normal operating conditions, the voltage at Pin 3 is maintained at 12V.
4. RT (Pin 4) - Frequency Setting Pin: A resistor connected between this pin and GND determines the IC’s operating frequency. This resistor is critical—any replacement must use the original resistance value to avoid performance issues.
5. FB (Pin 5) - Feedback Pin: Adjusts the output voltage by monitoring the feedback signal, ensuring the 5V output remains stable.
6. SS (Pin 6) - Soft-Start Control Pin: Connected to an external capacitor that sets the soft-start time. This circuit controls the slope of the output voltage, preventing excessive inrush current and avoiding unwanted voltage overshoots or drops during IC startup.
7. GND (Pin 7) - Ground Pin: Provides the necessary ground reference for the IC’s internal circuits.
8. SW (Pin 8) - Output Pin: Internally connected to the drain (D) of the switching MOSFET, serving as the terminal for the converted voltage output.
How does 5V power supply circuit work?
The conversion of 12V to 5V by the LMR14020 is a sequential, precision-controlled process. It can be divided into four key stages, each building on the previous one to ensure stable and efficient power output.
Stage 1: Power Supply and IC Activation.
The VBAT_PROT first passes through diode D19361 and fuse inductor FB017968 to supply 12V to Pin 2 (VIN) of the LMR14020. Simultaneously, a pull-up resistor R017955 provides the enable voltage to Pin 3 (EN).
Diode D19361: Acts as a one-way switch. It conducts forward to supply power to Pin 2 but blocks reverse current, preventing voltage at Pin 2 from flowing back to VBAT_PROT.
Fuse Inductor FB017968: Provides overcurrent protection. If a short circuit occurs in the subsequent stage, the large current flowing through FB017968 will cause it to blow, shielding the front-stage circuit from damage.
With 12V at both VIN (Pin 2) and EN (Pin 3), the IC is activated and enters the standby state for further operations.
Stage 2: Frequency Setting and Soft-Start Initialization.
After activation, the RT and SS pins immediately start their respective functions:
RT Pin (Pin 4): The external resistor to GND locks in the IC’s operating frequency. This parameter is pre-calibrated for the Model 3’s front control panel, so resistor replacement must strictly follow the original specifications.
SS Pin (Pin 6): The external capacitor begins charging, setting the soft-start time. This process ensures the output voltage rises gradually, avoiding inrush current that could damage sensitive components in the front control panel.
Stage 3: Bootstrap Boost (Critical for MOSFET Operation).
When the VIN, EN, SS, and RT pins are all functioning normally, the IC initiates the bootstrap boost process via the BOOT pin and its external capacitor. This stage is vital because the LMR14020 uses internal N-channel MOSFETs, and the upper MOSFET requires a higher gate voltage to conduct stably.
N-channel MOSFETs conduct when the gate-source voltage (Vgs) exceeds the threshold voltage (typically 4.5V). If only the 12V VIN were used to drive the upper MOSFET’s gate, the Vgs would drop to 0V once the MOSFET conducts, causing it to turn off immediately. The bootstrap capacitor solves this by boosting the gate voltage to 24V.
Stage 4: Stable 5V Output.
With successful bootstrap boosting, the IC drives the MOSFETs to switch, initiating the 12V-to-5V conversion and voltage regulation cycle:
1. MOSFET Conduction Phase: When the MOSFETs are fully on, they act as a low-resistance path. The inductor stores energy, with current flowing from left to right, while the output capacitor charges gradually.
2. Voltage Feedback and Regulation: Resistors R17441-R19440 and R19439 form a voltage divider, feeding a portion of the output voltage to the FB pin (Pin 5). The FB pin connects to the inverting input of the IC’s internal comparator, while the non-inverting input is tied to a 0.75V reference voltage.
3. If the output voltage exceeds 5V, the FB voltage exceeds 0.75V. The comparator’s inverting input voltage becomes higher than the non-inverting input, so it outputs a low level, and the IC stops driving the MOSFETs.
4. MOSFET Turn-Off Phase: When the MOSFETs turn off, the inductor following its characteristic that current cannot change abruptly generates a reverse electromotive force. This reverses the current direction, and the inductor releases stored energy to continue powering the load.
5. Stable Output Maintenance: The inductor discharges until the output voltage drops below 5V, which lowers the FB voltage below 0.75V. The IC then re-drives the MOSFETs to conduct, repeating the cycle. The output capacitor filters and stores energy, resulting in a stable 5V supply for the front control panel.
Tesla Model 3 front control board 5V power supply DC-DC circuit is a key link connecting the vehicle 12V low-voltage system and the internal precision control components. Its stable and reliable design ensures the normal operation of the front body control system and further guarantees the safety and comfort of the vehicle. With the continuous upgrading of Tesla’s electronic and electrical architecture, the integration and intelligence level of this circuit will also be further improved, which puts forward higher requirements for after-sales maintenance and technical research.
Tesla Model 3 Front Control Panel 5V Power Supply DC-DC Circuit Principle
