Automotive-grade Step-down Converter Improves Reliability for “Cold Cranking”
Supporting up to 6 A of load current, the MP4316A from Monolithic Power Systems offers a wide input range for automotive battery applications.
Monolithic Power Systems has released the MP4316A, the automotive-qualified version of the MP4316 synchronous step-down converter.
Key Features for Automotive Battery Use Cases
High-duty cycle capabilities, along with a low-dropout (LDO) mode and internal bootstrap charging circuit, allow the MP4316A to withstand low battery voltages during automotive cold-cranking conditions.
The MP4316A can support a wide operating input voltage range of 3.3 V to 45 V. The switching frequency (fSW) for the buck conversion is configurable from 350 kHz to 1000 kHz, well suited to car battery applications. Internal Frequency Spread Spectrum (FSS) modulation helps to mitigate EMI noise energy at the nominal switching frequency and associated harmonics.
MP4316A functional block diagram. Image used courtesy of MPS
Automotive Cold Crank Conditions
In most cars, the power architecture for the battery consists of a single buck converter that regulates the 12 V battery to 5 V or 3.3 V. This works great for nominal temperatures when the battery is operating at, or near, 12 V. But in cold weather when the temperature drops “cold cranking” can be problematic.
Car battery power conditioning. Image used courtesy of MPS
The car battery starts the car by using the battery energy to begin rotating, or cranking, the crankshaft until the car ignition cycle is completed and the engine can maintain this rotation on its own. During cranking, the battery delivers large load currents for several seconds, which can cause the output voltage to drop. In extremely cold weather, the voltage may drop below an acceptable operating range for the downstream load circuits, and the car may not start properly.
Car battery operation during cranking. Image used courtesy of PowerTech
Output Regulation
During normal operation, with moderate to high load currents and a stable input voltage, the MP4316A operates with traditional Pulse-Width Modulation (PWM) control. In this mode, the circuit cycles the internal high and low side FETs (HS-FET and LS-FET) to maintain proper output voltage based on the internal compensation voltage (VCOMP). VCOMP is derived from output feedback and voltage reference inputs to the Error Amplifier. The resistor divider connected to the FB pin sets the output voltage.
Configuring the output voltage for the MP4316(A). Image used courtesy of MPS
During cold crank conditions, when input voltages can drop significantly, the MP4316A will enter low-dropout (LDO) mode. In this mode, the chip operates at almost 100% duty cycle with the high side FET (HS-FET) turned on nearly continuously (VSW = VIN). This improves dropout and ensures the maximum energy is transferred to the output to maintain an adequate output voltage.
The internal boost charging circuit and external capacitor (CBST) serve as a reservoir of stored energy to assist during these low input voltage conditions. The chip cannot operate continuously in this mode as the boost capacitor has limited stored energy, but even a few seconds can be sufficient to get to a proper engine start and recover the battery voltage.
Operation during cold crank conditions. Image used courtesy of MPS
Selecting the Switching Frequency
The operating frequency can be selected by a resistor attached to the FREQ pin and set from 350 kHz to 1 MHz, well-suited to car battery applications.
The Frequency Spread Spectrum (FSS) feature uses a 12 kHz modulation waveform to spread the nominal switching frequency (fSW) across a 20% window. By modulating fSW, the energy of the fundamental frequency, and harmonics, is reduced, mitigating associated EMI interference. This feature can relax external filtering requirements, reducing BOM count, PCB area, and costs.
Protection features include Under Voltage Lock-Out (UVLO), frequency foldback, Overcurrent Protection (OCP), and thermal shutdown.
The MP4136A is available in a 4 mm x 4 mm QFN-20 package.
MP4316(A) typical application circuit. Image used courtesy of MPS
Featured images used courtesy of Adobe Stock
