When the battery in the car starts cold, the battery voltage will drop to a very low level; and in the case of a steep load drop, a high voltage spike may appear on the battery bus. At the same time, due to the current need to provide multiple power supply voltages for automotive electronic voltage rails, the industry urgently needs power supply devices that can provide a wide range of input voltages and multiple output voltages.
Tony Armstrong, Power Products Marketing Director of Linear Technology, pointed out that in recent years, the proportion of electronic devices in automobiles has increased rapidly. Automotive electronic loads require power supplies that can provide a very wide voltage operating range. In addition, the use of 5 to 6 power rails (such as 1.8V, 2.5V, 3.3V and 5V, etc.) in automotive electronics is very common. Linear Technology has introduced a LT3504 step-down switching regulator with a wide input voltage range of 3.2V to 40V for automotive electronics applications (Figure 1).
The voltage regulator can provide 4 independent channels of continuous 1A output. The cost is lower and the board area is smaller. One of its nice features is the 100% duty cycle. When the cold car starts, the battery voltage will drop very low. When the input voltage (battery voltage) drops very low and is very close to the output voltage, the buck converter may stop working. While the 100% duty cycle when the input voltage drops to a very low level, the output voltage can still maintain or follow the input voltage changes (depending on whether it is below the converter startup threshold). In the cold start or start-stop system, the battery works at a lower voltage level, and subsequent power supply and circuit can still guarantee normal operation is very useful.
Figure 1: Step-down switching regulator LT3504 application diagram.
The LT3504 relies on a unique control method to make the TON conduction time very small. In this way, higher switching frequency can still be obtained under the application of high step-down ratio. The switching frequency can be programmed by the user from 250kHz to 2.2MHz, which can be synchronized in the whole frequency range. In this way, you can avoid the AM radio frequency band that is sensitive to noise in the car radio, and control the switching frequency to run below 600kHz or higher than 1.8MHz, so that the volume and cost of peripheral LCD and other devices can be better.
The LT3504 has a conversion efficiency of 88% over a very wide input voltage range. The chip synchronizes the converter's operating frequency with an external clock. The 4-way output uses inverting switching technology to reduce ripple. It also provides a very accurate UVLO shutdown function, allowing it to avoid undesirable operating ranges. The overheat shutdown function can be turned off when the junction temperature is too high to play a self-protection effect.
Each channel of the device uses an independent pin to set the output voltage, and the voltage range from FB (feedback) pin 0.8V to VIN pin can be obtained. The independent RUN / SS (run / soft start) pin of each channel can control the RUN / SS enable of each channel output. Through the synchronization (RT / SYNC) pin, you can use a resistor to set the switching frequency, or you can use the MCU output clock signal to synchronize. Multi-channel output applications can be implemented with very compact solutions.
Figure 2: Schematic diagram of the LT3976 step-down switching regulator system.
Voltage regulator provides ultra-low quiescent current for cars
Many electronic devices (such as GPS systems and TelemaTIcs systems) are connected to batteries for long-term power consumption. If the quiescent current is not reduced, they will continue to "steal". After the car has been idle for a period of time, it may not catch fire. This may be caused by batteries that have not been maintained for a long time and the battery runs out of power. Therefore, it is necessary to control the quiescent current (IQ) as low as possible without load.
Linear Technology's 42V step-down switching regulator LT3975 can provide 2.5A continuous output current and only requires 2.7μA quiescent current. The other LT3976 (Figure 2) can provide up to 5A of output current at 40V input and requires only 3.3μA of quiescent current. Both devices provide 4.2V output in the nominal input voltage range up to 40V, making them ideal for automotive and industrial applications.
Their internal 75mΩ resistance switch can provide up to 90% efficiency. The LT3975 and LT3976's Burst Mode operation provides ultra-low quiescent current. This feature is very suitable for automotive and industrial system applications, because these applications need to always keep working and the best battery life. When the input voltage drops below the set output voltage, these devices can maintain a minimum dropout voltage of only 500mV, thus responding to the cold start requirements of automotive electronics.
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