Power supply for infotainment equipment in car start-stop system (II)

Extend battery run time:

For any battery-powered system, if you need a “always on” power bus and the rest of the system is off, you must conserve battery power. This state is often referred to as the "sleep", "standby" or "idle" mode and requires such systems to have very low quiescent current. In automotive applications that may have a variety of electrical circuits, such as in-vehicle information systems, CD/DVD players, remote-controlled door switches, and multiple electrical circuits that are always on and off, it is particularly important to achieve low quiescent current in order to conserve battery power. During the standby mode, the overall current consumption of these systems must be as low as possible, and as cars increasingly rely on the operation of electronic systems, the pressure on battery manufacturers to save battery energy continues to increase.

The LTC7815 consumes only 28μA in sleep mode and when the boost converter and one of the buck converters are turned on. When all three channels are on and in sleep mode, the LTC7815 consumes only 20μA, significantly extending battery run time in idle mode. This is accomplished by configuring the device to enter a high efficiency burst mode operating state in which the LTC7815 delivers a short current pulse to the output capacitor, followed by a sleep cycle, where only the output capacitor is provided to the load. Output Power.

In sleep mode, most of the internal circuitry is turned off except for critical circuits that require fast response. When the output voltage drops sufficiently, the sleep signal is activated and the controller resumes normal burst mode operation by turning on the external MOSFET at the top. On the other hand, in some cases, the user needs the device to operate in a forced continuous mode or a constant frequency pulse skip mode under light load current conditions. Both modes are easy to configure and have high quiescent current.

Efficiency / Solution Size:

If the operating frequency is reduced from 2.1MHz to 300kHz, the efficiency can be increased by 3% to 4%.

Protective function:

The LTC7815 can be configured to sense the output current using a DCR (inductor resistor) or a sense resistor. As for how to choose between two current detection schemes, a comprehensive trade-off between cost, power consumption and accuracy is required to a large extent. DCR detection methods are gaining popularity because they eliminate expensive current sense resistors and are more efficient, especially in high current applications. However, the sense resistor is a more accurate method of current detection.

The built-in comparator monitors the buck output voltage and indicates an overvoltage condition when the output is greater than 10% of its nominal value. When this condition is detected, the top MOSFET turns off and the bottom MOSFET turns on until the overvoltage condition is cleared. As long as the overvoltage condition persists, the bottom MOSFET will remain on continuously. If the output voltage returns to a safe level, normal operation resumes automatically.

The thermal shutdown circuit turns off the LTC7815 at higher temperature conditions or when internal power dissipation causes excessive self-heating inside the chip. When the junction temperature exceeds approximately 170°C, the overtemperature protection circuit disables the built-in bias LDO, causing the bias supply to drop to 0V and effectively shut down the entire LTC7815 in an orderly manner. Once the junction temperature drops back to around 155 °C, the LDO will turn back on.

In conclusion:

The fuel-saving car start-stop system will continue to evolve in the next few years. Care must be taken in the power supply of in-vehicle infotainment and navigation systems that require up to and even more than 5V. Such systems reset when the car battery voltage drops below 5V due to engine re-start. The LTC7815 provides a solution that boosts battery voltage to a safe operating level. Used in conjunction with two step-down controllers, it is ideal for powering many in-vehicle electronics in cars with start-stop systems.