Selecting the Right Power Supply Regulators for Automotive Secondary Rail Applications: Page 5 of 6

June 02, 2016 //By Jerome Johnston, Intersil Corporation
Selecting the Right Power Supply Regulators for Automotive Secondary Rail Applications
Today’s car manufacturers are focusing their innovations on the sophisticated cockpit electronics that improve driver safety and the overall driving experience. More and more consumers are factoring a car’s advanced driver assistance systems (ADAS) and infotainment features into their buying decisions. These systems combine several features that require heavy-duty signal processing, such as forward-looking smart cameras for detecting and classifying objects, back-up camera electronic control units (ECUs), and head-unit center information displays, to name a few. As a result, they require higher current power supply regulation at low voltages.
N-channel FET, the P-channel will be switched on for much less time, and will have less impact on efficiency.


Figure 6. Typical application for ISL78235 5A sync buck regulator


The ISL7823x devices shown in Figure 6 are designed to operate from lower input voltages (+5.5 V down to 2.7 V). As a result, most of their transistors use a smaller geometry, take up less die area and can switch at higher speeds. And since the gate of the upper P-channel FET is driven with the same supply, the signal swing is lower than if an N-channel arrangement is used. This also enables faster switching. The devices can operate with a clock as high as 4 MHz, and when set to switch at 2MHz, they can achieve a guaranteed minimum on time of only 100ns. Since 2MHz has a period of 500ns, the devices can down convert with a guaranteed 20% minimum duty ratio. This enables the regulators to output a wide range of voltages while operating at a high frequency. 

A 100% duty ratio is possible because no time is required to charge a boot capacitor (the ISL7823x devices do not use one). Moreover, no boot capacitor equates to no radiated fields. The high 2MHz switching frequency also enables the use of a smaller power inductor and lower valued capacitors on the regulator’s input and output. It’s also above the AM radio band and helps prevent EMI at these frequencies.

Pass Inspection with a Wettable Flank QFN Package

All three of the ISL7823x devices are available in 5mm x 5mm 16-lead wettable flank thin quad flat no-lead (WFQFN) packages with an exposed pad for improved thermal performance. Because they offer 3A, 4A or 5A output current options, it’s easy to upgrade a design just by dropping in a new IC with no change in the PCB layout. This saves development costs and time! In addition, the wettable flank

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