Low noise power supplies are essential for RF sections of navigation and radar-based systems for automotive products. The low power signals used in the receiver sections of these circuits are easily affected by noise, which increases the signal acquisition time and reduces accuracy of the desired signals. As camera systems become requirements in all new automobiles, the increased need for high quality video requires a cleaner and wider range operational power source. The DC-DC converters used extensively in automobile electronics also require ripple attenuation.
The Texas Instruments TPS717 and TPS7A81 devices are ultra-low noise and wide bandwidth power supply ripple rejection ratio (PSRR) low-dropout regulators (LDOs) that meet all of the current and future requirements mentioned above. They have very wide bandwidth PSRR characteristics making them especially suited for automotive applications using DC-DC voltage conversion and operating outside the AM band.
Sensitive automotive applications require noise-free regulators. LDO linear voltage regulators like the TI TPS717 and TPS7A81 devices have great advantages over switching regulators in these noise-critical applications, mainly in reduced RMS noise over great frequency ranges. Since one dominant source of noise in an LDO circuit is the amplified reference noise, a noise-reduction pin is added with a filter cap to reduce this noise in high-performance LDOs like the TI devices.
The wide-bandwidth error-amplifier of the LDO design leads to the best PSRR over less noise situations, while adding the NR pin in the above diagram for low noise applications.
Reject that ripple
Ripple rejection in linear regulators is explained in detail in "Understanding Power Supply Ripple Rejection in Linear Regulators" by John Teel. PSRR is a measurement of the circuit's ability to reject ripple from the power supply at various frequencies. RF and wireless circuits are increasingly finding their way into automotive applications. PSRR increases for an LDO are desirable and this can be accomplished by increasing open-loop gain or by decreasing VIN -to- VOUT. The formula for PSRR from the referenced document shows all of the circuit characteristics involved.
Where AV is open-loop gain and AVO is gain from VIN to VOUT.
Open loop gain is the desired goal but increasing it also makes stabilization of the PSRR hard to manage. This is where the low noise, wide bandwidth PSRR regulators, like the TPS717 and TPS7A81 LDOs from TI, make your automotive designs better by eliminating the negative effects of poorly operating power supplies for navigation and radar-based circuits in automotive applications.
Low Current, Easy to Integrate
The TPS717 family of LDOs has many advantages over competitor products. Their small 5-pin SC70 package type takes up little board space in your design. High PSRR is always desirable and these LDOs deliver in that area along with providing low ground current characteristics which make them ideal for many automotive applications. Just add a 1µF capacitor, and your circuit is up and running. Fast start-up also means you do not need to worry about power-on timing issues when designing with these parts.
The wide operating temperature range (-40°C to 125°C) of the TPS7A81 device means there is no need to worry when your design is placed into automobiles, even in the harshest of environments. It includes an enable signal to make output control simple, no need for output switching circuitry in the high current portion of the circuit. The wide-bandwidth high PSRR characteristics and low noise make it an excellent choice for the emerging automotive applications brought on by new regulations. Typical application circuits demonstrate low part counts for design implementations.
As expected, TI provides an evaluation module to get you started quickly and a PSpice model for modeling your designs. The TI E2E community supports LDO designs with training events, selection videos, and blogs to assist with the use of these new devices.
If your design needs are in the automotive area and you are working to add navigation, radar-based equipment or video, the TI TPS717 and TPS7A81 devices look like strong candidates to prevent future power supply issues.
Texas Instruments has sponsored promotion of their power solutions on ENGINEERING.com. They have no editorial input to this post - all opinions are mine. Bruce Schreiner