Fundamentals of designing with LDOs in automotive battery direct connect applications

Texas InstrumentsBy David Xu, Product Marketing Engineer, Mixed Signal Automotive, Texas Instruments

Introduction

In the automotive world, a low-dropout linear regulator (LDO) offers good voltage ripple suppression and electromagnetic compatibility (EMC) performance, especially when compared to DC/DC converters.

While either a system battery or a pre-stage power regulator
can supply an LDO, the requirements for a battery-direct- connect LDO are more rigorous: it must pass the International Organization for Standardization 7637 (ISO 7637) standard and survive a load-dump condition. During operation, the LDO powers the target load through a printed circuit board (PCB) trace in an on-board system or through a cable in an off-board system. For off-board systems, the LDO must protect itself from different kinds of potential cable failures.

This document describes various LDO speci cations in the context of automotive applications, with a key focus on battery-direct- connection and driving an off-board load system.

Linear voltage regulators

An LDO plays an important role in the electronic design world
and is crucial for systems functioning in harsh environments,
like automobiles. In today’s automotive designs, normally a 12-V battery powers the system. While a stable lower-voltage supply is required for system operation, constantly changing load conditions and other environmental factors cause variations in a 12-V supply. It is an automotive battery-direct-connect LDO that converts a harsh high-voltage supply down to a stable lower-voltage output.

Depending on its structure, most LDO outputs are set as 3.3 V or 5 V, or they are con gurable.

Compared to a DC/DC converter, a linear voltage regulator is easy to use; one output capacitor ensures the stability of the device. Meanwhile, passing EMC testing is not dif cult because the topology of a linear voltage regulator does not create any switching noise. Therefore, LDOs are the most popular electronic power supplies in automotive applications.

Internal topology

An LDO is a type of negative-feedback control system that consists of a signal sampling circuit, a signal processing circuit and a power control circuit.

A resistor divider samples the output voltage, which is then compared with an accurate internal reference voltage. The difference between these two signals represents the shift in output voltage from a target value. Systems use this difference to control a passing element, normally a low RDS(on) eld-effect transistor (FET), thus governing the output value.

LDOs implement necessary compensation circuits for system stability (Figure 1).

Figure 1: Internal topology of an LDO. Figure 2: LDO adjustable output application circuit.

Adjustable output voltage

A linear voltage regulator’s output voltage is adjustable by using an external resistor divider when a feedback (FB) pin exists (Figure 2). Equation 1 calculates the output voltage:

Equation 1 - where V(FB) is the internal reference voltage, R1 is the resistor connected between OUT and FB, and R2 is the resistor connected between FB and GND.

Let us use the selection of external resistors for the TPS7B6701-Q1 as an example. The internal reference voltage of this device is 1.233 V. In order to set the output voltage to 5 V, rst calculate the feedback resistor divider ratio based on Equation 2:

Equation 2 showing how to calculate the feedback resistor divider ratioTo balance the quiescent current and capacity of resisting disturbance, consider a 10- to 100-kΩ resistance. In such a case, you can select two resistors to achieve a 5-V output voltage: R1 = 55 kΩ and R2 = 18 kΩ. See Figure 3.

Figure 3: High-voltage, ultra-low voltage regulator in a typical application diagram.Automotive battery voltage characterizations

Today’s automobiles use a 12-V battery, while trucks and heavy- duty vehicles use a 24-V battery. In real applications, the alternator, driven by the engine, charges the battery. A load dump may occur if the battery becomes disconnected as a result of cable corrosion, a poor connection or an intentional disconnection with the engine running. According to ISO 7637-2 standard test pulse 5a, the battery’s maximum transient voltage may go as high as 99 V in a 12-V system, or 198 V in a 24-V system, lasting around hundreds of milliseconds. See the details in Figure 4 and Table 1 …


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