LINEAR Monitor & Protect Automotive Systems w/ Integrated Current Sensing – Design Note 374

Monitor and Protect Automotive Systems with Integrated Current Sensing – Design Note 374 John Munson

Introduction load and ground or an H-bridge. The circuit is program- mable to produce up to 1m A of full-scale output current

An automobile is an unforgiving environment for inte-

into ROUT, yet draws a mere 250µA supply current when

grated circuits, where under-the-hood operating tem-

the load is off.

peratures run from –40°C to 125°C and large transient excursions on the battery voltage bus are expected. In the

Figure 2 shows the LT6100 used as a combination

past, electronics were part of the well-protected and

current sensor and fuse monitor. This part includes on-

centralized Engine Control Unit (ECU), but the trend is

chip output buffering and was designed to operate with

toward more distributed electronics. Electrically driven

the low supply voltage (≥2.7V), typical of vehicle data

accessories and fault-protection monitoring functions are

acquisition systems, while the sense inputs monitor

leaving the protective umbrella of the ECU and migrating

signals at the higher battery bus potential. The LT6100

directly into vehicle subsystems.

, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.

For example, many functions formerly driven by the engine—via belt and pulley or hydraulics—are now elec-

BATTERY BUS

trically driven (motorized), such as water pumps, steering mechanisms, brake actuators and various body controls.

RSENSE

These functions can become a safety risk if they are not 100Ω 0.01Ω

continually monitored for operational readiness and/or

LOAD –+

have a back-up mode of operation. In either case, real- time monitoring becomes necessary and generally involves accurately measuring the current draw of each subsystem.

LT6101

4.99V = 10A ROUT

Simple Current Monitoring Solutions

The LT®6100 and LTC6101 are high side current-sense VOUT = ILOAD(RSENSE • ROUT/RIN) DN374 F01

amplifiers that have been developed specifically to address

Figure 1. Simple LTC6101 High Side

the automotive designers’ needs. These parts require a

Current Sense Amplifier

minimum number of support components to operate in

the harsh automotive battery-bus environment. RSENSE

TO LOAD FUSE

BATTERY BUS

Figure 1 shows a basic high side current monitor using the LTC6101. The selection of RIN and ROUT establishes the

VS VS

desired gain of this circuit, powered directly from the

POWER VCC A4

battery bus. The current output of the LTC6101 allows it

to be located remotely to ROUT. Thus, the amplifier can be

FIL A2

placed directly at the shunt, while ROUT is placed near the monitoring electronics without ground drop errors.

OUT OUTPUT

2.5V = 25AVEE

This circuit has a fast 1µs response time that makes it ideal LT6100 DN374 F02

for providing MOSFET load switch protection. The switch element may be the high side type connected between the Figure 2. Simple LT6100 High Side Current Sense sense resistor and the load, a low side type between the Amplifier and Fuse Monitor