LINEAR TECHNOLOGY LT6100 Technical data

Monitor and Protect Automotive Systems with Integrated
Current Sensing –

Design Note 374

John Munson

Introduction

An automobile is an unforgiving environment for inte­grated circuits, where under-the-hood operating tem­peratures run from –40°C to 125°C and large transient
excursions on the battery voltage bus are expected. In the
past, electronics were part of the well-protected and
centralized Engine Control Unit (ECU), but the trend is
toward more distributed electronics. Electrically driven
accessories and fault-protection monitoring functions are
leaving the protective umbrella of the ECU and migrating
directly into vehicle subsystems.

For example, many functions formerly driven by the
engine—via belt and pulley or hydraulics—are now elec­trically driven (motorized), such as water pumps, steering
mechanisms, brake actuators and various body controls.
These functions can become a safety risk if they are not
continually monitored for operational readiness and/or
have a back-up mode of operation. In either case, real­time monitoring becomes necessary and generally
involves accurately measuring the current draw of each
subsystem.

Simple Current Monitoring Solutions

The LT®6100 and LTC6101 are high side current-sense
amplifiers that have been developed specifically to address
the automotive designers’ needs. These parts require a
minimum number of support components to operate in
the harsh automotive battery-bus environment.

Figure 1 shows a basic high side current monitor using the
LTC6101. The selection of R
desired gain of this circuit, powered directly from the
battery bus. The current output of the LTC6101 allows it
to be located remotely to R
placed directly at the shunt, while R
monitoring electronics without ground drop errors.

This circuit has a fast 1µs response time that makes it ideal
for providing MOSFET load switch protection. The switch
element may be the high side type connected between the
sense resistor and the load, a low side type between the

and R

IN

. Thus, the amplifier can be

OUT

establishes the

OUT

is placed near the

OUT

load and ground or an H-bridge. The circuit is program­mable to produce up to 1mA of full-scale output current
into R

, yet draws a mere 250µA supply current when

OUT

the load is off.

Figure 2 shows the LT6100 used as a combination
current sensor and fuse monitor. This part includes on­chip output buffering and was designed to operate with
the low supply voltage (≥2.7V), typical of vehicle data
acquisition systems, while the sense inputs monitor
signals at the higher battery bus potential. The LT6100

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

BATTERY BUS

R

R

0.01Ω

POWER

Figure 2. Simple LT6100 High Side Current Sense
Amplifier and Fuse Monitor

IN

SENSE

LOAD

100Ω

4

+

2

LT6101

= I

V

OUT

LOAD(RSENSE

3

–

5

1

R

OUT

4.99k

• R

)

OUT/RIN

DN374 F01

Figure 1. Simple LTC6101 High Side
Current Sense Amplifier

R

SENSE

FUSE

2mΩ

A4

A2

OUT

DN374 F02

7

6

5

+

OUTPUT

2.5V = 25A

81

–

+

V

V

S

–

LT6100

S

+

2

V

CC

3

FIL

4

V

EE

ADC

≥2.7V

TO LOAD

C2

0.1µF

V

OUT

4.99V = 10A

BATTERY
BUS

10/05/374

inputs are tolerant of large input differentials, thus allow­ing the blown-fuse operating condition (this would be
detected by an output full-scale indication). The LT6100
can also be powered down while maintaining high imped­ance sense inputs, drawing less than 1µA max from the
battery bus.

Solving the H-Bridge Problem

Many of the newer electric drive functions, such as
steering assist, are bidirectional in nature. These func­tions are generally driven by H-bridge MOSFET arrays
using pulse-width-modulation (PWM) methods to vary
the commanded torque. In these systems, there are two
main purposes for current monitoring. One is to monitor
the current in the load, to track its performance against the
desired command (i.e., closed-loop servo law), and
another is for fault detection and protection features.

A common monitoring approach in these systems is to
amplify the voltage on a “flying” sense resistor, as shown
in Figure 3. Unfortunately, several potentially hazardous
fault scenarios go undetected, such as a simple short to
ground at a motor terminal. Another complication is the
noise introduced by the PWM activity. While the PWM
noise may be filtered for purposes of the servo law,
information useful for protection becomes obscured. The
best solution is to simply provide two circuits that indi­vidually protect each half-bridge and report the bidirec­tional load current. In some cases, a smart MOSFET

bridge driver may already include sense resistors and
offer the protection features needed. In these situations,
the best solution is the one that derives the load informa­tion with the least additional circuitry.

Figure 4 shows a differential load measurement for an
ADC using twin unidirectional sense measurements. Each
LTC6101 performs high side sensing that rapidly responds
to fault conditions, including load shorts and MOSFET
failures. Hardware local to the switch module (not shown
in the diagram) can provide the protection logic and fur­nish a status flag to the control system. The two LTC6101
outputs taken differentially produce a bidirectional load
measurement for the control servo. The ground-refer­enced signals are compatible with most ∆Σ ADCs. The ∆Σ
ADC circuit also provides a “free” integration function that
removes PWM content from the measurement. This
scheme also eliminates the need for analog-to-digital
conversions at the rate needed to support switch protec­tion, thus reducing cost and complexity.

Conclusion

The LT6100 and LTC6101 high side current-sense ampli­fiers simplify designs in the automotive environment.
High transient voltage tolerance (105V for the LTC6101HV)
and ground-referenced outputs make it possible to
improve robustness and substantially reduce the parts­count over traditional solutions.

R

–

DIFF
OUTPUT
TO ADC

+

OUT

BATTERY BUS

+

BATTERY BUS

R

LTC6101

R

OUT

IN

R

S

R

IN

LTC6101

R

S

+

RANGE = ±100A,

+

R

S

I

M

DN374 F03

DIFF
AMP

–

I

M

FOR I
DIFF OUT = ±2.5V

R
R
R

DN374 F04

= 1mΩ

S

= 200Ω

IN
OUT

M

= 4.99k

Figure 3. Limited Performance H-Bridge Current Monitor Figure 4. Practical H-Bridge Current Monitor Offers

Fault Detection and Bidirectional Load Information

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dn374f LT/TP 1005 409K • PRINTED IN THE USA

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