LINEAR dn407f Data Sheet

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Dual Current-Sense Amplifi ers Simplify H-Bridge Load Monitoring Design Note 407 Jon Munson Introduction A far more practical method is to monitor the supply

The H-bridge power-transistor topology is increasingly current fed to each half-bridge as shown in Figure 2. popular as a means of driving motors and other loads bi- This scheme provides several benefi ts that simplify and directionally from a single supply potential. In most cases improve the circuit performance. The main improvement there is great benefi t in monitoring the current delivered comes from having the sense resistors at a relatively to the load and utilizing this information in real-time to constant common mode voltage (i.e., the power supply provide operational feedback to a control system. In most voltage) so that fi delity of the PWM current waveform new designs, pulse-width-modulation (PWM) techniques can be preserved. Additionally, by monitoring each half- are used to provide highly effi cient variable power-delivery, bridge individually at the supply side, both failed power but this places extremely fast voltage transitions at both device operation and load shorts to ground are readily terminals of the load and therefore complicates the instru- detected and manageable.

mentation problem. New high side current sense amplifi ers

By using PWM logic that generates “sign-magnitude”

from Linear Technology can simplify this problem.

control, one of the half-bridges is in a 100% pull high Measuring Load Current in the H-Bridge condition (depending on the direction or polarity of drive). The load current equals the current delivered through the

The classical approach to load monitoring is to place a

100% (fully on) switch, unaffected by the duty cycle of

small value sense-resistance in series with the load that

the PWM activity on the other half-bridge. This permits

can develop a measurable voltage drop representing the

simple reconstruction of the load current waveform using

load current (see Figure 1). The diffi culty here is that with

suitable high side sense amplifi cation techniques.

PWM activity, the common mode voltage at the sense resistor has nasty voltage transitions that can corrupt the

The Simple Solution

sense amplifi er operation with high frequency hash. While

The LTC6103 and LTC6104 dual high side sense amplifi ers

this hash can be fi ltered to recover useful low frequency

are ideal for performing the H-bridge monitoring function.

information, the ability to provide fast fault protection

Both parts include two current sense input channels and

is then lost. Additionally, this “fl ying” sense resistor

, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.

confi guration is unable to monitor switch shoot-through All other trademarks are the property of their respective owners. current, leaving many important fault modes undetected or unmanaged (failed switch function, for example).

RS RS

DIFF AMP

DN407 F01 DN407 F02

Figure 1. Classical Load Sensing Problematic with PWM Figure 2. PWM-Compatible H-Bridge Load Sensing