5

NR031306

HV7800

Application Information

General

The HV7800 offers high side current sensing featuring

accuracy, small size, low component count, low power

consumption, ease of use and low cost. The input voltage

rating of 450V extends the use to off-line applications.

The HV7800 allows measurement of line or load current

for purpose of overcurrent protection, current metering and

current regulation.

High side current sensing, as opposed to ground or low-side

sensing, is desirable or required when:

•

The current to be measured does not flow in a circuit

associated with ground.

•

The addition of resistance in the grounding system

will cause issues with safety, EMI, or will lead to

signal degradation due to common impedance

coupling.

Principle of Operation

The operational amplifier and associated MOSFET, as

shown in the block diagram, force a current through resistor

R

flows to ground through R

V

equal resistance.

Note that the output is not buffered, having an output

resistance of 3.3kΩ nominal. Loading of the output will affect

gain accuracy and / or bandwidth.

Assume in the following that R

To limit gain error to say 1 part in 1000, the external load

impedance should exceed 4MΩ.

Capacitive loading forms a load pole. The break frequency

follows from:

where

E.g. let R

located at 38.8kHz, giving rise and fall times of 8.8us.

Sense Resistor Considerations

The choice of value of the sense resistor entails a trade-

off between power dissipation from the sense resistor and

accuracy. Power dissipation from the sense resistor is

equal to (V

V

dissipation from the sense resistor becomes objectionable.

Consider the use of Kelvin connections for applications with

a large load current. A typical layout pattern, which minimizes

voltage drops across the sense lines is shown below:

V

SENSE

Preservation of bandwidth requires careful attention to trace

layout and choice of sense resistor. Note that the resistance

of the sense resistor is typically small, and that both the

impedence of printed circuit board traces and the equivalent

series inductance of the sense resistance can be a major part

of the total sense/impedence at high frequency operation.

An inductance of just 1nH contributes an impedence of 3mΩ

at 500kHz.

f

=

••

1

2

πτ

τ =

RR

C

B

LOAD

LOAD

//

HV7800

IN

LOAD

GND

OUT

Sense

Resistor