HV7800 Supertex, Inc., HV7800 Datasheet - Page 5

• Manufacturer Part Number: HV7800
• Description: High Side Current Monitor 8v To 450v, Voltage Gain Of 1
• Manufacturer: Supertex, Inc.

Block Diagram
Application Information
General
The HV7800 high side current monitor IC features accurate
current sensing, small size, low component count, low power
consumption, exceptional input voltage range, ease of use
and low cost.
The part typically performs the measurement of line or
load current for overcurrent protection, metering or current
regulation.
High side current sensing, as opposed to ground referenced
or low side current sensing, is desirable or required when:
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►
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The current to be measured does not fl ow in a circuit
associated with ground.
The measurement at ground level can lead to ambiguity
due to changes in the grounding arrangement during
fi eld use.
The introduction of a sense resistor in the system ground
is undesirable due to issues with safety, EMI, or signal
degradation due to common impedance coupling.
Circuits
Bias
IN
V
GND
R
SENSE
SENSE
R
R
HV7800
LOAD
A
B
R
I
SENSE
P
OUT
V
OUT
5
Principle of Operation
The operational amplifi er and MOSFET forces the voltage
across R
of the opamp, i.e. V
The current through R
R
tracking. By design, R
Consequently, V
of 1.
OUT Pin Loading Effects
Note that the OUT pin has a typical output resistance of
3.6kΩ. Loading the output causes the voltage gain to drop
and rise/fall times to increase.
For example, assuming an output resistance of 3.6kΩ, the
load resistance should exceed 3.6MΩ in order to limit the
drop in gain to 1 part in 1000.
Again assuming an output resistance of 3.6kΩ, capacitive
loading of 30pF results in a response pole with a time
constant of 100ns, not yet high enough to materially affect
the output rise and fall times (about 700ns).
Sense Resistor Considerations
Choose a sense resistor that will not exceed 500mV during
normal operating conditions. Limit the power dissipation in
the sense resistor to whatever is practical; a high sense
voltage benefi ts accuracy, but increases power dissipation.
Consider the use of Kelvin connections for applications
where considerable voltage drops may occur in the PCB
traces that carry the current to be measured to the sense
resistor. A layout pattern that minimizes voltage across the
sense lines is shown below.
Choose a low inductance type sense resistor if preservation
of bandwidth is important. The use of Kelvin connections
helps by excluding the inductive voltage drop across the
traces leading to the sense resistor. The inductive voltage
drop may be substantial when operating at high frequencies.
A trace or component inductance of just 10nH contributes
an impedance of 6.2mΩ at 100kHz, which constitutes a 6%
error when using a 100mΩ sense resistor.
B
are integrated, exhibiting tight matching and excellent
A
to track V
RA
is equal to V
RA
SENSE
= V
A
A
returns to ground through R
+ V
and R
within the limit of the offset voltage
IN
SENSE
SENSE
.
LOAD
B
RB
have the same resistance.
, resulting in a voltage gain
-
R
SENSE
HV7800
B
. R
A
and