HV7800(2006) Ver la hoja de datos (PDF) - Supertex Inc
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HV7800 Datasheet PDF : 6 Pages
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HV7800
Application Information
General
RSENSE
ILOAD
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:
IN LOAD
HV7800
GND
OUT
ROUT = 3.3kΩ ± 30%
• The current to be measured does not flow in a circuit
associated with ground.
• The addition of resistance in the grounding system
RLOAD CLOAD
will cause issues with safety, EMI, or will lead to
signal degradation due to common impedance Sense Resistor Considerations
coupling.
Principle of Operation
The operational amplifier and associated MOSFET, as
shown in the block diagram, force a current through resistor
RA such that VRA is equal to VSENSE. The current through RA
flows to ground through RB and develops an output voltage
VOUT, which is equal to VSENSE, given that RA and RB have
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.
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 (VSENSE • ILOAD). It is generally desirable to maximize
VSENSE to enhace accuracy, but not to the extent that power
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:
Assume in the following that ROUT = 4kΩ.
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:
( ) f = 1
2•π •τ
where
τ = RB // RLOAD • CLOAD
E.g. let RLOAD = ∞ and CLOAD = 1nF, then the load pole is
located at 38.8kHz, giving rise and fall times of 8.8us.
Sense
Resistor
VSENSE
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.
NR031306
5
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