OAR1R01JP Datasheet, PDF (8/58 Pages) Token Electronics Industry Co., Ltd. – Chip Current Sense / Open-Air Current Shunts / Axial Current Sense Resistors

TOKEN
Terminology & Glossary
Current Detecting Resistors
Due to the increase in sales of notebook PCs, the demand for the DC/DC converter has shown rapid
growth because of its high-energy conversion efficiency, and its precise current-limiting capability. How-
ever, to ensure the performance of the multiple output DC/DC converter, the current limiting voltage must
be detected precisely to protect an expensive notebook PC from an overload, which is generally due to
shorts within the capacitors used in these devices.
For high-energy conversion efficiency various control ICs have been developed that utilize resistive com-
ponents. To achieve the perfect cutoff mode of the DC load current at the programmed current-limiting
voltage of the control IC, a very stable and accurate sense resistor with the following characteristics is
required for precise voltage comparison.
● Very Low Ohmic Value should be below 25mΩ for minimizing power consumption at the current
sensing resistor.
● Tight Tolerance must be ±1% or tighter for maximizing the current supply within the limit of accept
able current.
● Low TCR is required for current sensing across the ambient temperature range of 0°C to 60°C.
● Low Thermal EMF for an accurate comparison between the programmed current-limiting voltage of
the control IC and the detected voltage.
Furthermore, the self-inductance should be for high frequency applications. Recommended types are gen-
eral purpose current sensing products or a flame retardant type.
How Four Terminal Sense Resistor Work
High precision resistors used for current sensing are usually low ohmic value devices suitable for four
terminal connection. To use a 4-terminal resistor, we force a current from Terminal 1 to Terminal 2. It’s
current, so the unknown resistances attached to Terminal 1 and Terminal 2 don’t affect the amount of the
current. The same number of electrons per second flow through from T1 to T2, regardless of the resistance.
A voltmeter measures the resulting voltage drop across the ultra-precise resistor, measuring through the
unknown resistors attached to Terminal 3 and Terminal 4. The voltmeter’s input impedance is very, very
high compared to the unknown resistors, so the unknown resistors have essentially zero effect (typically
less than 0.1 parts-per-million).
So the current flows through the 0.100 ohm resistor, unaffected by the unknown resistors, and we measure
the voltage across the 0.100 ohm resistor, unaffected by the unknown resistors. And that’s how a 4-terminal
resistor works!
Terminal 3
(Sense terminal)
Vs
Voltmeter Measured
Terminal 4
(Sense terminal)
(Current terminal)
Terminal 1
? ohm
Rs
Known ohm
? ohm
(Current terminal)
Terminal 2
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Version 2010