Tocurrentsense

circuitry

InductorPCBpads

InductorPCBpads

Tocurrentsense

circuitry

Right!

Wrong!

UCD7232

SLUSAH3

– MAY 2011

www.ti.com

INDUCTOR CURRENT SENSE TRACE LAYOUT

Since matching of the L/DCR to RC time constants is important to obtain an accurate replica of the inductor

current, the PCB layout must be done correctly to insure that the voltage drop across the inductor is sensed

properly. For best results, the current sensing connections should be made by separate, non-current-carrying

traces that connect directly to the inductor solder pads. The sensing connections should not be made to current

carrying traces that lead to the switching node or the output capacitors. An example of a correct and incorrect

layout is given in Figure 8.

Figure 8. Inductor Current Sense Trace Layout

The current carrying traces have finite resistance that exhibit an additional voltage drop which will contaminate

the sensed readings. It represents an additional DCR that is not taken into account in the current sensing

equations. The trace resistance varies with the thickness of the PCB copper used on the board. This thickness

can vary from batch to batch of pc boards, so the additional resistance of the traces is not a tightly controlled

value. Even a short length of PCB trace can introduce a significant amount of added resistance. Remember,

milliohms matter. By making a Kelvin connection to the inductor pads, the effects of PCB trace resistance can be

minimized.

LIMITATIONS OF DCR CURRENT SENSING

The accuracy of the DCR current sense method is limited by the stability of the DCR and L values of the power

inductor. In practice, the inductance value of the power inductor decreases with increasing load current. Most

inductors will exhibit a 20% to 30% reduction in inductance as load current changes from no load to full rated

current. The DCR sense method cannot detect inductor saturation or a cracked core, both of which cause greatly

increased ac current to flow in the inductor.

The resistance of the inductor windings is strongly affected by temperature. Most inductors use copper wire, and

copper has a resistance temperature coefficient of approximately +3800ppm/

°C. This means that if the winding

temperature of the inductor rises by 40

°C, its DCR will increase by 15.2%. This will cause the sensed voltage at

CSP and CSN to increase by 15.2% as well for the same current flow. If high accuracy of measured current is

important, then some form of temperature correction needs to be applied to the DCR sensed reading. This

requires some form of temperature sensing and a method to correlate the sensed temperature to the actual

winding temperature.

Since it is impractical to place a temperature sensor inside the inductor to sense the winding temperature, a

practical alternative is to sense the high-side FET device temperature. Tests have shown that a small analog-

output temperature sensor placed under the high-side FET on the back side of the board works well as a

substitute. Its temperature output correlates strongly to the inductor winding temperature. The voltage

proportional to temperature can be fed to the Temp input of the UCD92xx family of Digital Power Controllers. The

firmware internal to the controller can use the temperature reading to correct for the temperature effects on the

DCR current sense readings.

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© 2011, Texas Instruments Incorporated