Data Sheet

LT9890

APPLICATIONS INFORMATION

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Rev. 0 | 9 of 11

The LT9890 is a high-side current-sense amplifier with an integrat-

ed current-sense element. The current-sense element is copper (a

proprietary method that corrects for the nonideal resistance traits

of the copper). The sense voltage across the sense element is

amplified and level shifted to a ground-referred IOUT. The output

signal is analog and can be used as is or processed with an output

filter.

SELECTION OF ROUT

ROUT determines how IOUT is converted to the IOUT voltage (VIO-

UT). VIOUT is the output current, IIOUT, × ROUT. In choosing ROUT, the

maximum output voltage, VOUT (MAX), must first be considered. If the

circuit that is driven by the output does not limit VOUT, ROUT must

be chosen such that the VOUT (MAX) does not exceed the LT9890

IOUT maximum VOUT rating. If the following circuit is a buffer or

analog-to-digital converter (ADC) with a limited input range, ROUT

must be chosen so that IOUT (MAX) × ROUT is less than the allowed

maximum input range of this circuit.

In addition, the output impedance is determined by ROUT. If the

circuit driven has a high enough input impedance (RI), most ROUT

values are acceptable. However, if the driven circuit has relatively

low RI or draws spikes of current, such as an ADC does, a lower

ROUT value is required to preserve the accuracy of the output. For

example, if the RI of the driven circuit is 100 x ROUT, the accuracy of

VIOUT is reduced by 1% because of the following:

VIOUT=

IIOUT×ROUT×R1

ROUT+R1

≈0.99×IIOUT×ROUT (1)

OUTPUT FILTERING

The high-frequency performance of the LT9890 is useful for some

applications, but other applications require less bandwidth. In these

instances, an output capacitor (COUT) can be placed in parallel with

ROUT to reduce noise and to help keep the output steady while

driving a switching circuit, such as a multiplexer or ADC. This COUT

in parallel with the ROUT creates a pole in the output response at

the following:

f−3 dB= 1

2×π×ROUT×COUT

(2)

PCB LAYOUT

The high currents through the LT9890 require careful PCB routing.

The wide package size was chosen to allow 18 mm wide copper

into and out of the device. Use at least 70 μm thick board copper.

All current flows through the large IP (Pin 103 to Pin 108) and IM

pins (Pin 97 to Pin 102) beneath the package. The many IP pins on

the top (Pin 49 to Pin 84) and IM pins on the bottom (Pin 1 to Pin

36) of the PCB carry no current. These pins exist for mechanical

stability. Solder these pins to IP and IM as labeled.

A wide copper sense element exists within the package. Current

density variations through that shunt can cause errors.

Figure 16 shows three possible routing methods of copper through

the LT9890. The left-most method is best because it places the

LT9890 directly in the path from the connector to the downstream

circuitry. The other two methods create additional TUE. To minimize

trace resistance, use multiple wide planes to the LT9890. Reduce

the width of these wide planes to 18 mm approximately 4 mm

before the large IP and IM pins to force the current to run straight

into the LT9890 and avoid crowding. Figure 17 depicts this recom-

mended power layout for one side of the LT9890. Note that the

other side mirrors it. The row of vias near the power pads connects

the multiple planes right before current enters the LT9890. The vias

also operate as thermal vias to dissipate the heat of the LT9890

to all power planes. PCB path resistance often exceeds the 150

μΩ path resistance of the LT9890, which raises the TJ beyond

typical calculations. Use short distances, multiple power planes,

and thermal vias to reduce self heating. For heat dissipation, the

planes on the top and the bottom of the PCB are most important.

For minimum PCB resistance, the top planes are most important.

Figure 16. Three Routing Methods of Copper

Figure 17. Recommended Power Layout

FACTORY CALIBRATION

The LT9890 is factory calibrated to correct for the copper sense

nonideal resistivity of the element.