11.2.1.1 Design Requirements

A detailed design procedure is based on a design example. For this design example, use the parameters listed

in Table 11-2 as the input parameters.

Table 11-2. Design Example Parameters

DESIGN PARAMETER

VALUE

12 V

5 V

REF3450-Q1 input capacitor

1 µF

REF3450-Q1 output capacitor

10 µF

11.2.1.2 Detailed Design Procedure

11.2.1.2.1 Input and Output Capacitors

A 1-μF to 10-μF electrolytic or ceramic capacitor can be connected to the input to improve transient response in

applications where the supply voltage may fluctuate. Connect an additional 0.1-μF ceramic capacitor in parallel

to reduce high frequency supply noise.

A ceramic capacitor of at least a 0.1 μF must be connected to the output to improve stability and help filter out

high frequency noise. An additional 1-μF to 10-μF electrolytic or ceramic capacitor can be added in parallel to

improve transient performance in response to sudden changes in load current; however, keep in mind that doing

so increases the turnon time of the device.

Best performance and stability is attained with low-ESR, low-inductance ceramic chip-type output capacitors

(X5R, X7R, or similar). If using an electrolytic capacitor on the output, place a 0.1-μF ceramic capacitor in

parallel to reduce overall ESR on the output.

11.2.1.2.2 4-Wire Kelvin Connections

Current flowing through a PCB trace produces an IR voltage drop, and with longer traces, this drop can reach

several millivolts or more, introducing a considerable error into the output voltage of the reference. A 1-inch long,

5-millimeter wide trace of 1-ounce copper has a resistance of approximately 100 mΩ at room temperature; at a

load current of 10 mA, this can introduce a full millivolt of error. In an ideal board layout, the reference must be

mounted as close as possible to the load to minimize the length of the output traces, and, therefore, the error

introduced by voltage drop. However, in applications where this is not possible or convenient, force and sense

connections (sometimes referred to as Kelvin sensing connections) are provided as a means of minimizing the

IR drop and improving accuracy.

Kelvin connections work by providing a set of high impedance voltage-sensing lines to the output and ground

nodes. Because very little current flows through these connections, the IR drop across their traces is negligible,

and the output and ground.

It is always advantageous to use Kelvin connections whenever possible. However, in applications where the IR

and GND can simply be tied together, and the device can be used in the same fashion as a normal 3-terminal

reference (as shown in Figure 9-6).

In applications with slow-rising input voltage signals, the reference exhibits overshoot or other transient

anomalies that appear on the output. These phenomena also appear during shutdown as the internal circuitry

loses power.

To avoid such conditions, ensure that the input voltage wave-form has both a rising and falling slew rate close to

6 V/ms.

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REF34-Q1

SBAS901C – JULY 2018 – REVISED OCTOBER 2020

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