10.2.2 Detailed Design Procedure

10.2.2.1 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.

10.2.2.2 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 voltage information can be obtain with minimum IR drop error.

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

drop is negligible or an extra set of traces cannot be routed to the load, the force and sense pins for both VOUT

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 9-1).

10.2.2.3 VIN Slew Rate Considerations

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.

10.2.2.4 Shutdown/Enable Feature

The REF34xx references can be switched to a low power shut-down mode when a voltage of 0.5 V or lower is

input to the EN pin. Likewise, the reference becomes operational for EN voltages of 1.6 V or higher. During

shutdown, the supply current drops to less than 2 μA, useful in applications that are sensitive to power

consumption.

If using the shutdown feature, ensure that the EN pin voltage does not fall between 0.5 V and 1.6 V because

this causes a large increase in the supply current of the device and may keep the reference from starting up

correctly. If not using the shutdown feature, however, the EN pin can simply be tied to the IN pin, and the

reference remains operational continuously.

REF3425, REF3430, REF3433, REF3440, REF3450

SBAS804F – SEPTEMBER 2017 – REVISED JUNE 2021

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