However, they sometimes have an ESR values lower than

the minimum required by the LM4140, and relatively large

capacitance change with temperature. The manufacturer’s

datasheet for the capacitor should be consulted before se-

lecting a value. Test results of LM4140 stability using multi-

layer ceramic capacitors show that a minimum of 0.2µF is

usually needed.

Multilayer ceramic capacitors that have been verified as suit-

able for use with the LM4140 are shown in

Table 2.

TABLE 2. Surface-Mount Ceramic Capacitors Selection

Guide

2.2µF Surface-Mount Ceramic

Manufacturer

Part Number

Tokin

1E225ZY5U-C203

Murata

GRM42-6Y5V225Z16

4.7µF Surface-Mount Ceramic

Tokin

1E475ZY5U-C304

Reverse Current Path

The P-channel Pass transistor used in the LM4140 has an

inherent diode connected between the V

(see diagram below).

Forcing the output to voltages higher than the input, or pull-

ing V

than a V

from the V

occur under these conditions as long as the current flowing

into the output pin does not exceed 50mA.

ON/OFF Operation

The LM4140 is designed to quickly reduce both V

to zero when turned-off. V

when turned-on. During the turn-off, the charge across the

output capacitor is discharged to ground through internal cir-

cuitry.

The LM4140 is turned-off by pulling the enable input low, and

turned-on by driving the input high. If this feature is not to be

used, the enable pin should be tied to the V

erence on at all times (the enable pin must not be left float-

ing).

To ensure proper operation, the signal source used to drive

the enable pin must be able to swing above and below the

specified high and low voltage thresholds which guarantee

an ON or OFF state (see Electrical Characteristics).

The ON/OFF signal may come from either a totem-pole out-

put, or an open-collector output with pull-up resistor to the

LM4140 input voltage. This high-level voltage may exceed

the LM4140 input voltage, but must remain within the Abso-

lute Maximum Rating for the enable pin.

Output Accuracy

Like all references, either series or shunt, the after assembly

accuracy is made up of primarily three components: initial

accuracy itself, thermal hysteresis and effects of the PCB as-

sembly stress.

LM4140 provides an excellent output initial accuracy of 0.1%

and temperature coefficient of 6ppm/˚C (B Grade).

For best accuracy and precision, the LM4140 junction tem-

perature should not exceed 70˚C.

The thermal hysteresis curve on this datasheet are perfor-

mance characteristics of three typical parts selected at ran-

dom from a sample of 40 parts.

Parts are mounted in a socket to minimize the effect of

PCB’s mechnical expansion and contraction. Readings are

taken at 25˚C following multiple temperature cycles to 0˚C

and 70˚C. The labels on the X axis of the graph indicates the

device temperature cycle prior to measurement at 25˚C.

The mechanical stress due to the PCB’s mechanical and

thermal stress can cause an output voltage shift more than

the true thermal coefficient of the device. References in sur-

face mount packages are more susceptible to these stresses

because of the small amount of plastic molding which sup-

port the leads.

Following the recommendations on PCB Layout Consider-

ation section can minimize the mechanical stress on the de-

vice.

PCB Layout Consideration

The simplest ways to reduce the stress related shifts are:

1.

Mounting the device near the edges or the corners of the

board where mechanical stress is at its minimum. The

center of the board generally has the highest mechani-

cal and thermal expansion stress.

2.

Mechanical isolation of the device by creating an island

by cutting a U shape slot on the PCB for mounting the

device. This approach would also provide some thermal

isolation from the rest of the circuit.

Figure 5 is a recommended printed board layout with a slot

cut on three sides of the circuit layout to serve as a strain

relief.

DS101079-3

DS101079-33

FIGURE 4. Typical Thermal Hysteresis

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