VRE4100 datasheet(6/7 Pages) ETC

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VRE4100DS REV. A MAY 01

OPERATIONAL NOTES

Input Capacitor

An input capacitor is recommended on the VRE4100

device. A supply bypass capacitor on the input will

assure that the reference is working from a low

impedance source which will improve stability.

can improve the transient response when the load

current is suddenly increased.

Output Capacitor

The VRE4100 requires a 1µF output capacitor for

loop

stabilization

(compensation)

well

transient response. When the load current changes,

the output capacitor must source or sink current

during the time it takes the control loop of the

VRE4100 to respond.

The output capacitor must meet the requirements of

minimum

capacitance

and

equivalent

series

resistance (ESR) range.

See Capacitor Selection

below.

Capacitor Selection

A minimum value of 0.2µF over the operating

temperature range is recommended. For a 0.22µF

capacitor the ESR range for 0

°C -70°C is 0.9 to 6.0,

1.0µF is 0.8 to 6.0, and 10µF is 0.4 to 7.0.

Surface mount tantalum capacitors offer small size

for the value and ESR in the range required for the

VRE4100. The optimum performance for the output

capacitor is achieved with a 1.0µF value.

Aluminum electrolytic capacitors have a relatively

large size for the value.

They meet the ESR

requirements at 1.0µF as long as the temperature is

above 0

°C. Below 0°C, the ESR increases and it

may exceed the limits indicated in the figures.

Multilayer ceramic capacitors have a small size for

the value, are available in surface mount, and have

excellent RF characteristics. They may not meet the

minimum ESR requirements and have a large

change in value with temperature.

Reverse Current Path

The P-channel pass transistor used in the VRE4100

has an inherent diode connected between the V

and V

OUT pins. Forcing the output to voltages higher

than the input or pulling V

in below the voltage stored

in the output capacitor by more than the V

be will

forward bias this diode and current will flow from the

out pin to Vin. This will not damage the VRE4100 as

long as the current does not exceed 50mA.

ON/OFF Operation

The VRE4100 features a sleep mode that is

activated by pulling the enable pin low. To turn the

reference on, the enable pin is pulled high. If this

feature is not used, the the enable pin should be tied

to V

in to keep the reference on at all times.

The

enable pin must not be left unconnected (floating).

When powered off, the VRE4100 will quickly reduce

both V

out and IQ to zero.

During power down, the

charge across the output capacitor is discharged to

ground through the internal circuitry. On power up,

the V

out is restored in less than 200µs.

The signal source used to drive the enable pin can

come from either a totem-pole output or an open

collector output with a pull-up resistor to the

VRE4100 input voltage. The signal source must be

able to swing above and below the voltage

thresholds to guarantee an ON or OFF state.

must not exceed the absolute maximum rating for

the enable pin.

Output Accuracy

The output accuracy after assembly at room

temperature is made up of three components: initial

accuracy of the device, thermal hysterisis, and

mechanical stress. The initial accuracy is measured

at the factory and may not reflect the actual output

voltage when the devices are mounted to a PCB.

The effects of mechanical stress and thermal

hysterisis can shift the output voltage.

Thermal Hysterisis

Thermal hysterisis is a change in output voltage

as a result of a temperature change.

When

references experience a temperature change and

return to the initial temperature, they do not always

have the same initial voltage. Thermal hysterisis is

difficult to correct and is a major error source in

systems

that

experience

temperature

changes

greater than 25°C. Reference vendors are starting

to include this important specification in their

datasheets

Mechanical Hysterisis

Recommendations to minimize mechanical stress:

1) Mount the VRE4100 near the edges or corners of

the PCB. The center of the board generally has the

highest mechanical and thermal stress.

Mechanically isolate the device by cutting a U

shaped slot around the VRE4100.

This provides

some mechanical and thermal isolation from the rest

of the circuit.

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