61CI33 View Datasheet(PDF) - Linear Technology

Part Name

Description

Manufacturer

61CI33

Micropower Precision Low Dropout Series Voltage Reference Family

Linear Technology 

LT1461

APPLICATIO S I FOR ATIO

Precision Regulator

The LT1461 will deliver 50mA with VIN = VOUT + 2.5V and

higher load current with higher VIN. Load regulation is

typically 12ppm/mA, which means for a 50mA load step,

the output will change by only 1.5mV. Thermal regulation,

caused by die temperature gradients and created from

load current or input voltage changes, is not measurable.

This often overlooked parameter must be added to normal

line and load regulation errors. The load regulation photo,

on the first page of this data sheet, shows the output

response to 200mW of instantaneous power dissipation

and the reference shows no sign of thermal errors. The

reference has thermal shutdown and will turn off if the

junction temperature exceeds 150°C.

Shutdown

The shutdown (Pin 3 low) serves to shut off load current

when the LT1461 is used as a regulator. The LT1461

operates normally with Pin 3 open or greater than or equal

to 2.4V. In shutdown, the reference draws a maximum

supply current of 35µA. Figure 3 shows the transient

response of shutdown while the part is delivering 25mA.

After shutdown, the reference powers up in about 200µs.

5V

PIN 3

0V

VOUT

0V

1461 F03

Figure 3. Shutdown While Delivering 25mA, RL = 100Ω

PC Board Layout

In 13- to 16-bit systems where initial accuracy and tem-

perature coefficient calibrations have been done, the me-

chanical and thermal stress on a PC board (in a card cage

for instance) can shift the output voltage and mask the true

temperature coefficient of a reference. In addition, the

mechanical stress of being soldered into a PC board can

cause the output voltage to shift from its ideal value.

Surface mount voltage references are the most suscep-

tible to PC board stress because of the small amount of

plastic used to hold the lead frame.

A simple way to improve the stress-related shifts is to

mount the reference near the short edge of the PC board,

or in a corner. The board edge acts as a stress boundary,

or a region where the flexure of the board is minimum. The

package should always be mounted so that the leads

absorb the stress and not the package. The package is

generally aligned with the leads parallel to the long side of

the PC board as shown in Figure 5a.

A qualitative technique to evaluate the effect of stress on

voltage references is to solder the part into a PC board and

deform the board a fixed amount as shown in Figure 4. The

flexure #1 represents no displacement, flexure #2 is

concave movement, flexure #3 is relaxation to no dis-

placement and finally, flexure #4 is a convex movement.

This motion is repeated for a number of cycles and the

1

2

3

4

1461 F04

Figure 4. Flexure Numbers

10

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