LT1460-5 Linear Technology, LT1460-5 Datasheet - Page 7

• Manufacturer Part Number: LT1460-5
• Description: Micropower Precision Series Reference
• Manufacturer: Linear Technology

APPLICATIONS
Output Accuracy
Like all references, either series or shunt, the error budget
of the LT1460-5 is made up of primarily three compo-
nents: initial accuracy, temperature coefficient and load
regulation. Line regulation is neglected because it typically
contributes only 30ppm/V, or 150 V for a 1V input change.
The LT1460-5 typically shifts less than 0.01% when
soldered into a PCB, so this is also neglected (see PC
Board Layout section). The output errors are calculated as
follows for a 100 A load and 0 C to 70 C temperature
range:
LT1460AC
Initial accuracy = 0.075%
For I
which is 0.035%.
capacitance on the input and no output load. In this case
the supply dv/dt is 7.5V in 30ns which causes internal
overslew, and the output does not bias to 5V until 45 s.
Although 45 s is a typical turn-on time, it can be much
longer. A 0.1 F input capacitor guarantees the part always
starts quickly as shown in Figure 5.
I
0
100 A
10mA
20mA
OUT
O
V
OUT
= 100 A,
V
OUT
V
IN
LT1460AC
0.145%
0.180%
0.325%
0.425%
3500
mA
Figure 5. C
ppm
U
INFORMATION
20 s/DIV
U
0 1
LT1460BI
0.225%
0.260%
0.405%
N/A
IN
mA V
= 0.1 F
5
W
LT1460CC
0.205%
0.240%
0.385%
0.485%
1 75
1460-5 F04
mV
0V
U
0V
7.5V
LT1460DC
0.240%
0.275%
0.420%
0.520%
For temperature 0 C to 70 C the maximum T = 70 C,
which is 0.07%.
Total worst-case output error is:
Table 1 gives worst-case accuracy for the LT1460AC, CC,
DC, FC, GC from 0 C to 70 C and the LT1460BI, EI, GI
from – 40 C to 85 C.
PC Board Layout
In 13- to 16-bit systems where initial accuracy and tem-
perature coefficient calibrations have been done, the
mechanical and thermal stress on a PC board (in a cardcage
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 (MS8 and S8) are the
most susceptible 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 7a.
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 6. 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.
0.075% + 0.035% + 0.070% = 0.180%.
V
OUT
LT1460EI
0.375%
0.410%
0.555%
N/A
10
ppm
C
LT1460FC
0.325%
0.360%
0.505%
0.605%
70
C
5
V
LT1460GC
0.425%
0.460%
0.605%
0.705%
3 5
mV
LT1460-5
LT1460GI
0.562%
0.597%
0.742%
N/A
7