LT1790 datasheet(22/26 Pages) LINER

22 / 26 page

LT1790

1790fb

APPLICATIONS INFORMATION

the shift after 1 cycle, while Figure 12 shows shift after

3 cycles. In the worst case, shifts are typically 150ppm,

but may be as high as 290ppm. Shifts in output voltage

are proportional to temperature and dwell time.

In general, the output shift can be reduced or fully recov-

ered by a long (12-24 hour) bake of the completed PC

Board assembly at high temperature (100°C to 150C°)

after soldering to remove mechanical stress that has been

induced by thermal shock. Once the PC Boards have cooled

to room temperature, they may continue to shift for up to

3 times the bake time. This should be taken into account

before any calibration is performed.

Assuming 80μA max supply current for the LT1790, a

25μA load, 120mV max dropout and a 4V to 30V input

speciﬁcation, the largest that R1 can be is (4V – 3.3V

– 120mV)/(80μA + 25μA) = 5.5k. Furthermore, assum-

ing 220mW of dissipation in the 18V SOT-23 Zener, this

gives a max current of (220mW)/(18V) = 12.2mA. So the

smallest that R1 should be is (30V – 18V)/12.2mA = 1k,

rated at 150mW.

With R1 = 1k, and assuming a 450mV worst-case drop-

out, the LT1790 can deliver a minimum current of (4V

– 3.3V–450mV)/(1k) = 250μA. In Figure 13, R1 and C1

provide ﬁltering of the Zener noise when the Zener is in

its noisy V-I knee.

There are other variations for higher voltage operation that

use a pass transistor shown in Figures 14 and 15. These

circuits allow the input voltage to be as high as 160V while

maintaining low supply current.

LT1790-3.3

4V TO 30V

0.1μF

BZX84C18

1μF

VOUT

1790 F13

Figure 13. Extended Supply Range Reference

PPM

290

270

250

230

210

190

170

150

130

110

2.0

2.5

3.0

1790 F12

1.5

1.0

0.5

3.5

Figure 12. 3X IR Reﬂow Peak Temperature = 250°C,

Delta Output Voltage (ppm)

LT1790

0.1μF

1μF

ON SEMI

MMBT5551

6V TO 160V

VOUT

1790 F14

4.7k

330k

BZX84C12

LT1790

1μF

BAV99

VOUT

1790 F15

0.1μF

330k

6.5V TO 160V

ON SEMI

MMBT5551

Figure 14. Extended Supply Range Reference

Figure 15. Extended Supply Range Reference

Higher Input Voltage

The circuit in Figure 13 shows an easy way to increase the

input voltage range of the LT1790. The Zener diode can be

anywhere from 6V to 18V. For equal power sharing between

R1 and the Zener (at 30V), the 18V option is better. The

circuit can tolerate much higher voltages for short periods

and is suitable for transient protection.

LT1790 Datasheet(PDF) 22 Page - Linear Technology

LT1790 Datasheet(HTML) 22 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

Datasheet Download

Link URL