LT1790 Просмотр технического описания (PDF) - Linear Technology

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LT1790

Micropower SOT-23 Low Dropout Reference Family

Linear Technology 

LT1790

APPLICATIONS INFORMATION

VGEN

8V

6V

4V

VOUT

(AC Coupled)

2V

0V

1790 F06

Figure 6. LT1790-2.5 Sinking – 4mA to – 5mA

Positive or Negative Operation

Series operation is ideal for extending battery life. If an

LT1790 is operated in series mode it does not require an

external current setting resistor. The specifications guar-

antee that the LT1790 family operates to 18V. When the

circuitry being regulated does not demand current, the

series connected LT1790 consumes only a few hundred

µW, yet the same connection can sink or source 5mA of

load current when demanded. A typical series connection

is shown on the front page of this data sheet.

The circuit in Figure 7 shows the connection for a – 2.5V

reference, although any LT1790 voltage option can be

configured this way to make a negative reference. The

LT1790 can be used as very stable negative references,

however, they require a positive voltage applied to Pin 4

to bias internal circuitry. This voltage must be current

limited with R1 to keep the output PNP transistor from

turning on and driving the grounded output. C1 provides

4

6

LT1790-2.5

1, 2

RL

=

VEE – VOUT

125µA

VEE

R1

10k

3V

C1

0.1µF

VOUT = –2.5V

CL

1µF

1790 F07

Figure 7. Using the LT1790-2.5 to Build a –2.5V Reference

stability during load transients. This connection main-

tains nearly the same accuracy and temperature coeffi-

cient of the positive connected LT1790.

Long-Term Drift

Long-term drift cannot be extrapolated from acceler-

ated high temperature testing. This erroneous tech-

nique gives drift numbers that are widely optimistic. The

only way long-term drift can be determined is to mea-

sure it over the time interval of interest. The LT1790S6

drift data was taken on over 100 parts that were soldered

into PC boards similar to a “real world” application. The

boards were then placed into a constant temperature oven

with TA = 30°C, their outputs scanned regularly and

measured with an 8.5 digit DVM. Long-term drift curves

are shown in the Typical Performance Characteristics.

Hysteresis

Hysteresis data shown in Figures 8 and 9 represent the

worst-case data taken on parts from 0°C to 70°C and from

– 40°C to 85°C. Units were cycled several times over these

temperature ranges and the largest change is shown. As

expected, the parts cycled over the higher temperature

range have higher hysteresis than those cycled over the

lower range.

When an LT1790 is IR reflow soldered onto a PC board, the

output shift is typically just 150ppm (0.015%).

Higher Input Voltage

The circuit in Figure 10 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 be-

tween 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.

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, assuming

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.

1790fa

19

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