LM8207 datasheet(13/22 Pages) NSC

13 / 22 page

Application Section (Continued)

To calculate the output voltage of the Voltage Reference

Amplifier (OUT_V

REF) use the following equation:

(1)

As can be seen in the Electrical Characteristics Table on

page 3, I

IN_VREF has a typical value of −10 nA. Using resistor

values for R

1 =9k

Ω and R

2 =1k

Ω this results in a gain of

10 and OUT_V

REF = 12.95 V an error will be introduced of

−10 nA*9 k

Ω = −90 µV. This error can be neglected. The

simplified formula for calculating the OUT_V

REF is:

(2)

Example:

DD = 16V

OUT_V

REF = 14.4V

Choose R

2 =5k

Ω. Using Equation (2), this will result in

1 = 50.6 k

Ω

THE OPERATING RANGE OF THE VOLTAGE

REFERENCE

The output of the Voltage Reference Amplifier has a mini-

mum of 1.295V (R

1 = 0). This is determined by the value of

the internal reference. The maximum output voltage

(OUT_V

REFMAX) can approach the positive supply rail VDD.

The voltage is limited by the output resistance (R

OUT)ofthe

output stage of the internal amplifier and depends on the

load current. Figure 5 shows the operating output voltage

range.

The minimum headroom (OUT_V

REF with respect to the

positive supply rail V

DD) can be measured using the test

circuit shown in Figure 6.

The headroom is measured by varying both the supply volt-

age and the output current (I

LOAD) for a fixed programmed

value of OUT_V

REF. As shown in Figure 7, the minimum

headroom slightly increases for a constant V

DD when the

load current increases.

20137925

FIGURE 4. Typical Application Circuit for V

REF

20137921

FIGURE 5. Operating Output Voltage Range

20137924

FIGURE 6. Headroom Test Circuit with Variable Output

Current Load

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