REV. B

ADP3334

–7–

To have the lowest possible sensitivity of the output voltage to

temperature variations, it is important that the value of the parallel

resistance of R1 and R2 be kept as close as possible to 50 k

W.

RR

RR

k

12

12

50

¥

+

=W

(1)

Also, for the best accuracy over temperature, the feedback volt-

age should be set for 1.178 V:

VV

R

RR

FB

OUT

=¥

+

Ê

ËÁ

ˆ

¯˜

2

12

(2)

FB pin due to loading by the internal PTAT current.

Combining the above equations and solving for R1 and R2 gives

the following formulas:

Rk

V

V

OUT

FB

150

=¥ Ê

ËÁ

ˆ

¯˜

W

(3)

R

k

V

V

FB

OUT

2

50

1

=

-

Ê

ËÁ

ˆ

¯˜

W

(4)

Table I. Feedback Resistor Selection

) R1 (1% Resistor) (k ) R2 (1% Resistor) (k )

1.5

63.4

232.0

1.8

76.8

147.0

2.2

93.1

107.0

2.7

115.0

88.7

3.3

140.0

78.7

5.0

210.0

64.9

10.0

422.0

56.2

Using standard 1% values, as shown in Table I, will sacrifice

some output voltage accuracy. To estimate the overall output

voltage accuracy, it is necessary to take into account all sources

of error. The accuracy given in the specifications table does not

take into account the error introduced by the feedback resistor

divider ratio or the error introduced by the parallel combination

of the feedback resistors.

The error in the parallel combination of the feedback resistors

causes the reference to have a wider variation over temperature.

To estimate the variation, calculate the worst-case error from

50 k

W, and then use the graph in Figure 4 to estimate the

additional change in the output voltage over the operating

temperature range.

For example:

R1 = 140 k

W, 1%

R2 = 78.7 k

W, 1%

Rp ERROR – %

3.0

2.5

2.0

1.5

1.0

0.5

0

02

3

456

Figure 4. Output Voltage Error vs.

Parallel Resistance Error

The actual output voltage can be calculated using the following

equation.

V.

V

R

R

V.

V

OUT

OUT

=¥

+

Ê

ËÁ

ˆ

¯˜

=

1 178

1

2

1

3 274

(5)

So worst-case error will occur when R1 has a –1% tolerance and

R2 has a +1% tolerance. Recalculating the output voltage, the

parallel resistance and error are:

V.

V

.

.

V.

V

Resistor Divider Error

.

.

OUT

OUT

=¥

+

Ê

ËÁ

ˆ

¯˜

=

=-

Ê

ËÁ

ˆ

¯˜

¥= -

1 178

138 6

79 5

1

3 232

3 232

33

1

100

2 1

%. %

(6)

R

RR

RR

..

..

.

k

R

Error

.

PARALLEL

PARALLEL

=

¥

+

=

¥

+

=

=-

Ê

ËÁ

ˆ

¯˜

¥=

12

12

138 6

79 5

138 6

79 5

50 51

50 51

50

1

100

1 02

W

%.

%

(7)

So, from the graph in Figure 4, the output voltage error is

estimated to be an additional 0.25%. The error budget is

1.8% (the initial output voltage accuracy over temperature),

plus 2.1% (resistor divider error), plus 0.25% (parallel resis-

tance error) for a worst-case total of 4.15%.

Thermal Overload Protection

The ADP3334 is protected against damage from excessive power

dissipation by its thermal overload protection circuit, which limits

the die temperature to a maximum of 165°C. Under extreme

conditions (i.e., high ambient temperature and power dissipation)

where die temperature starts to rise above 165°C, the output

current is reduced until the die temperature has dropped to a safe

level. The output current is restored when the die temperature

is reduced.