11

LT3028

3028f

APPLICATIO S I FOR ATIO

Figure 1. Adjustable Operation

IN1/IN2

3024 F01

R2

LT3028

OUT1/OUT2

ADJ1/ADJ2

GND

R1

+

VV

R

R

IR

VV

InA AT

C

OUTPUT RANGE

V TO

V

OUT

ADJ

ADJ

ADJ

=+

⎛

⎝

⎜

⎞

⎠

⎟ +

()( )

=

=°

122

1

2

1

2

122

30

25

122

20

.

.

= .

than 1µA in shutdown. In addition to the low quiescent

current, the LT3028 regulator incorporates several pro-

tection features which make it ideal for use in battery-

powered systems. The device is protected against reverse

input voltages. Additionally, in dual supply applications

where the regulator load is returned to a negative supply,

the output can be pulled below ground by as much as 20V

and still allow the device to start and operate.

Adjustable Operation

The LT3028 has an output voltage range of 1.22V to 20V.

The output voltage is set by the ratio of two external resis-

tors as shown in Figure 1. The device servos the output to

maintain the corresponding ADJ pin voltage at 1.22V ref-

erenced to ground. The current in R1 is then equal to

1.22V/R1 and the current in R2 is the current in R1 plus the

ADJ pin bias current. The ADJ pin bias current, 30nA at

25°C, flows through R2 into the ADJ pin. The output volt-

age can be calculated using the formula in Figure 1. The

value of R1 should be no greater than 250k to minimize

errors in the output voltage caused by the ADJ pin bias

current. Note that in shutdown the output is turned off and

the divider current will be zero. Curves of ADJ Pin Voltage

vs Temperature and ADJ Pin Bias Current vs Temperature

appear in the Typical Performance Characteristics.

The device is tested and specified with the ADJ pin tied to

the corresponding OUT pin for an output voltage of 1.22V.

Specifications for output voltages greater than 1.22V will

be proportional to the ratio of the desired output voltage

Output 2 for an output current change of 1mA to 100mA

regulation is:

(12V/1.22V)(–1mV) = – 9.8mV

Bypass Capacitance and Low Noise Performance

The LT3028 regulator may be used with the addition of a

to lower output voltage noise. A good quality low leakage

capacitor is recommended. This capacitor will bypass the

reference of the regulator, providing a low frequency noise

pole. The noise pole provided by this bypass capacitor will

the addition of a 0.01µF bypass capacitor. Using a bypass

capacitor has the added benefit of improving transient

response. With no bypass capacitor and a 10µF output

capacitor, a 10mA to 100mA load step on Output 2 will

settle to within 1% of its final value in less than 100µs. With

the addition of a 0.01µF bypass capacitor, the output will

stay within 1% for the same load step. Both outputs exhibit

this improvement in transient response (see Transient

Reponse in Typical Performance Characteristics section).

However, regulator start-up time is inversely proportional

to the size of the bypass capacitor, slowing to 15ms with

a 0.01µF bypass capacitor and 10µF output capacitor.

Output Capacitance and Transient Response

The LT3028 regulator is designed to be stable with a wide

range of output capacitors. The ESR of the output capaci-

tor affects stability, most notably with small capacitors.

A minimum output capacitor of 1µF with an ESR of 3Ω or

less is recommended for Output 2 to prevent oscillations.

A minimum output capacitor of 3.3µF with an ESR of 3Ω

or less is recommended for Output 1. The LT3028 is a

micropower device and output transient response will be

a function of output capacitance. Larger values of output

capacitance decrease the peak deviations and provide

improved transient response for larger load current

changes. Bypass capacitors, used to decouple individual

components powered by the LT3028, will increase the

effective output capacitor value. With larger capacitors