13

LT1761 Series

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 voltage 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 adjustable device is tested and specified with the ADJ

pin tied to the 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 to

output current change of 1mA to 100mA is –1mV typical

(12V/1.22V)(–1mV) = – 9.8mV

IN

1761 F01

R2

LT1761

OUT

ADJ

GND

R1

+

VV

R

R

IR

VV

InA

OUT

ADJ

ADJ

ADJ

=+









+

()( )

=

=°

122

1

2

1

2

122

30

.

.

AT 25 C

OUTPUT RANGE = 1.22V TO 20V

Figure 1. Adjustable Operation

APPLICATIO S I FOR ATIO

Bypass Capacitance and Low Noise Performance

The LT1761 regulators may be used with the addition of a

voltage noise. A good quality low leakage capacitor is rec-

ommended. This capacitor will bypass the reference of the

regulator, providing a low frequency noise pole. The noise

pole provided by this bypass capacitor will lower the out-

put voltage noise to as low as 20

of a 0.01

µF bypass capacitor. Using a bypass capacitor

has the added benefit of improving transient response.

With no bypass capacitor and a 10

µF output capacitor, a

10mA to 100mA load step will settle to within 1% of its final

value in less than 100

µs. With the addition of a 0.01µF

bypass capacitor, the output will stay within 1% for a 10mA

to 100mA load step (see LT1761-5 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 0.01

µF

bypass capacitor and 10

µF output capacitor.

Output Capacitance and Transient Response

The LT1761 regulators are designed to be stable with a

wide range of output capacitors. The ESR of the output

capacitor affects stability, most notably with small

capacitors. A minimum output capacitor of 1

µF with an

ESR of 3

Ω or less is recommended to prevent oscilla-

tions. The LT1761-X is a micropower device and output

transient response will be a function of output capaci-

tance. 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

LT1761-X, will increase the effective output capacitor

value. With larger capacitors used to bypass the refer-

ence (for low noise operation), larger values of output

capacitors are needed. For 100pF of bypass capacitance,

2.2

µF of output capacitor is recommended. With a 330pF

bypass capacitor or larger, a 3.3

µF output capacitor is

recommended. The shaded region of Figure 2 defines the

region over which the LT1761 regulators are stable. The

minimum ESR needed is defined by the amount of

bypass capacitance used, while the maximum ESR is 3

Ω.

Extra consideration must be given to the use of ceramic

capacitors. Ceramic capacitors are manufactured with a

variety of dielectrics, each with different behavior across

temperature and applied voltage. The most common

Figure 2. Stability

OUTPUT CAPACITANCE (

µF)

1

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0

310

1761 F02

24

5

6 78 9

STABLE REGION