LTC1258 Series

6

1258sfb

BLOCK DIAGRAM

–

+

IN

1

1258 BD

2

4

OUT

3 (LTC1258)

FB

2.385V

GND

R1

R2

NOTE: R1 AND R2 ARE NOT CONNECTED FOR LTC1258

APPLICATIONS INFORMATION

Longer Battery Life

Series references have an advantage over shunt style

references. To operate, shunt references require a resistor

between the power supply and the output. This resistor

must be chosen to supply the maximum current that is

demanded by the circuit being regulated. When the circuit

being controlled is not operating at this maximum current,

the shunt reference must always sink this current, resulting

in high power dissipation and short battery life.

The LTC1258 series low dropout references do not require

a current setting resistor and can operate with any supply

circuitry being regulated does not demand current, the

LTC1258 series reduces its dissipation and battery life is

extended. If the reference is not delivering load current it

dissipates only 10.8μW when operating on a 2.7V supply

for LTC1258-2.5, yet the same connection can deliver

10mA of load current when demanded.

Output Bypass Capacitor

The LTC1258 series is designed to be stable with or without

capacitive loads. With no capacitive load, the reference

is ideal for fast settling applications, or where PC board

space is at a premium.

In applications with significant output loading changes,

an output bypass capacitor of up to 1μF can be used to

improve the output transient response. Figure 1 shows

the response of the reference to a 1mA to 0μA load step

with a 1μF output capacitor. If more than 1μF of output

capacitance is required, a resistor in series with the capaci-

tor is recommended to reduce the output ringing. Figure 2

illustrates the use of a damping resistor for capacitive loads

greater than 1μF. Figure 3 shows the resistor and capacitor

values required to achieve critical damping.

1mA

0mA

10ms/DIV

1258 F01a

100mV/DIV

1mA

0mA

0.5ms/DIV

1258 F01b

100mV/DIV

Figure 1. Reference Output Load Transient Response, 1μF Output Capacitor