6

LTC1258 Series

Figure 1. Reference Output Load Transient Response, 1

µF Output Capacitor

1mA

0mA

1mA

0mA

100mV/DIV

100mV/DIV

0.5ms/DIV

1258 F01b

10ms/DIV

1258 F01a

Figure 2. Adding a Damping Resistor with

Output Capacitors Greater Than 1

µF

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, result-

ing 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 10.8

µW 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 1

µF can be used to

improve the output transient response. Figure 1 shows the

response of the reference to a 1mA to 0

µA load step with

a 1

µF output capacitor. If more than 1µF of output capaci-

tance is required, a resistor in series with the capacitor is

recommended to reduce the output ringing. Figure 2

illustrates the use of a damping resistor for capacitive

loads greater than 1

µF. Figure 3 shows the resistor and

capacitor values required to achieve critical damping.

Figure 3. Damping Resistance vs Output Capacitor Value

LTC1258-5

OUT

IN

GND

4

2

5.2V

TO

12.6V

1

5V

1258 F02

0.1

µF

>1

µF

DAMPING RESISTOR

<1k

OUTPUT CAPACITOR VALUE (

µF)

1

10

100

1k

10k

10

100

1000

1258 F03