RC1585

PRODUCT

SPECIFICATION

6

Applications Information

General

The RC1585 and RC1585-1.5 are three-terminal regulators

applications. These devices are short-circuit protected, safe

area protected, and offer thermal shutdown to turn off the

regulator when the junction temperature exceeds about

150

°C. The RC1585 series provides low dropout voltage

and fast transient response. Frequency compensation uses

capacitors with low ESR while still maintaining stability.

This is critical in addressing the needs of low voltage high

speed microprocessor buses like GTL+.

Stability

The RC1585 series requires an output capacitor as a part

of the frequency compensation. It is recommended to use a

22

µF solid tantalum or a 100 µF aluminum electrolytic on

the output to ensure stability. The frequency compensation

of these devices optimizes the frequency response with low

ESR capacitors. In general, it is suggested to use

capacitors with an ESR of

<1Ω. It is also recommended to

use bypass capacitors such as a 22

µF tantalum or a 100µF

aluminum on the adjust pin of the RC1585 for low ripple

and fast transient response. When these bypassing

capacitors are not used at the adjust pin, smaller values of

output capacitors provide equally good results.

Protection Diodes

In normal operation, the RC1585 series does not require

any protection diodes. For the RC1585, internal resistors

limit internal current paths on the adjust pin. Therefore,

even with bypass capacitors on the adjust pin, no

protection diode is needed to ensure device safety under

short-circuit conditions.

A protection diode between the input and output pins is

usually not needed. An internal diode between the input

and the output pins on the RC1585 series can handle

microsecond surge currents of 50A to 100A. Even with

large value output capacitors it is difficult to obtain those

values of surge currents in normal operation. Only with

large values of output capacitance, such as 1000

µF to

5000

µF, and with the input pin instantaneously shorted to

ground can damage occur. A crowbar circuit at the input

can generate those levels of current; a diode from output to

input is then recommended, as shown in Figure 10.

Usually, normal power supply cycling or system “hot

plugging and unplugging” will not generate current large

enough to do any damage.

Ripple Rejection

In applications that require improved ripple rejection, a

bypass capacitor from the adjust pin of the RC1585 to

ground reduces the output ripple by the ratio of

the ripple frequency should be less than the value of R1

(typically in the range of 100

Ω to 120Ω) in the feedback

divider network in Figure 10. Therefore, the value of the

required adjust pin capacitor is a function of the input

ripple frequency. For example, if R1 equals 100

Ω and

the ripple frequency equals 120Hz, the adjust pin

capacitor should be 22

µF. At 10kHz, only 0.22µF is

needed.

D1

1N4002

(OPTIONAL)

R2

R1

C1

10

µF

C2

22

µF

RC1585

IN

OUT

ADJ

+

+

+

C2

22

µF

+

D1

1N4002

(OPTIONAL)

C1

10

µF

RC1585-1.5

IN

OUT

GND

+

Figure 10. Optional Protection

The adjust pin can be driven on a transient basis

±7V

with respect to the output, without any device

degradation. As with any IC regulator, exceeding the

maximum input-to-output voltage differential causes

the internal transistors to break down and none of the

protection circuitry is then functional.

Output Voltage

The RC1585 regulator develops a 1.25V reference

voltage between the output pin and the adjust pin (see

Figure 11). Placing a resistor R1 between these two

terminals causes a constant current to flow through R1

and down through R2 to set the overall output voltage.

Normally, this current is the specified minimum load

current of 10mA.