LT1585A

5

MOTOROLA ANALOG IC DEVICE DATA

The use of capacitors with low ESR, low ESL and good

high frequency characteristics is critical in meeting the output

voltage tolerances of these high speed microprocessors.

These requirements dictate a combination of high quality,

surface mount tantalum capacitors and ceramic capacitors.

The location of the decoupling network is critical to

transient response performance. Place the decoupling

network as close as possible to the processor pins because

trace runs from the decoupling capacitors to the processor

pins are inductive. The ideal location for the decoupling

network is actually inside the microprocessor socket cavity.

In addition, use large power and ground plane areas to

minimize distribution drops.

A possible stability problem that occurs in monolithic linear

regulators is current limit oscillations. The LT1585A

essentially has a flat current limit over the range of input

supply voltage. The lower current limit rating and 12V

maximum supply voltage rating for these devices permit this

characteristic.

Current limit oscillations are typically nonexistent, unless

the input and output decoupling capacitors for the regulators

are mounted several inches from the terminals.

Protection Diodes

In normal operation, the LT1585A does not require any

protection diodes. Older 3–terminal regulators require

protection diodes between the output pin and the input pin or

between the adjust pin and the output pin to prevent die

overstress.

Built–in 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

output pins on the LT1585A 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 500µF, and with the input pin

instantaneously shorted to ground can damage occur. A

crowbar circuit at the input of the LT1585A can generate

those levels of current, and a diode from output to input is

then recommended. This is 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.

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.

Vout

Vin

LT1585A

ADJ

IN

OUT

D1

1N4002

(Optional)

+

+

+

R2

R1

C2

10

mF

C1

10

mF

C

Figure 10.

Ripple Rejection

A bypass capacitor from the adjust pin to ground reduces

the output ripple by the ratio of VOUT /1.25 V. The impedance

of the adjust pin capacitor at the ripple frequency should be

less than the value of R1 (typically in the rage of 100

Ω to

120

Ω) 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.

Output Voltage

The LT1585A adjustable 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.

The current out of the adjust pin adds to the current from R1

and is typically 55

µA. Its output voltage contribution is small

and only needs consideration when very precise output

voltage setting is required.