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LT1580

7A, Very Low Dropout Regulator

Linear Technology 

LT1580/LT1580 -2.5

APPLICATIONS INFORMATION

load current changes. Output capacitance can increase

without limit and larger values of output capacitance

further improve the stability and transient response of the

LT1580.

Modern microprocessors generate large high frequency

current transients. The load current step contains higher

order frequency components that the output coupling

network must handle until the regulator throttles to the

load current level. Capacitors are not ideal elements and

contain parasitic resistance and inductance. These para-

sitic elements dominate the change in output voltage at the

beginning of a transient load step change. The ESR of the

output capacitors produces an instantaneous step in out-

put voltage (∆V = ∆I)(ESR). The ESL of the output capaci-

tors produces a droop proportional to the rate of change

of the output current (V = L)(∆I/∆t). The output capaci-

tance produces a change in output voltage proportional to

the time until the regulator can respond (∆V = ∆t)(∆I/ C).

These transient effects are illustrated in Figure 4 .

ESR

EFFECTS

ESL

EFFECTS

SLOPE,

V

t

=

∆I

C

POINT AT WHICH REGULATOR

TAKES CONTROL

CAPACITANCE

EFFECTS

1580 F04

Figure 4

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 micropro-

cessors. These requirements dictate a combination of

high quality, surface mount, tantalum and ceramic capaci-

tors. The location of the decoupling network is critical to

transient performance. Place the decoupling network as

close to the processor pins as possible 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 addi-

tion, use large power and ground plane areas to minimize

distribution drops.

Output Voltage

The adjustable version of the LT1580 develops a 1.25V

reference voltage between the SENSE pin and the ADJ pin

(see Figure 5). 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 R1 is chosen so that this current is the specified

minimum load current of 10mA. The current out of the ADJ

pin adds to the current from R1. The ADJ pin current is

small, typically 50µA. The output voltage contribution of

the ADJ pin current is small and only needs to be consid-

ered when very precise output voltage setting is required.

Note that the top of the resistor divider should be con-

nected directly to the SENSE pin for best regulation. See

the section on grounding and Kelvin sensing above.

+

VPOWER +

VCONTROL

VCONTROL

VPOWER

LT1580

VOUT

SENSE

ADJ

VREF

+

R1

VOUT

IADJ = 50µA

( ) VOUT = VREF

1 + R2

R1

+ IADJ (R2)

R2

1580 F05

Figure 5. Setting Output Voltage

Protection Diodes

In normal operation the LT1580 does not require protec-

tion diodes. Older 3-terminal regulators require protection

diodes between the VOUT pin and the Input pin or between

the ADJ pin and the VOUT pin to prevent die overstress.

On the LT1580, internal resistors limit internal current

paths on the ADJ pin. Therefore even with bypass capaci-

tors on the ADJ pin, no protection diode is needed to

ensure device safety under short-circuit conditions. The

ADJ pin can be driven on a transient basis ±7V with

respect to the output without any device degradation.

9

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