LT1580CR-2.5#PBF Datasheet LT1580IQ#PBF, LT1580CQ#PBF, LT1580CR-2.5#PBF | P7-P9

LT1580/LT1580-2.5

APPLICATIONS INFORMATION

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The LT1580 has improved frequency compensation which

permits the use of capacitors with very low ESR. This is

critical in addressing the needs of modern, low voltage,

high speed microprocessors. Current generation micro-

processors cycle load current from several hundred mil-

liamperes to several amperes in tens of nanoseconds.

Output voltage tolerances are tighter and include transient

response as part of the specification. The LT1580 is

designed to meet the fast current load step requirements

of these microprocessors and saves total cost by needing

less output capacitance to maintain regulation.

Careful design has eliminated any supply sequencing

issues associated with a dual supply system. The output

voltage will not turn on until both supplies are operating.

If the control voltage comes up first, the output current will

be limited to a few milliamperes until the power input

voltage comes up. If the power input comes up first the

output will not turn on at all until the control voltage comes

up. The output can never come up unregulated. The

LT1580 can also be operated as a single supply device by

tying the control and power inputs together. Dropout in

single supply operation will be determined by the mini-

mum control voltage.

The LT1580 includes several innovative features that

require additional pins over the traditional 3-terminal

regulator. Both the fixed and adjustable devices have

remote SENSE pins, permitting very accurate regulation of

output voltage at the load, where it counts, rather than at

the regulator. As a result the typical load regulation over

an output current range of 100mA to 7A with a 2.5V output

is typically less than 1mV. For the fixed voltage devices the

ADJ pin is also brought out. This allows the user to

improve transient response by bypassing the internal

resistor divider. In the past fixed output voltage devices

did not provide this capability. Bypassing the ADJ pin with

a capacitor in the range of 0.1µF to 1µF will provide

optimum transient response. The value chosen will de-

pend on the amount of output capacitance in the system.

In addition to the enhancements mentioned above the

reference accuracy has been improved by a factor of two

with a guaranteed initial tolerance of ±0.6% at 25°C.

Temperature drift is also very well controlled. When com-

bined with ratiometrically accurate internal divider resis-

tors the part can easily hold 1% output accuracy over the

full temperature range and load current range, guaran-

teed, while operating with an input/output differential of

well under 1V.

Typical applications for the LT1580 include 3.3V to 2.5V

conversion with a 5V control supply, 5V to 4.2V conver-

sion with a 12V control supply or 5V to 3.6V conversion

with a 12V control supply. It is easy to obtain dropout

voltages of less than 0.5V at 4A along with excellent static

and dynamic specifications. The LT1580 is capable of 7A

of output current with a maximum dropout of 0.8V. The

LT1580 has fast transient response that allows it to handle

the large current changes associated with today’s micro-

processors. The device is fully protected against

overcurrent and overtemperature conditions. Both fixed

voltage (2.5V) and adjustable output versions are avail-

able. The device is available in a multilead TO-220 package

with five leads for the adjustable device and seven leads for

the fixed voltage device.

Grounding and Output Sensing

The LT1580 allows true Kelvin sensing for both the high

and low side of the load. This means that the voltage

regulation at the load can be easily optimized. Voltage

drops due to parasitic resistances between the regulator

and the load which would normally degrade regulation can

be placed inside the regulation loop of the LT1580. Figures

1 through 3 illustrate the advantages of remote sensing.

Figure 1 shows the LT1580 connected as a conventional

3-terminal regulator with the SENSE lead connected di-

rectly to the output of the device. R

represents the

parasitic resistance of the connections between the LT1580

and the load. The load is typically a microprocessor and

is made up of the PC traces and/or connector resis-

tances, in the case of a modular regulator, between the

regulator and the processor. The effect of R

can be seen

in trace A of Figure 3. Very small resistances cause

significant load regulation steps. For example, at 7A out-

put current the output voltage will shift by 7mV for every

0.001Ω of resistance. In Figure 2 the LT1580 is connected

to take advantage of the remote sense feature. The SENSE

LT1580/LT1580-2.5

APPLICATIONS INFORMATION

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values of R

. Trace B of Figure 3 illustrates the effect on

output regulation. It is important to note that the voltage

drops due to R

are not eliminated. They will add to the

dropout voltage of the regulator regardless of whether

they are inside the loop as in Figure 2 or outside the loop

as in Figure 1. This means that the LT1580 can control the

voltage at the load as long as the input-output voltage is

greater than the total of the dropout voltage of the LT1580

plus the voltage drop across R

Stability

The LT1580 requires the use of an output capacitor as part

of the device frequency compensation. The device re-

quires a minimum of 22µF tantalum or 150µF of aluminum

electrolytic to ensure stability. Larger capacitor values

increase stability and improve transient performance.

Many different types of capacitors are available and have

widely varying characteristics. These capacitors differ in

capacitor tolerance (sometimes up to ±100%), equivalent

series resistance, equivalent series inductance and ca-

pacitance temperature coefficient. The LT1580 frequency

compensation optimizes frequency response with low

ESR capacitors. In general, use capacitors with an ESR of

less than 1Ω.

For microprocessor applications larger value capacitors

will be needed to meet the transient requirements of the

processor. Processor manufacturers require tight voltage

tolerances on the power supply. High quality bypass

capacitors must be used to limit the high frequency noise

generated by the processor. Multiple small ceramic ca-

pacitors in addition to high quality bulk tantalum capaci-

tors are typically required to limit parasitic inductance

(ESL) and resistance (ESR) in the capacitors to acceptable

levels. The LT1580 is stable with the type of capacitors

recommended by processor manufacturers.

Bypassing the adjust terminal on the LT1580 improves

ripple rejection and transient response. The ADJ pin is

brought out on the fixed voltage device specifically to

allow this capability.

Capacitor values on the order of several hundred microfar-

ads are used to ensure good transient response with heavy

pin and the top of the resistor divider are connected to the

top of the load. The bottom of the resistor divider is

connected to the bottom of the load. R

is now effectively

connected inside the regulating loop of the LT1580 and the

load regulation at the load will be negligible for reasonable

OUT

SENSE

ADJ

1580 F02

LT1580

POWER

3.3V

CONTROL

LOAD

OUT

–

OUT

SENSE

ADJ

1580 F01

LT1580

POWER

3.3V

CONTROL

LOAD

OUT

–

Figure 1. Conventional Load Sensing

Figure 3. Remote Sensing Improves Load Regulation

OUT

FIGURE 1

(∆I

OUT

)(R

)

TIME

1580 F03

OUT

FIGURE 2

OUT

Figure 2. Remote Load Sensing

LT1580/LT1580-2.5

APPLICATIONS INFORMATION

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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

1580 F04

ESL

EFFECTS

CAPACITANCE

EFFECTS

POINT AT WHICH REGULATOR

TAKES CONTROL

SLOPE, =

∆I

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.

OUT

SENSE

ADJ

= 50µA

1580 F05

LT1580

POWER

CONTROL

REF

CONTROL

OUT

= V

REF

1 + + I

ADJ

(R2)

(

)

Figure 4

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 V

OUT

pin and the Input pin or between

the ADJ pin and the V

OUT

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.

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LT1580CR-2.5#PBF

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Description:

LDO Voltage Regulators 7A L/Dropout HS 2.5V Volt Reg

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LT1580CR-2.5#PBF

LT1580CR-2.5#PBF

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