LP2975 datasheet(8/19 Pages) NSC

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Reference Designs (Continued)

DESIGN #2: V

OUT =3V @ 0.5A (Refer to Typical

Application Circuits, Adjustable Voltage Regulator)

COMPONENTS:

IN = 68 µF Tantalum

OUT =2X68µF Tantalum

C = 470 pF

R1 = 237 k

Ω,1%

R2 = NOT USED

SC = 0.1Ω

Tie feedback pin to V

OUT

P-FET = NDT452P

Heatsink: Tab of N-FET is soldered down to 0.6 in

2 copper

area on PC board.

Output Voltage Adjustment: For this application, a 3.3V part

is “trimmed” down to 3V by using a single external 237 k

Ω re-

sistor at R1, which parallels the internal 39.9 k

Ω resistor (re-

ducing the effective resistance to 34.2 k

Ω).

Because the tempco of the external resistor will not match

the tempco of the internal resistor (which is typically 3000

ppm), this method of adjusting V

OUT by using a single resis-

tor is only recommended in cases where the output voltage

is adjusted

≤ 10% away from the nominal value.

PERFORMANCE DATA:

Dropout Voltage

Dropout voltage is defined as the minimum input-to-output

differential voltage required by the regulator to keep the out-

put in regulation. It is measured by reducing V

IN until the out-

put voltage drops below the nominal value (the nominal

value is the output voltage measured with V

IN = 5V). IL =

0.5A for this test.

DROPOUT VOLTAGE = 141 mV

Load Regulation

Load regulation is defined as the maximum change in output

voltage as the load current is varied. It is measured by

changing the load resistance and recording the minimum/

maximum output voltage. The measured change in output

voltage is divided by the nominal output voltage and ex-

pressed as a percentage. V

IN = 3.5V for this test.

≤ I

L ≤ 0.5A: LOAD REGULATION = 0.034%

Line Regulation

Line regulation is defined as the maximum change in output

voltage as the input voltage is varied. It is measured by

changing the input voltage and recording the minimum/

maximum output voltage. The measured change in output

voltage is divided by the nominal output voltage and ex-

pressed as a percentage. I

L = 0.5A for this test.

3.5V

≤ V

IN ≤ 6V: LINE REGULATION = 0.017%

Output Noise Voltage

Output noise voltage was measured by connecting a wide-

band AC voltmeter (HP 400E) directly across the output ca-

pacitor. V

IN = 5V and IL = 0.5A for this test.

NOISE = 85 µV (rms)

Transient Response

Transient response is defined as the change in output volt-

age which occurs after the load current is suddenly changed.

IN = 3.5V for this test.

The load resistor is connected to the regulator output using a

switch so that the load current increases from 0 to 0.5A

abruptly. The change in output voltage is shown in the scope

photo (the vertical scale is 20 mV/division and the horizontal

scale is 50 µs/division). The regulator nominal output (3V) is

located on the center line of the photo. A maximum change

of about −50 mV is shown.

Minimizing C

OUT

It is often desirable to decrease the value of C

OUT to save

cost and reduce size. The design guidelines suggest select-

ing C

OUT to set the first pole ≤ 200 Hz (see later section Out-

put Capacitor), but this is not an absolute requirement in all

cases.

The effect of reducing C

OUT is to decrease phase margin. As

phase margin is decreased, the output ringing will increase

when a load step is applied to the output. Eventually, if C

OUT

is made small enough, the regulator will oscillate.

To demonstrate these effects, the value of C

OUT in reference

design #2 is halved by removing one of the two 68 µF output

capacitors and the transient response test is repeated (see

DS100034-37

Transient Response for 0–5A Load Step

DS100034-38

Transient Response for 0–0.5A Load Step

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