LP2975 datasheet(9/19 Pages) NSC

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Reference Designs (Continued)

photo below). The total overshoot increases from −50 mV to

about −75 mV, and the second “ring” on the transient is no-

ticeably larger.

The design is next tested with only a 4.7 µF output capacitor

(see scope photo below). Observe that the vertical scale has

been increased to 100 mV/division to accommodate the

−250 mV undershoot. More important is the severe ringing

as the transient decays. Most designers would recognize

this immediately as the warning sign of a marginally stable

design.

The reason this design is marginally stable is that the 4.7 µF

output capacitor (along with the 6

Ω output load) sets the pole

p at 5 kHz. Analysis shows that the unity-gain frequency of

the loop is increased to about 100 kHz, allowing the FET’s

gate capacitance pole f

pg to cause significant phase shift be-

fore the loop gain goes below unity. Also, because of the low

output voltage, the feedforward capacitor provides less than

10˚ of positive phase shift. For good stability, the output

capcitor needs to be larger than 4.7 µF.

For detailed information on stability and phase margin, see

the

Application Hints section.

DESIGN #3: V

OUT = 1.5V @ 6A. (Refer to Typical

Application Circuits, Adjustable Voltage Regulator)

COMPONENTS:

IN = 1000 µF Aluminum Electrolytic

OUT = 4 X 330 µF OSCON Aluminum Electrolytic

C = NOT USED

R1 = 261

Ω,1%

R2 = 1.21 k

Ω,1%

SC =6mΩ

P-FET = NDP6020P

Heatsink: (Assuming V

IN ≤ 3.3V and TA ≤ 60˚C) if protection

against a

continuous short-circuit is required, a heatsink with

S-A < 2.5 ˚C/W must be used. However, if continuous short-

circuit survivability is not needed, a heatsink with

S-A <

7 ˚C/W is adequate.

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 = 3.3V). IL =

6A for this test.

DROPOUT VOLTAGE = 0.68V

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 = 3.3V for this test.

≤ I

L ≤ 6A: LOAD REGULATION = 0.092%

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 = 6A for this test.

3.3V

≤ V

IN ≤ 5V: LINE REGULATION = 0.033%

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 = 3.3V and IL = 6A for this test.

NOISE = 60 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 = 3.3V for this test.

The load resistor is connected to the regulator output using a

switch so that the load current increases from 0 to 6A

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

photo (the vertical scale is 50 mV/division and the horizontal

scale is 20 µs/division. The regulator nominal output (1.5V)

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

of about −80 mV is shown.

DS100034-39

Transient Response with Output Capacitor Halved

DS100034-40

Transient Response with Only 4.7 µF Output Cap

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