LP2975AIMM-12 National Semiconductor, LP2975AIMM-12 Datasheet - Page 8

• Manufacturer Part Number: LP2975AIMM-12
• Description: MOSFET LDO Driver/Controller
• Manufacturer: National Semiconductor

www.national.com
Reference Designs
DESIGN # 2: V
Application Circuits, Adjustable Voltage Regulator)
COMPONENTS:
C
C
C
R1 = 237 k , 1%
R2 = NOT USED
R
Tie feedback pin to V
P-FET = NDT452P
Heatsink: Tab of N-FET is soldered down to 0.6 in
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 re-
sistor at R1, which parallels the internal 39.9 k resistor (re-
ducing the effective resistance to 34.2 k ).
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
tor is only recommended in cases where the output voltage
is adjusted
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
put voltage drops below the nominal value (the nominal
value is the output voltage measured with V
0.5A for this test.
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/
IN
OUT
C
SC
= 470 pF
= 68 µF Tantalum
= 0.1
= 2 X 68 µF Tantalum
Transient Response for 0–5A Load Step
DROPOUT VOLTAGE = 141 mV
10% away from the nominal value.
OUT
= 3V
OUT
@
0.5A (Refer to Typical
OUT
(Continued)
by using a single resis-
IN
IN
until the out-
= 5V). I
2
DS100034-37
copper
L
=
8
maximum output voltage. The measured change in output
voltage is divided by the nominal output voltage and ex-
pressed as a percentage. V
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
Output Noise Voltage
Output noise voltage was measured by connecting a wide-
band AC voltmeter (HP 400E) directly across the output ca-
pacitor. V
Transient Response
Transient response is defined as the change in output volt-
age which occurs after the load current is suddenly changed.
V
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
It is often desirable to decrease the value of C
cost and reduce size. The design guidelines suggest select-
ing C
put Capacitor ), but this is not an absolute requirement in all
cases.
The effect of reducing C
phase margin is decreased, the output ringing will increase
when a load step is applied to the output. Eventually, if C
is made small enough, the regulator will oscillate.
To demonstrate these effects, the value of C
design # 2 is halved by removing one of the two 68 µF output
capacitors and the transient response test is repeated (see
IN
= 3.5V for this test.
OUT
3.5V
0
Transient Response for 0–0.5A Load Step
IN
to set the first pole
I
L
= 5V and I
V
OUT
IN
0.5A: LOAD REGULATION = 0.034%
NOISE = 85 µV (rms)
6V: LINE REGULATION = 0.017%
L
OUT
= 0.5A for this test.
L
IN
is to decrease phase margin. As
= 0.5A for this test.
200 Hz (see later section Out-
= 3.5V for this test.
OUT
OUT
in reference
to save
DS100034-38
OUT