LP2975AIMM-12 National Semiconductor, LP2975AIMM-12 Datasheet - Page 9

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

Reference Designs
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 output load) sets the pole
f
the loop is increased to about 100 kHz, allowing the FET’s
gate capacitance pole f
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.
p
at 5 kHz. Analysis shows that the unity-gain frequency of
Transient Response with Output Capacitor Halved
Transient Response with Only 4.7 µF Output Cap
pg
to cause significant phase shift be-
(Continued)
DS100034-39
DS100034-40
9
DESIGN # 3: V
Application Circuits, Adjustable Voltage Regulator)
COMPONENTS:
C
C
C
R1 = 261 , 1%
R2 = 1.21 k , 1%
R
P-FET = NDP6020P
Heatsink: (Assuming V
against a continuous short-circuit is required, a heatsink with
circuit survivability is not needed, a heatsink with
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
put voltage drops below the nominal value (the nominal
value is the output voltage measured with V
6A 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/
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 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.
S-A
IN
IN
OUT
C
SC
= NOT USED
= 1000 µF Aluminum Electrolytic
= 3.3V for this test.
= 6 m
<
3.3V
= 4 X 330 µF OSCON Aluminum Electrolytic
2.5 ˚C/W must be used. However, if continuous short-
0
IN
I
= 3.3V and I
L
V
DROPOUT VOLTAGE = 0.68V
IN
OUT
6A: LOAD REGULATION = 0.092%
NOISE = 60 µV (rms)
5V: LINE REGULATION = 0.033%
= 1.5V
IN
L
= 6A for this test.
@
3.3V and T
L
IN
= 6A for this test.
6A. (Refer to Typical
= 3.3V for this test.
A
60˚C) if protection
IN
IN
= 3.3V). I
until the out-
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S-A
L
<
=