LP2975 Datasheet, PDF (9/19 Pages) National Semiconductor (TI) – MOSFET LDO Driver/Controller

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.
DS100034-39
Transient Response with Output Capacitor Halved
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.
DS100034-40
Transient Response with Only 4.7 µF Output Cap
The reason this design is marginally stable is that the 4.7 µF
output capacitor (along with the 6Ω output load) sets the pole
f_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:
C_IN = 1000 µF Aluminum Electrolytic
C_OUT = 4 X 330 µF OSCON Aluminum Electrolytic
C_C = NOT USED
R1 = 261Ω, 1%
R2 = 1.21 kΩ, 1%
R_SC = 6 mΩ
P-FET = NDP6020P
Heatsink: (Assuming V_IN ≤ 3.3V and T_A ≤ 60˚C) if protection
against a continuous short-circuit is required, a heatsink with
θ_S-A < 2.5 ˚C/W must be used. However, if continuous short-
circuit survivability is not needed, a heatsink with θ_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). I_L =
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.
0 ≤ 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 I_L = 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.
V_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.
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