LP2975 Datasheet, PDF (8/19 Pages) National Semiconductor (TI) – MOSFET LDO Driver/Controller

Reference Designs (Continued)
DS100034-37
Transient Response for 0–5A Load Step
DESIGN #2: V_OUT = 3V @ 0.5A (Refer to Typical
Application Circuits, Adjustable Voltage Regulator)
COMPONENTS:
C_IN = 68 µF Tantalum
C_OUT = 2 X 68 µF Tantalum
C_C = 470 pF
R1 = 237 kΩ, 1%
R2 = NOT USED
R_SC = 0.1Ω
Tie feedback pin to V_OUT
P-FET = NDT452P
Heatsink: Tab of N-FET is soldered down to 0.6 in² 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 kΩ re-
sistor at R1, which parallels the internal 39.9 kΩ resistor (re-
ducing the effective resistance to 34.2 kΩ).
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.
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.
0 ≤ 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 I_L = 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.
V_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.
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). I_L =
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/
DS100034-38
Transient Response for 0–0.5A Load Step
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
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