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參數(shù)資料
| 型號(hào)： | LP2975IMMX-5.0 |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 模擬信號(hào)調(diào)理 |
| 英文描述： | MOSFET LDO Driver/Controller |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PDSO8 |
| 封裝： | MINI, SOP-8 |
| 文件頁數(shù)： | 8/19頁 |
| 文件大?。?/td> | 715K |
| 代理商： | LP2975IMMX-5.0 |
Reference Designs
(Continued)
DESIGN
#
2: V
= 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 k
, 1%
R2 = NOT USED
R
SC
= 0.1
Tie feedback pin to V
OUT
P-FET = NDT452P
Heatsink: Tab of N-FET is soldered down to 0.6 in
2
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 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
by using a single resis-
tor is only recommended in cases where the output voltage
is adjusted
≤
10% away from the nominal value.
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
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 regulationis 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
= 3.5V 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.
Minimizing C
OUT
It is often desirable to decrease the value of C
to save
cost and reduce size. The design guidelines suggest select-
ing C
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
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
in reference
design
#
2 is halved by removing one of the two 68 μF output
capacitors and the transient response test is repeated (see
DS100034-37
Transient Response for 0–5A Load Step
DS100034-38
Transient Response for 0–0.5A Load Step
www.national.com
8
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