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參數(shù)資料
| 型號(hào)： | SPX2954 |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | 250 mA Low Dropout Voltage Regulator（Output Programmable From 1.24V to 29V）(250mA，低壓差穩(wěn)壓器（可編程輸出從1.24V-29V)) |
| 中文描述： | 250毫安低壓差穩(wěn)壓器（輸出可編程從兩種器件的29V）（250mA的，低壓差穩(wěn)壓器（可編程輸出從兩種器件- 29V）） |
| 文件頁(yè)數(shù)： | 5/7頁(yè) |
| 文件大?。?/td> | 204K |
| 代理商： | SPX2954 |
Rev. 11/21/00
SPX2954
Also, it may be programmed for any output voltage between its
1.235V reference and its 30V maximum rating. As seen in
Figure 2, an external pair of resistors is required.
Refer to the below equation for the programming of the output
voltage:
V
OUT
= V
REF
×
( 1 + R
1
\ R
2
)+ I
FB
R
1
The V
REF
is 1.235 and I
FB
is the feedback bias current, nominally
-20nA. The minimum recommended load current of 1
μ
A forces
an upper limit of 1.2 M
on value of R
2
.
If no load is presented
the I
FB
produces an error of typically 2% in V
OUT
, which may be
eliminated at room temperature by trimming R
1
. To improve the
accuracy choose the value of R
2
= 100k this reduces the error by
0.17% and increases the resistor program current by 12
μ
A. Since
the LP2951 typically draws 60
μ
A at no load with Pin 2 open-
circuited this is a small price to pay
HEATSINK REQUIREMENTS
Depending on the maximum ambient temperature and maximum
power dissipation a heatsink may be required with the SPX2954.
The junction temperature range has to be within the range
specified under Absolute Maximum Ratings under all possible
operating conditions. To find out if a heatsink is required, the
maximum power dissipation of the device needs to be calculated.
This is the maximum specific AC voltage that must be taken into
consideration at input. Figure 3 shows the condition and power
dissipation which should be calculated with the following
formula:
P
TOTAL
= (V
IN
- 5) I
L
+ (V
IN
)I
G
Next step is to calculate the temperature rise T
R
(
MAX
). T
J
(
MAX
)
maximum allowable junction temperature, T
A
(
MAX
) maximum
ambient temperature :
T
R
(
MAX
) = T
J
(
MAX
) - T
A
(
MAX
)
Junction to ambient thermal resistance
θ
(j-A)
can be calculated
after determining of P
TOTAL &
T
R
(
MAX
):
θ
(J-A)
= T
R
(max)/P
(MAX)
If the
θ
(J-A)
is 60
°
C/W or higher, the device could be operated
without a heatsink. If the value is below 60
°
C/W then the
heatsink is required and the thermal resistance of the heatsink can
be calculated by the following formula,
θ
(J-C)
junction to case,
θ
(C-H)
case to heatsink,
θ
(H-A)
heatsink to ambient:
θ
(J-A)
=
θ
(J-C)
+
θ
(C-H)
+
θ
(H-A)
REDUCING OUTPUT NOISE
It may be an advantage to reduce the AC noise present at the output.
One way is to reduce the regulator bandwidth by increasing the size of
the output capacitor. This is the only way that noise can be reduced
on the 3 lead SPX2954 but is relatively inefficient, as increasing the
capacitor from 1
μ
F to 220
μ
F only decreases the noise from 430
μ
V to
160
μ
V rms for a 100kHz bandwidth at 5V output.
Noise could also be reduced fourfold by a bypass capacitor across R
1
,
since it reduces the high frequency gain from 4 to unity. Pick
C
BYPASS
1 / 2
π
R
1
×
200 Hz
or choose 0.01
μ
F. When doing this, the output capacitor must be
increased to 3.3
μ
F to maintain stability. These changes reduce the
output noise from 430
μ
V to 100
μ
V
RMS
for a 100kHz bandwidth at 5V
output. With the bypass capacitor added, noise no longer scales with
output voltage so that improvements are more dramatic at higher
output voltages.
+
2.2 uF
+
SPX2954
I
IN
V
IN
I
G
I
L
5V
IN
OUT
GND
LOAD
I
IN
=
I
L
+
I
G
Figure 3. 5V Regulator Circuit
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