SPX5940AT-3.3 Sipexoration, SPX5940AT-3.3 Datasheet - Page 5

• Manufacturer Part Number: SPX5940AT-3.3
• Description: 1A Low Drop Out Voltage Regulator
• Manufacturer: Sipexoration

Also, it may be programmed for any output voltage between its
1.235V reference and its 30V maximum rating.
Figure 2, an external pair of resistors is required.
Refer to the below equation for the programming of the output
voltage::
The V
-20nA. The minimum recommended load current of 1 µA forces
an upper limit of 1.2 MΩ on value of R
the I
eliminated at room temperature by trimming R
accuracy choose the value of R2 = 100k this reduces the error by
0.17% and increases the resistor program current by 12µA. Since
the LP2951 typically draws 60 µA at no load with Pin 2 open-
circuited this is a small price to pay
HEAT SINK REQUIREMENTS
Depending on the maximum ambient temperature and maximum
power dissipation a heat sink may be required with the
SPX3940/41/42. The junction temperature range has to be within
the range specified under Absolute Maximum Ratings under all
possible operating conditions.
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:
Next step is to calculate the temperature rise T
maximum allowable junction temperature, T
ambient temperature :
Junction to ambient thermal resistance θ
after determining of P
If the θ
without a heat sink. If the value is below 60°C/W then the heat
sink is required and the thermal resistance of the heat sink can be
calculated by the following formula, θ
case to heat sink, θ
FB
REF
produces an error of typically 2% in V
(J-A)
is 1.235 and I
is 60°C/W or higher, the device could be operated
T
P
θ
V
R
(H-A)
TOTAL
(J-A)
OUT
(max) = T
θ
TOTAL &
(J-A)
heat sink to ambient:
= θ
FB
= V
= (V
is the feedback bias current, nominally
= T
(J-C)
REF
T
IN
J
R
R
(max) - T
+ θ
- 5) I
× ( 1 + R
(max):
(max)/P
To find out if a heat sink is
(C-H)
L
+ (V
(J-C)
+ θ
(max)
2
A
1
. If no load is presented
\ R
(max)
(j-A)
IN
junction to case, θ
(H-A)
)I
2
A
)+ I
G
OUT
1
R
can be calculated
. To improve the
(max) maximum
(max). T
, which may be
FB
R
As seen in
1
J
(max)
(C-H)
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 SPX3940/41/42 but is relatively inefficient, as
increasing the capacitor from 1µF to 220µF only decreases the noise
from 430µV to 160µV Vrms for a 100kHz bandwidth at 5V output.
Noise could also be reduced fourfold by a bypass capacitor across R
since it reduces the high frequency gain from 4 to unity. Pick
or choose 0.01µF. When doing this, the output capacitor must be
increased to 3.3µF to maintain stability. These changes reduce the
output noise from 430µV to 100µV Vrms 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.
V
IN
I
IN
= I
L
+ I
G
I
IN
+
Figure 3. 3.3V Regulator Circuit
C
BYPASS
IN
SPX3940
GND
≅ 1 / 2πR
I
SPX3940/41/42
G
OUT
1
× 200 Hz
+
3.3V
2.2 uF
Rev. 12/19/00
LOAD
I
L
1
,