NCP781
www.onsemi.com
12
APPLICATIONS INFORMATION
The NCP781 is very high input voltage regulator with
internal thermal shutdown and internal current limit. Typical
application circuits are shown in Figure 23.
NCP781
adj
C
in
C
out
R
1
R
2
1 uF
1 uF
Vin = 6 V – 150 V
Vout = 1.23 V – 15 V
NCP781
fix
IN
EN
OUT
GND
C
in
C
out
1 uF
1 uF
Vin = 6 V – 150 V
IN
EN
GND
OUT
ADJ
Vout = 1.5 V – 15 V
Figure 23. Typical Application Circuits
ENABLE
ENABLE
SEN
Input Decoupling (C
in
)
A ceramic or tantalum 0.1 mF capacitor is recommended
and should be connected close to the NCP781 package.
Higher capacitance and lower ESR will improve the overall
line and load transient response.
Output Decoupling (C
out
)
The NCP781 is a stable component and does not require
a minimum Equivalent Series Resistance (ESR) for the
output capacitor. The minimum output decoupling value is
0.1 mF and can be augmented to fulfill stringent load
transient requirements. The regulator works with ceramic
chip capacitors as well as tantalum devices up to 10 mF. The
larger values improve noise rejection, load regulation and
transient response.
Enable Operation
The enable pin will turn the regulator on or off. The
threshold limits are covered in the electrical characteristics
table in this data sheet. The turn−on/turn−off transient
voltage being supplied to the enable pin should exceed a
slew rate of 150 mV/ms to ensure correct operation. If the
enable function is not to be used then the pin should be
connected directly to V
in
.
Output Voltage Adjust
The output voltage can be adjusted from 1.23 V to 15 V
using resistors between the output and the ADJ input. The
output voltage and resistors are chosen using Equation 1 and
Equation 2.
V
OUT
+ 1.23
ǒ
1 )
R
1
R
2
Ǔ
)
ǒ
I
ADJ
R
1
Ǔ
(eq. 1)
R
2
^ R
1
1
V
OUT
1.25
* 1
(eq. 2)
Input bias current I
ADJ
is typically less than 5 nA. Choose
R1 arbitrarily to minimize errors due to the bias current and
to minimize noise contribution to the output voltage. Use
Equation 2 to find the required value for R2. This device
does not require a minimal load.
Thermal Considerations
As power in the NCP781 increases, it might become
necessary to provide some thermal relief. The maximum
power dissipation supported by the device is dependent
upon board design and layout. Mounting pad configuration
on the PCB, the board material, and the ambient temperature
affect the rate of junction temperature rise for the part. When
the NCP781 has good thermal conductivity through the
PCB, the junction temperature will be relatively low with
high power applications. The maximum dissipation the
NCP781 can handle is given by:
P
D(MAX)
+
ƪ
T
J(MAX)
* T
A
ƫ
R
qJA
(eq. 3)
The power dissipated by the NCP781 can be calculated
from the following equations:
P
D
[ V
in
ǒ
I
GND
@I
OUT
Ǔ
) I
OUT
ǒ
V
IN
* V
OUT
Ǔ
(eq. 4)
or
V
IN(MAX)
[
P
D(MAX)
)
ǒ
V
OUT
I
OUT
Ǔ
I
OUT
) I
GND
(eq. 5)
Hints
V
in
and GND printed circuit board traces should be as
wide as possible. When the impedance of these traces is
high, there is a chance to pick up noise or cause the regulator
to malfunction. Place external components, especially the
output capacitor, as close as possible to the NCP781, and
make traces as short as possible.
