NCP3335A
http://onsemi.com
18
APPLICATIONS INFORMATION
Reverse Bias Protection
Reverse bias is a condition caused when the input voltage
goes to zero, but the output voltage is kept high either by a
large output capacitor or another source in the application
which feeds the output pin.
Normally in a bipolar LDO all the current will flow from
the output pin to input pin through the PN junction with
limited current capability and with the potential to destroy
the IC.
Due to an improved architecture, the NCP3335A can
withstand up to 7.0 V on the output pin with virtually no
current flowing from output pin to input pin, and only
negligible amount of current (tens of mA) flowing from the
output pin to ground for infinite duration.
Input Capacitor
An input capacitor of at least 1.0 mF, any type, is
recommended to improve the transient response of the
regulator and/or if the regulator is located more than a few
inches from the power source. It will also reduce the circuit’s
sensitivity to the input line impedance at high frequencies.
The capacitor should be mounted with the shortest possible
track length directly across the regular’s input terminals.
Output Capacitor
The NCP3335A remains stable with any type of capacitor
as long as it fulfills its 1.0 mF requirement. There are no
constraints on the minimum ESR and it will remain stable up
to an ESR of 5.0 W. Larger capacitor values will improve the
noise rejection and load transient response.
Noise Reduction Pin
Output noise can be greatly reduced by connecting a 10 nF
capacitor (C
nr
) between the noise reduction pin and ground
(see Figure 1). In applications where very low noise is not
required, the noise reduction pin can be left unconnected.
For the adjustable version, in addition to the 10 nF C
nr
, a
68 pF capacitor connected in parallel with R1 (see Figure 2)
is recommended to further reduce output noise and improve
stability.
Adjustable Operation
The output voltage can be set by using a resistor divider
as shown in Figure 2 with a range of 1.25 to 10 V. The
appropriate resistor divider can be found by solving the
equation below. The recommended current through the
resistor divider is from 10 mA to 100 mA. This can be
accomplished by selecting resistors in the kW range. As
result, the I
adj
*R2 becomes negligible in the equation and
can be ignored.
V
out
+ 1.25 *
ǒ
1 )
R1
R2
Ǔ
) I
adj
*R2
(eq. 1)
Example:
For V
out
= 2.9 V, can use R
1
= 36 kW and R
2
= 27 kW.
1.25 *
ǒ
1 )
36 kW
27 kW
Ǔ
+ 2.91 V
(eq. 2)
Dropout Voltage
The voltage dropout is measured at 97% of the nominal
output voltage.
Thermal Considerations
Internal thermal limiting circuitry is provided to protect the
integrated circuit in the event that the maximum junction
temperature is exceeded. This feature provides protection
from a catastrophic device failure due to accidental
overheating. This protection feature is not intended to be used
as a substitute to heat sinking. The maximum power that can
be dissipated, can be calculated with the equation below:
P
D
+
T
J(max)
* T
A
R
qJA
(eq. 3)
For improved thermal performance, contact the factory
for the DFN package option. The DFN package includes an
exposed metal pad that is specifically designed to reduce the
junction to air thermal resistance, R
qJA
.
