EUP7981
Application Note
capacitance can drop by more than 50% as the
temperature goes from 25°C to 85°C. Therefore, X7R
is recommended over Z5U and Y5V in applications
where the ambient temperature will change
significantly above or below 25°C.
External Capacitors
Like any low-dropout regulator, the EUP7981 requires
external capacitors for regulator stability. The
EUP7981 is specifically designed for portable
applications requiring minimum board space and
smallest components. These capacitors must be
correctly selected for good performance.
Noise Bypass Capacitor
Connecting a 0.01µF capacitor between the CBYPASS
pin and ground significantly reduces noise on the
regulator output. This cap is connected directly to a
high impedance node in the bandgap reference circuit.
Any significant loading on this node will cause a
change on the regulated output voltage. For this reason,
DC leakage current through this pin must be kept as
low as possible for best output voltage accuracy. The
types of capacitors best suited for the noise bypass
capacitor are ceramic and film.
Input Capacitor
An input capacitance of 1µF is required between the
EUP7981 input pin and ground (the amount of the
capacitance may be increased without limit).
This capacitor must be located a distance of not more
than 1cm from the input pin and returned to a clean
analog ground. Any good quality ceramic, tantalum, or
film capacitor may be used at the input.
If a tantalum capacitor is used at the input, it must be
guaranteed by the manufacturer to have asurge current
rating sufficient for the application.
Unlike many other LDO’s, addition of a noise
reduction capacitor does not effect the load transient
response of the device.
There are no requirements for the ESR on the input
capacitor, but tolerance and temperature coefficient
must be considered when selecting the capacitor to
ensure the capacitance will be 1µF over the entire
operating temperature range.
Adjustable Operation
The adjustable version of the EUP7981 has an output
voltage range of 1.2V to 4.5V. The output voltage of
the EUP7981 adjustable regulator is programmed
using an external resistor divider as shown in Figure3.
The output voltage is calculated using:
Output Capacitor
The EUP7981 is designed specifically to work with
very small ceramic output capacitors. A ceramic
capacitor (temperature characteristics X7R, X5R, Z5U,
or Y5V) in 1 to 22µF range with 5mΩ to 500mΩ ESR
range is suitable in the EUP7981 application circuit.
The output capacitor must meet the requirement for
minimum amount of capacitance and also have an
ESR (Equivalent Series Resistance) value which is
within a stable range (5mΩ to 500mΩ)
R
R
1
V = V ×(1+
)
O
ref
2
Where:
Vref= 1.2V typ. (the internal reference voltage)
Utilize the following equation for adjusting the output
to a particular voltage:
No-Load Stability
V
1.2V
O
The EUP7981 will remain stable and in regulation
with no external load. This is specially important in
CMOS RAM keep-alive applications.
P = R
−1
I
2
Capacitor Characteristics
Choose R2=50k to optimize accuracy, power supply
rejection, noise and power consumption.
The EUP7981 is designed to work with ceramic
capacitors on the output to take advantage of the
benefits they offer: for capacitance values in the range
of 1µF to 4.7µF range, ceramic capacitors are the
smallest, least expensive and have the lowest ESR
values (which makes them best for eliminating high
frequency noise). The ESR of a typical 1µF ceramic
capacitor is in the range of 20mΩ to 40mΩ, which
easily meets the ESR requirement for stability by the
EUP7981.
The ceramic capacitor’s capacitance can vary with
temperature. The capacitor type X7R, which operates
over a temperature range of -55°C to +125°C, will
only vary the capacitance to within ±15%. Most large
value ceramic capacitors ( 2.2µF) are manufactured
≈
Figure3. Adjustable Regulator with Resistors
with Z5U or Y5V temperature characteristics. Their
DS7981 Ver1.0 Jan. 2008
8
EUTECH [ EUTECH MICROELECTRONICS INC ]
