EUP7987
Application Information
External Capacitors
Like any low-dropout regulator, the EUP7987 requires
external capacitors for regulator stability. The EUP7987
is specifically designed for portable applications
requiring minimum board space and smallest components.
These capacitors must be correctly selected for good
performance.
Input Capacitor
Noise Bypass Capacitor
An external 10nF bypass capacitor at BP pin conjunction
with an internal resistor creates lowpass filter. Increasing
the capacitance will decrease the output noise. 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.
Unlike many other LDO’s , addition of a noise reduction
capacitor does not effect the load transient response of
the device. However, it does affect start-up time. The
smaller the capacitance value, the quicker the start-up
time.
Power Dissipation and Junction Temperature
A minimum input capacitance of 1µF is required between
the EUP7987 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 a surge current rating sufficient for the application.
No-Load Stability
The EUP7987 will remain stable and in regulation with
no external load. This is specially important in CMOS
RAM keep-alive applications.
Output Capacitance
The EUP7987 is specifically designed to employ ceramic
output capacitors as low as 2.2µF. Ceramic capacitors
below 10µF offer significant cost and space savings,
along with high frequency noise filtering. Higher values
and other types and of capacitor may be used, but their
equivalent series resistance (ESR) should be maintained
below 0.5Ω. Ceramic capacitor of the value required by
the EUP7987 are available in the following dielectric
types: Z5U, Y5V, X5R, and X7R. The Z5U and Y5V
types exhibit a 50% or more drop in capacitance value as
their temperature increase from 25°C, an important
consideration. The X5R generally maintain their
capacitance value within ± 20%. The X7R type are
desirable for their tighter tolerance of 10% over
temperature.
Specified regulator operation is assured to a junction
temperature of 125°C; the maximum junction
temperature should be restricted to 125°C under normal
operating conditions. This restriction limits the power
dissipation the regulator can handle in any given
application. To ensure the junction temperature is within
acceptable limits, calculate the maximum allowable
dissipation, P
D(max)
, and the actual dissipation, P
D
, which
must be less than or equal to P
D(max)
The maximum-power-dissipation limit is determined
using the following equation:
P
T max
−
T
A
=
J
D(max)
R
θJA
Where:
T
JMAX
is the maximum allowable junction temperature.
R
θJA
is the thermal resistance junction-to-ambient for the
package
T
A
is the ambient temperature.
The regulator dissipation is calculated using:
P
D
=(V
I
-V
O
)
×
I
O
Power dissipation resulting from quiescent current is
negligible. Excessive power dissipation triggers the
thermal protection circuit.
DS7987 Ver 1.5 May .2005
8
EUTECH [ EUTECH MICROELECTRONICS INC ]
