AS2810
APPLICATION HINTS
Reducing parasitic resistance and inductance
One solution to minimize parasitic resistance and inductance is
to connect in parallel capacitors. This arrangement will
improve the transient response of the power supply if your
system requires rapidly changing current load condition.
The AS2810 incorporates protection against over-current
faults, reversed load insertion, over temperature operation, and
positive and negative transient voltage. However, the use of
an output capacitor is required in order to improve the stability
and the performances.
Thermal Consideration
Stability
Although the AS2810 offers some limiting circuitry for
overload conditions, it is necessary not to exceed the
maximum junction temperature, and therefore to be careful
about thermal resistance. The heat flow will follow the lowest
resistance path, which is the Junction-to-case thermal
resistance. In order to insure the best thermal flow of the
component, a proper mounting is required. Note that the case
of the device is electrically connected to the output. In case
the case has to be electrically isolated, a thermally conductive
spacer can be used. However do not forget to consider its
contribution to thermal resistance.
The output capacitor is part of the regulator’s frequency
compensation system. Either a 22µF aluminum electrolytic
capacitor or a 10µF solid tantalum capacitor between the
output terminal and ground guarantees stable operation for all
operating conditions.
However, in order to minimize overshoot and undershoot, and
therefore optimize the design, please refer to the section
‘Ripple Rejection’.
Ripple Rejection
Assuming:
Ripple rejection can be improved by adding a capacitor
between the ADJ pin and ground as shown in figure 6. When
ADJ pin bypassing is used, the value of the output capacitor
required increases to its maximum (22µF for an aluminum
electrolytic capacitor, or 10µF for a solid tantalum capacitor).
If the ADJ pin is not bypass, the value of the output capacitor
can be lowered to 10µF for an electrolytic aluminum capacitor
or 4.7µF for a solid tantalum capacitor.
V
IN = 10V, VOUT = 5V, IOUT = 1.5A, TA = 50°C/W,
θ Heatsink Case= 6°C/W, θ Heatsink Case = 0.5°C/W, θ JC = 3°C/W
Power dissipation under this condition
PD = (VIN – VOUT) * IOUT = 7.5W
Junction Temperature
TJ = TA + PD * (θ Case - HS+ θ HS + θ JC
)
However the value of the ADJ-bypass capacitor should be
chosen with respect to the following equation:
For the Control Sections
C = 1 / ( 6.28 * FR * R1 )
TJ = 50 + 7.5*(0.5+6=3) = 121.25°C
121.25°C < TJ (max) for the Control & Power Sections.
Where C
= value of the capacitor in Farads (select an
equal or larger standard value),
FR = ripple frequency in Hz,
R1 = value of resistor R1 in Ohms.
In both case reliable operation is insured by adequate junction
temperature.
If an ADJ-bypass capacitor is use, the amplitude of the output
ripple will be independent of the output voltage. If an ADJ-
bypass capacitor is not used, the output ripple will be
proportional to the ratio of the output voltage to the reference
voltage:
M = VOUT / VREF
Where M = multiplier for the ripple seen when the ADJ pin
is optimally bypassed.
V
REF = Reference Voltage
Rev. 10/2/00
SIPEX [ SIPEX CORPORATION ]
