SPX1585
APPLICATION HINTS
The SPX1585 incorporates protection against over-current
faults, reversed load insertion, over temperature operation, and
positive and negative transient voltage
s. However, the use of an
output capacitor is required in order to insure the stability and
the performance
of the device.
Stability
The output capacitor is part of the regulator’s frequency
compensation system. Either a 22
µ
F aluminum electrolytic
capacitor or a 10
µ
F solid tantalum capacitor between the output
terminal and ground guarantees stable operation for all operat-
ing conditions.
The recommended value for ESR is 0.5Ohms or
less.
However, in order to minimize overshoot and undershoot, and
therefore optimize the design, please refer to the section ‘Ripple
Rejection’.
Ripple Rejection
Ripple rejection can be improved by adding a capacitor between
the ADJ pin and ground. When ADJ pin bypassing is used, the
value of the output capacitor required increases to its maximum
(220
µ
F for an aluminum electrolytic capacitor, or 47
µ
F for
a
solid tantalum capacitor). If the ADJ pin is not bypass
ed, the
value of the output capacitor can be lowered to 10
µ
F for an elec-
trolytic aluminum capacitor or 4.7
µ
F for a solid tantalum
capacitor.
However the value of the ADJ-bypass capacitor should be
chosen with respect to the following equation:
C = 1 / ( 6.28 * F
R
* R
1
)
Where C
= value of the capacitor in Farads
(select an
equal or larger standard value),
F
R
= ripple frequency in Hz,
R
1
= value of resistor R
1
in Ohms.
If an ADJ-bypass capacitor is use
d, the amplitude of the output
ripple will be i
ndependent 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 = V
OUT
/ V
REF
Where M = multiplier for the ripple seen when the ADJ pin
is optimally bypassed.
V
REF
= Reference Voltage
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.
Thermal Consideration
Although the SPX1585 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 in-
sure the best thermal flow of the component, a proper mounting
is required. Note that the case of the device is electrically con-
nected to the output. If the case m
ust be electrically isolated, a
thermally conductive spacer can be used. However do not for-
get to consider its contribution to thermal resistance.
Assuming:
V
IN
= 10V, V
OUT
= 5V, I
OUT
= 1.5A, T
A
= 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
P
D
= (V
IN
– V
OUT
) * I
OUT
= 7.5W
Junctio
n Temperature
T
J
= T
A
+ P
D
* (
θ
Case – HS
+
θ
HS
θ
JC
)
For the Control Section
T
J
= 50 + 7.5*(0.5 + 6=3) = 121.25
°
C
121.25
°
C < T
J (max)
for the Control & Power Sections.
In both
conditions, reliable operation is insured by adequate
junction temperature.
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