CLM2815
CORPORATION
APPLICATION HINTS
The Calogic Semiconductor CLM2815 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 insure the stability and the performances.
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.
Stability
Thermal Consideration
The output capacitor is part of the regulator’s frequency
compensation system. Either a 220µF aluminum electrolytic
capacitor or a 47µF solid tantalum capacitor between the
output terminal and ground guarantees stable operation for all
operating conditions.
Although the CLM2815 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.
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 (220µF for an aluminum electrolytic capacitor, or
47µF for a solid tantalum capacitor). If the ADJ pin is not
bypass, the value of the output capacitor can be lowered to
100µF for an electrolytic aluminum capacitor or 15µF for a
solid tantalum capacitor.
Assuming:
V
IN = 10V, VOUT = 5V, IOUT = 1.5A, TA = 90oC, ΘCASE = 1oC/W
(no external heat sink, no wind)
Power dissipation under these conditions
•
PD = (VIN - VOUT) IOUT = 7.5W
However the value of the ADJ-bypass capacitor should be
chosen with respect to the following equation:
Junction Temperature
TJ = TA + PD (ΘCASE + ΘJC
•
)
•
•
C = 1 / ( 6.28 FR R1 )
Where C = value of the capacitor in Farads (select an equal or
larger standard value),
For the Control Section
TJ = 90 C + 7.5W (1 C/W + 0.6 C/W) = 102 C
114oC TJUNCTION MAX for the control section.
o
o
o
o
•
FR = ripple frequency in Hz,
R1 = value of resistor R1 in Ohms.
For theoPower Section
o
o
o
•
TJ = 90 C + 7.5W (1 C/W + 1.6 C/W) = 104.5 C
109.5oC TJUNCTION MAX for the power transistor.
If an ADJ-bypass capacitor is used, 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:
In both cases reliable operation is insured by adequate
junction temperature.
M = VOUT / VREF
Where M = multiplier for the ripple seen when the ADJ pin is
optimally bypassed.
VREF = Reference Voltage
CALOGIC CORPORATION, 237 Whitney Place, Fremont, California 94539, Telephone: 510-656-2900, FAX: 510-651-3025
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