MC7900 Series
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10
Protection Diodes
When external capacitors are used with MC7900 series
regulator it is sometimes necessary to add protection diodes
to prevent the capacitors from discharging through low
current points into the regulator or from output polarity
reversals. Generally, no protection diode is required for
values of output capacitance less then 10mF. Figure 11 shows
the MC7915 with the recommended protection diodes.
• Opposite Polarity Protection
Diode D
1 protects the regulator from output polarity
reversals during startup, power off and short-circuit
operation.
• Reverse-bias Protection
Diode D
2 prevents output capacitor from discharging thru
the MC7915 during an input short circuit or fast switch off
of power supply.
Figure 11. Protection Diodes
Gnd
Gnd
+
-20 V
Input
-15 V
Output
MC7915
+
1.0 mF 1.0 mF
MBRS140
or Equiv
D1
MBRS140
or Equiv
D2
DEFINITIONS
Line Regulation − The change in output voltage for a
change in the input voltage. The measurement is made under
conditions of low dissipation or by using pulse techniques
such that the average chip temperature is not significantly
affected.
Load Regulation − The change in output voltage for a
change in load current at constant chip temperature.
Maximum Power Dissipation − The maximum total
device dissipation for which the regulator will operate
within specifications.
Input Bias Current − That part of the input current that
is not delivered to the load.
Output Noise Voltage − The rms AC voltage at the
output, with constant load and no input ripple, measured
over a specified frequency range.
Long Term Stability − Output voltage stability under
accelerated life test conditions with the maximum rated
voltage listed in the devices’ electrical characteristics and
maximum power dissipation.
Figure 12. D
2
PAK Thermal Resistance and Maximum
Power Dissipation versus P.C.B. Copper Length
R , THERMAL RESISTANCE
JAθ
JUNCTION‐TO‐AIR ( C/W)°
P
D(max)
for T
A
= +50°C
P
D
, MAXIMUM POWER DISSIPATION (W)
30
40
50
60
70
80
010203025155.0
L, LENGTH OF COPPER (mm)
Minimum
Size Pad
2.0 oz. Copper
L
L
Free Air
Mounted
Vertically
R
q
JA
1.0
1.5
2.0
2.5
3.0
3.5
