MCT7800

9

MCT7800

MOTOROLA

APPLICATIONS INFORMATION

Design Considerations

The MCT7800 Series of fixed voltage regulators are

designed with thermal overload protection that shuts down

the circuit when subjected to an excessive power overload

condition, internal short circuit protection that limits the

maximum current the circuit will pass, and output transistor

safe-area compensation that reduces the output short circuit

current as the voltage across the pass transistor is increased.

In many low current applications, compensation capacitors

are not required. However, it is recommended that the

regulator input be bypassed with a capacitor if the regulator is

connected to the power supply filter with long wire lengths, or

if the output load capacitance is large. An input bypass

capacitor should be selected to provide good high frequency

characteristics to insure stable operation under all load

conditions. A 0.33

µF or larger tantalum, mylar, or other

capacitor having low internal impedance at high frequencies,

should be chosen. The bypass capacitor should be mounted

with the shortest possible leads directly across the regulators’

input terminals. Normally, good construction techniques

should be used to minimize ground loops and lead resistance

drops since the regulator has no external sense lead.

Figure 6. Worst Case Power Dissipation versus

Ambient Temperature (Case 221A)

Figure 7. Input Output Differential as a

Function of Junction Temperature

20

16

12

8.0

4.0

0

– 50

– 25

0

25

50

75

100

125

150

TA, AMBIENT TEMPERATURE (°C)

No Heatsink

θHS = 0°C/W

θJC = 5°C/W

θJA = 65°C/W

TJ(max) = +150°C

2.5

2.0

1.5

1.0

–75

– 50

– 25

0

25

50

75

100

125

TJ, JUNCTION TEMPERATURE (°C)

IO = 1.0 A

200 mA

500 mA

0 mA

20 mA

0

0.5

θHS = 15°C/W

θHS = 5°C/W

∆VO = 2% of VO

Figure 8. D2PAK Thermal Resistance and Maximum

Power Dissipation versus P.C.B. Copper Length

30

40

50

60

70

80

1.0

1.5

2.0

2.5

3.0

3.5

0

10

20

30

25

15

5.0

L, LENGTH OF COPPER (mm)

PD(max) for TA = 50°C

Minimum

Size Pad

2.0 oz. Copper

L

L

Free Air

Mounted

Vertically

R

θJA

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.

Quiescent 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.