brought low, the converter will be in shutdown, and will draw

less than 1 µA from the source.

Notes:

A. Drive this input with a logic signal (see Application Hints).

If the shutdown function is not to be used, tie the ON/OFF

pin directly to the V

B. Capacitor is required for regulator stability. Minimum size

is shown, and may be increased without limit.

C. Increasing the output capacitance improves transient re-

sponse and increases phase margin.

D. A heatsink may be required for this transistor. The maxi-

mum allowable value for thermal resistance of the heatsink

is dependent on ambient temperature and load current (see

curves in ). Once the value is obtained from the graph, a

heatsink must be selected which has a thermal resistance

equal to or lower than this value. If the value is above

60˚C/W, no heatsink is required (the TO-220 package alone

will safely dissipate this).

For these curves, a maximum junction temperature of 150˚C

is assumed for the pass transistor. The case-to-heatsink at-

tachment thermal resistance is assumed to be 1.5˚C/W. All

calculations are for 5.5V input voltage (which is worst-case

for power dissipation).

DS012078-54

FIGURE 8. Heatsink Thermal Resistance Requirements

for

Figure 7

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