Function Description(Cont.)

Therefore,

power dissipated by the part is:

PD = ILOAD 2 • RDS(ON) = 187.2mW

For the SOT-23 package, the qJA is 250°C/ W. Thus,

the junction temperature of the regulator is:

TJ = 70°C + (0.1872)(250) = 116.8°C

which is below the maximum junction temperature of

125°C.

Note that at higher supply voltages, the junction

temperature is lower due to reduced switch resistance

(RDS(ON)).

Checking Transient Response

The regulator loop response can be checked by looking

at the load transient response. Switching regulators take

several cycles to respond to a step in load current.

When a load step occurs, VOUT immediately shifts by

an amount equal to (∆ILOAD • ESR), where ESR is the

effective series resistance of COUT. ∆ILOAD also

begins to charge or discharge COUT, which generates a

feedback error signal. The regulator loop then acts to

return VOUT to its steadystate value. During this

recovery time VOUT can be monitored for overshoot or

ringing that would indicate a stability problem. For a

detailed explanation of switching control loop theory.

A second, more severe transient is caused by switching

in loads with large (>1µF) supply bypass capacitors. The

discharged bypass capacitors are effectively put in

parallel with COUT, causing a rapid drop in VOUT. No

regulator can deliver enough current to prevent this

problem if the load switch resistance is low and it is

driven quickly. The only solution is to limit the rise time of

the switch drive so that the load rise time is limited to

approximately (25 • CLOAD).Thus, a 10µF capacitor

charging to 3.3V would require a 250µs rise time, limiting

the charging current to about 130mA.

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SEMICONDUCTOR

TECHNICAL D ATA

FTD6821

Revision No : 0