EC3217 Datasheet, PDF (15/16 Pages) E-CMOS Corporation – 1.5MHz, 2A, Synchronous Step-Down Regulator Dropout

1.5MHz, 2A, Synchronous
Step-Down Regulator Dropout
Function Description(Cont.)
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 (ΔILOAD • ESR), where ESR is the
effective series resistance of COUT. ΔILOAD also begins
to charge or discharge COUT, which generates a
feedback error signal. The regulator loop then acts to
return VOUT to its steady state 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.
EC3217
E-CMOS Corp. (www.ecmos.com.tw)
Page 15 of 16
5F05N-Rev.F005