SC471AEVB datasheet(12/27 Pages) SEMTECH

12 / 27 page

SC471/SC471A

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POWER MANAGEMENT

For the case where the down transition is less than

8%, and the load is light such that power-save is active,

the Smart Power Save detector will not activate. In this

case, with FB already above the 0.75V reference there

is no switching activity. DL and DH will remain off, and

the output voltage will slowly fall as the output capacitors

discharge into the load, see Figure 6.

Note: at light loads it can take many msec for the output to fall

to the new value. This should have no adverse effect. Many

loads such as graphics chipsets can have a minimum load of

several hundred mA, which will naturally pull VOUT down to the

next level.

VOUT

D0/D1

RTN

< 810mV

(FB threshold)

COUT Discharge due to load

Initial

VOUT

Final

VOUT

R3/R4

(Smart Psave threshold)

Figure 6

The time needed to reach the ﬁnal voltage is found from

the following equation, where COUT is in μF, and LOAD is

in Amps:

Time (μsec) = COUT • (VINITIAL – VFINAL)/LOAD

Note: the preceding equation applies only to the condition

where the VOUT downward change is less that the 8% limit

for Smart Psave, and also the load is light such that Psave is

active.

When doing an up transition (from lower to higher VOUT),

the G0/G1 change will affect D0/D1 and cause FB to drop

below the 0.75V internal reference. This quickly trips the

FB comparator regardless of whether psave is active or

not, generating a DH on-time and a subsequent DL high

time. At the end of the minimum off-time (350nsec), if FB

is still below 0.75V then another DH on-time is started,

Figure 7. This continues until FB reaches the normal

operating point.

VOUT

D0/D1

RTN

750mV

(FB threshold)

VOUT

Final

VOUT

Initial

VOUT

R3/R4

Figure 7

If the VOUT change is signiﬁcant, there can be several

consecutive cycles of DH on-time followed by minimum DL

time. This can cause a rapid increase in inductor current:

typically it only takes a few switching cycles for the induc-

tor current to rise up to the Current Limit. At some point

the FB voltage will rise up to the 0.75V reference and the

DH pulses will cease, but the inductor’s LI2 energy must

then ﬂow into the output cap. This can create a signiﬁcant

overshoot as shown in Figure 8.

Applications Information (continued)

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