SC488 datasheet(12/24 Pages) SEMTECH

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POWER MANAGEMENT

SC488

Switching Frequency

Switching frequency determines the trade-off between

size and efﬁ ciency. Higher frequency increases switch-

ing losses in the MOSFETs, since losses are a function of

F*VIN2. Knowing the maximum input voltage and budget

for MOSFET switches usually dictates the ﬁnal design.

Inductor Ripple Current

Low inductor values result in smaller size, but create high-

er ripple current and are less efﬁcient because of the high

AC current ﬂowing in the inductor. Higher inductor values

do reduce the ripple current and are more efﬁcient, but

are larger and more costly. The selection of the ripple cur-

rent is based on the maximum output current and tends

to be between 20% to 50% of the maximum load current.

Again, cost, size and efﬁciency all play a part in the selec-

tion process.

Stability Considerations

Unstable operation shows up in two related but distinctly

different ways: double pulsing and fast-feedback loop in-

stability. Double-pulsing occurs due to noise on the output

or because the ESR is too low, causing insufﬁcient voltage

ramp in the output signal. This causes the error ampliﬁer to

trigger prematurely after the 400ns minimum off-time has

expired. Double-pulsing will result in higher ripple voltage at

the output, but in most cases is harmless. In some cases,

however, double-pulsing can indicate the presence of loop

instability, which is caused by insufﬁcient ESR. One simple

way to solve this problem is to add some trace resistance

in the high current output path. A side effect of doing this

is output voltage droop with load. Another way to eliminate

doubling-pulsing is to add a 10pF capacitor across the

upper feedback resistor divider network. This is shown in

Figure 6, by capacitor C4 in the schematic. This capacitance

should be left out until conﬁrmation that double-pulsing ex-

ists. Adding this capacitance will add a zero in the transfer

function and should eliminate the problem. It is best to

leave a spot on the PCB in case it is needed.

Loop instability can cause oscillations at the output as a

response to line or load transients. These oscillations can

trip the over-voltage protection latch or cause the output

voltage to fall below the tolerance limit.

FBK

+5V

PGND

VDDP

ILIM

BST

+VIN

0.5V - 5.5V

10pF

SC488

Figure 6

The best way for checking stability is to apply a zero to

full load transient and observe the output voltage ripple

envelope for overshoot and ringing. Over one cycle of ring-

ing after the initial step is a sign that the ESR should be

increased.

SC488 ESR Requirements

The constant on-time control used in the SC488 regulates

the ripple voltage at the output capacitor. This signal

consists of a term generated by the output ESR of the

capacitor and a term based on the increase in voltage

across the capacitor due to charging and discharging

during the switching cycle. The minimum ESR is set to

generate the required ripple voltage for regulation. For most

applications the minimum ESR ripple voltage is dominated

by PCB layout and the properties of SP or POSCAP type

output capacitors. For applications using ceramic output

capacitors, the absolute minimum ESR must be considered.

If the ESR is low enough the ripple voltage is dominated

by the charging of the output capacitor. This ripple voltage

lags the on-time due to the LC poles and can cause double

pulsing if the phase delay exceeds the off-time of the

converter. Referring to Figure 5 on Page 10, the equation

for the minimum ESR as a function of output capacitance

and switching frequency and duty cycle is:

Cout

200000

1.5V

VOUT

ESR

Application Information (Cont.)

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