SP6656_06 Datasheet, PDF (12/16 Pages) Sipex Corporation – High Efficiency 400mA Synchronous Buck Regulator with Dynamically Adjustable Voltage Output

not produce as stable an output ripple. For
larger inductors with the SP6656, which has
a peak inductor current of 0.625A, most
15µH or 22µH inductors would have to be
larger physical sizes, limiting their use in
small portable applications. Smaller values
like 10µH would more easily meet the 0.625A
limit and come in small case sizes, and the
increased inductor current ripple of almost
200mA would produce very stable regula-
tion and fast load transient response at the
expense of slightly reduced efficiency.
Other inductor parameters are important: the
inductor current rating and the DC resistance.
When the current through the inductor reaches
the level of I_SAT, the inductance drops to 70%
of the nominal value. This nonlinear change
can cause stability problems or excessive
fluctuation in inductor current ripple. To avoid
this, the inductor should be selected with
saturation current at least equal to the maxi-
mum output current of the converter plus half
the inductor current ripple. To provide the
best performance in dynamic conditions such
as start-up and load transients, inductors
should be chosen with saturation current close
to the SP6656 inductor current limit of 0.625A.
DC resistance, another important inductor
characteristic, directly affects the efficiency of
the converter, so inductors with minimum DC
resistance should be chosen for high effi-
ciency designs. Recommended inductors
with low DC resistance are listed in Table 2.
Preferred inductors for on board power sup-
plies with the SP6656 are magnetically
shielded types to minimize radiated magnetic
field emissions.
Capacitor Selection
The SP6656 has been designed to work with
very low ESR output capacitors (listed in
Table 2 Component Selection) which for the
typical application circuit are 10µF ceramic
capacitors. These capacitors combine small
size, low ESR and good value. To regulate the
output with low ESR capacitors of 0.01Ω or
APPLICATION INFORMATION
less, an internal ramp voltage V_RAMP has been
added to the FB signal to reliably trip the loop
comparator (as described in the Operations
section).
The output capacitor is required to keep the
output voltage ripple small and to ensure
regulation loop stability. The output capaci-
tor must have low impedance at the switch-
ing frequency. Ceramic capacitors with X5R
or X7R dielectrics are recommended due to
their low ESR and high ripple current. The
output ripple ∆VOUT is determined by:
∆VOUT <=
{ } ^{VOUT * (VINMax - VOUT)}
(VINMax _{* L * ƒOSC)}
*
ESR
+
(8
*
1
ƒOSC _*
.
COUT)
To improve stability, a small ceramic capaci-
tor, C_F = 22pF should be paralleled with the
feedback voltage divider RF, as shown on the
typical application schematic on page 1. An-
other function of the output capacitance is to
hold up the output voltage during the load
transients and prevent excessive overshoot
and undershoot. The typical performance char-
acteristics curves show very good load step
transient response for the SP6656 with the
recommended output capacitance of 10µF
ceramic.
The input capacitor will reduce the peak cur-
rent drawn from the battery, improve effi-
ciency and significantly reduce high frequency
noises induced by a switching power supply.
The typical input capacitor for the SP6656 is
10µF ceramic. These capacitors will provide
good high frequency bypassing and their low
ESR will reduce resistive losses for higher
efficiency. An RC filter is recommended for
the V_IN pin 2 to effectively reduce the noise for
the ICs analog supply rail which powers sen-
sitive circuits. This time constant needs to be
at least 5 times greater than the switching
period, which is calculated as 1/FLP during
the CCM mode. The typical application sche-
matic uses the values of R_VIN = 10Ω and C_VIN
= 1µF to meet these requirements.
Date: 3/6/06
SP6656, 400mA Synchronous Buck Regulator with Dynamically Adjustable Voltage Output
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© Copyright 2006 Sipex Corporation