SP6655ER-L Datasheet, PDF (12/16 Pages) Sipex Corporation – High Efficiency 400mA Synchronous Buck Regulator

APPLICATION INFORMATION
inductor current ripple of almost 200mA would
produce very stable regulation and fast load
transient response at the expense of slightly
reduced efficiency.
Other inductor parameters are important: the in-
ductor 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 non-linear change can cause
stability problems or excessive fluctuation in in-
ductor current ripple. To avoid this, the inductor
should be selected with saturation current at least
equal to the maximum output current of the con-
verter plus half the inductor current ripple. To
provide the best performance in dynamic condi-
tions such as start-up and load transients, inductors
should be chosen with saturation current close to
the SP6655 inductor current limit of 0.625A.
DC resistance, another important inductor charac-
teristic, directly affects the efficiency of the con-
verter, so inductors with minimum DC resistance
should be chosen for high efficiency designs.
Recommended inductors with low DC resistance
are listed in Table 2. Preferred inductors for on
board power supplies with the SP6655 are mag-
netically shielded types to minimize radiated mag-
netic field emissions.
Capacitor Selection
The SP6655 has been designed to work with very
low ESR output capacitors (listed in Table 2
Component Selection) which for the typical appli-
cation circuit are 10µF ceramic capacitors. These
capacitors combine small size, low ESR and good
value. To regulate the output with low ESR capaci-
tors of 0.01Ω or less, an internal ramp voltage
V_RAMP has been added to the FB signal to reliably
trip the loop comparator (as described in the Op-
erations section).
Output ripple for a buck regulator is determined
mostly by output capacitor ESR, which for the
SP6655 with a constant inductor current ripple can
be expressed as:
V_OUT (ripple) = I_{LR *} R_ESR
For the 10µF Ceramic Output Capacitor with
0.003Ω ESR, and a 10µH inductor yielding 200mA
inductor current ripple I_LR, the V_OUT ripple would
be 0.6mVpp. Since 0.6mV is a very small signal
level, the actual value would probably be as large
as 10mV due to noise and layout issues, but this
illustrates that the SP6655 output ripple can be
very low indeed. To improve stability, a small
ceramic capacitor, 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 characteristics curves show
very good load step transient response for the
SP6655 with the recommended output capaci-
tance of 10µF ceramic.
The input capacitor will reduce the peak current
drawn from the battery, improve efficiency and
significantly reduce high frequency noises in-
duced by a switching power supply. The typical
input capacitor for the SP6655 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 recom-
mended for the V_IN pin 2 to effectively reduce the
noise for the ICs analog supply rail which powers
sensitive 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 schematic uses the
values of R_VIN = 10Ω and C_VIN = 1µF to meet these
requirements.
Date: 09/27/05
SP6655 High Efficiency 400mA Synchronous Buck Regulator
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