SP6654 Datasheet, PDF (12/17 Pages) Sipex Corporation – High Efficiency 800mA Synchronous Buck Regulator Ideal for portable designs powered with Li Ion battery

APPLICATION INFORMATION
current ripple of almost 300mA 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 SP6654 inductor current limit of 1.25A.
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 SP6654 are mag-
netically shielded types to minimize radiated mag-
netic field emissions.
Capacitor Selection
The SP6654 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 22µF ceramic, POSCAP or Alu-
minum Polymer. These capacitors combine small
size, low ESR and good value. To regulate the
output with low ESR capacitors 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 Operations section).
Output ripple for a buck regulator is determined
mostly by output capacitor ESR, which for the
SP6654 with a constant inductor current ripple can
be expressed as:
V_OUT (ripple) = I_{LR *} R_ESR
For the 22µF POSCAP with 0.04Ω ESR, and a
10µH inductor yielding 200mA inductor current
ripple I_LR, the V_OUT ripple would be 8mVpp.
Since 8mV is a very small signal level, the actual
value would probably be larger due to noise and
layout issues, but this illustrates that the SP6654
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. Another function of the
output capacitance is to hold up the output voltage
during the load transients and prevent excessive
overshoot and undershoot. The typical perfor-
mance characteristics curves show very good load
step transient response for the SP6654 with the
recommended output capacitance of 22µF ce-
ramic.
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 SP6654 is 22µF ceramic,
POSCAP or Aluminum Polymer. These capaci-
tors 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 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 require-
ments.
Date: 2/1/05
SP6654 High Efficiency 800mA Synchronous Buck Regulator
12
© Copyright 2005 Sipex Corporation