SP6650 Datasheet, PDF (11/17 Pages) Sipex Corporation – High Efficiency 600mA Synchronous Buck Regulator Ideal for portable designs powered with Li Ion battery

APPLICATION INFORMATION
External Component Selection
Inductor
According to the pulse frequency modulation
(PFM) algorithm, the peak to peak output ripple
current can be calculated as:
I_LR
≈
K_ON
L
K_ON = 2.7µs*V is a constant for SP6650 and is
set by the parameters of the internal ON-time
calculation circuitry. For the recommended 22µH
inductor, typical ripple currents are I_LR = 123mA
in discontinuous conduction mode (DCM) op-
eration. During continuous conduction mode,
the speed of the loop comparator determines the
current ripple. It is approximately equal to 200mA
with a 22µH inductor.
The value of the inductor is chosen based on the
constant K_ON and acceptable current ripple. Two
additional inductor parameters are important: its
current rating and its DC resistance.
When the current through the inductor reaches
the level of I_sat, inductance drops down to 70%
from the nominal. This non-linear change can
cause stability problems or excessive fluctuation
in current ripple. To avoid this, the inductor
should be chosen with saturation current at least
equal to the maximum output current of the
converter plus half of the ripple. To provide the
best converter performance in dynamic condi-
tions such as start-up and load transients, induc-
tors with saturation current close to the chosen
I_LIM are recommended.
The second important inductor parameter, DC
resistance, directly defines the efficiency of the
converter, therefore, the inductor should be cho-
sen with the minimum possible DC resistance
for a particular design. Recommended types of
the inductors for different applications are given
in Table 1. Preferred inductors for on board
power supplies with the SP6650 converter are
shielded inductors to minimize radiated mag-
netic fields emissions.
All components recommended for typical de-
signs like those shown in the applications sche-
matics are given in Table 1.
Input and Output Capacitors
Output capacitor is often selected based on the
requirement on the output ripple voltage. In a
Buck regulator, the output ripple is determined
by ESR (equivalent series resistor) of the output
capacitors and inductor ripple current
V_OR = ESR * I_LR,
where V_OR = peak to peak output ripple voltage.
SP6650’s adaptive on-time scheme provides a
constant inductor ripple that is independent of
input voltages and thus makes it easier to select
the output capacitor. In many power supply
designs, the ripple voltage needs to be less than
3% of the DC output voltage. Using low ESR
tantalum or electrolytic capacitors to reduce the
output ripple.
Due to the nature of the PFM control, certain
output ripple is required for stable operation.
The loop comparator requires minimum of 15mV
ripple on the FB pin to reliably toggle the com-
parator output. That translates to an output ripple
of
V_OR(MIN)
=
15mV * V_OUT
V_REF
where V_REF = 1.25V is the internal reference
voltage.
To reduce the output ripple and improve stabil-
ity, a small capacitor can be paralleled with the
feedback voltage divider as shown on page 1.
This capacitor forms a high pass filter with
feedback resistor to increase the ripple voltage
seen by the FB pin. The value of the capacitor
should be in the range of 100pF to 500pF.
Although the 3.3V output can be programmed
simply by connecting the FB pin to the ground,
using this external feedback scheme can signifi-
cantly reduce the output ripple. For output ripple
less than 15mV, for instance when ceramic
capacitors are used, an artificial ramp can be
generated and superimposed onto the output.
Date: 5/25/04
SP6650 High Efficiency 600mA Synchronous Buck Regulator
11
© Copyright 2004 Sipex Corporation