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参数资料
| 型号： | SP6650UEB |
| 厂商： | Exar Corporation |
| 文件页数： | 11/17页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD FOR SP6650 |
| 标准包装： | 1 |
| 系列： | * |
S P 6 6 5 0
H i g h E f f i c i e n c y 6 0 0 m A S y n c h r o n o u s B u c k R e g u l a t o r
APPLICATION INFORMATION
shielded inductors to minimize
magnetic fields emissions.
radiated
E XTERNAL C OMPONENT S ELECTION
Inductor
According to the pulse frequency modulation
(PFM) algorithm, the peak to peak output
ripple current can be calculated as:
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 ar e ILR
= 123mA in discontinuous conduction mode
(DCM) operation. 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 KON 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 conditions
such as start-up and load transients, inductors
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 chosen 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
All components recommended for typical
designs like those shown in the applications
schematics 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
where VOR = 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 comparator output. That translates to an
output ripple of
where V REF = 1.25V is the internal reference
voltage.
To reduce the output ripple and improve
stability, 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 significantly reduce the output
ripple. For output ripples less than 15mV, for
instance when ceramic capacitors are used, an
? 2012 Exar Corporation
11/17
Rev. 2.0.0
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