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| 型号： | SP6669EB |
| 厂商： | Exar Corporation |
| 文件页数： | 8/11页 |
| 文件大小： | 0K |
| 描述： | EVAL BOARD FOR SP6669 |
| 标准包装： | 1 |
| 系列： | * |
| 工具箱： | 1016-1749-ND - LOW POWER DESIGN KIT 732-3616-ND - LOW POWER DESIGN KIT |
S P 6 6 6 9
6 0 0 m A 1 . 5 M H z S y n c h r o n o u s S t e p D o w n C o n v e r t e r
THEORY OF OPERATION
A PPLICATIONS
The ESR rating of the capacitor is an important
parameter to select C OUT . The output ripple
V OUT is determined by:
? V OUT ? ? I L ? ? ESR ?
? ?
The typical application circuit of the adjustable
output voltage option and the fixed output
voltage option are shown below.
Eq. 3:
?
?
1
8 ? f ? C OUT
?
?
Higher values, lower cost ceramic capacitors
are now available in smaller sizes. These
capacitors have high ripple currents, high
voltage ratings and low ESR that makes them
ideal for switching regulator applications. As
Fig. 22: Adjustable Output Voltage Version
C OUT does not affect the internal control loop
stability, its value can be optimized to balance
very low output ripple and circuit size. It is
recommended to use an X5R or X7R rated
capacitors which have the best temperature
and voltage characteristics of all the ceramics
for a given value and size.
O UTPUT V OLTAGE – A DJUSTABLE V ERSION
V OUT ? 0 . 6 V ? ? ? 1 ? 2 ? ?
Fig. 23: Fixed Output Voltage Version
I NDUCTOR S ELECTION
Inductor ripple current and core saturation are
The adjustable
determined by:
Eq. 4:
output voltage version
? R ?
? R 1 ?
is
V OUT ? ? 1 ? OUT
? ?
Eq. 1: ? I L ?
f ? L
V OUT ? V IN ? V OUT ?
two factors considered to select the inductor
value.
1 ? V ?
? V IN ?
Equation 1 shows the inductor ripple current
as a function of the frequency, inductance, V IN
and V OUT . It is recommended to set the ripple
current between 30% to 40% of the maximum
load current. A low ESR inductor is preferred.
C IN AND C OUT S ELECTION
A low ESR input capacitor can prevent large
voltage transients at V IN . The RMS current
rating of the input capacitor is required to be
larger than I RMS calculated by:
Eq. 2: I RMS ? I OMAX
V IN
T HERMAL C ONSIDERATIONS
Although the SP6669 has an on board over
temperature circuitry, the total power
dissipation it can support is based on the
package thermal capabilities. The formula to
ensure safe operation is given in note 1.
PCB L AYOUT
The following PCB layout guidelines should be
taken into account to ensure proper operation
and performance of the SP6669:
1- The GND, SW and V IN traces should be kept
short, direct and wide.
2- V FB pin must be connected directly to the
feedback resistors. The resistor divider
network must be connected in parallel to the
C OUT capacitor.
3- The input capacitor C IN must be kept as
close as possible to the V IN pin.
? 2012 Exar Corporation
8/11
Rev. 2.2.0
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