SP6669_12 Datasheet, PDF (8/11 Pages) Exar Corporation – 600mA 1.5MHz Synchronous Step Down Converter

SP6 6 6 9
6 0 0 m A 1 .5 M H z Sy n ch r o n o u s St e p Do w n Co n v e r t e r
THEORY OF OPERATION
A PPLICATIONS
The typical application circuit of the adjustable
output voltage option and the fixed output
voltage option are shown below.
Fig. 22 : Adjustable Output Voltage Version
Fig. 23 : Fixed Output Voltage Version
I NDUCTOR SELECTION
Inductor ripple current and core saturation are
two factors considered to select the inductor
value.
Eq. 1: 'I _L
f
1
˜L
V_OUT
¨¨©§^{1 VVOIUNT}
¸¸¹·
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 SELECTION
A low ESR input capacitor can prevent large
voltage transients at V _IN . The RMS current
rating of the input capacit or is required to be
larger than I _RMS calculated by:
Eq. 2:
I _RMS #I_OMAX
  V_OUT V_IN V_OUT
V_IN
The ESR rating of the capacitor is an important
parameter to select C _OUT. The output ripple
V_OUT is determined by:
Eq. 3:
'V_OUT
#'I _L
¨ ^¨ _© ^§ESR

8
˜f
1
˜C_OUT
¸ ^¸ _¹ ^·
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
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.
OUTPUT V OLTAGE ±A DJUSTABLE V ERSION
The adjustable output voltage version is
determined by:
Eq. 4: V_OUT 0.6V ˜¨¨©§1^RR_{12 ¸¸¹·}
THERMAL CONSIDERATIONS
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 LAYOUT
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 shoul d 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