SP6669 Datasheet, PDF (7/10 Pages) Exar Corporation – 1.5MHZ, 600mA SYNCHRONOUS STEP DOWN CONVERTER

Detailed Description
Applications
The typical application circuit of the
adjustable output voltage option and the
fixed output voltage option are shown
below.
Fig. 18: Adjustable Output Voltage Version
Typical Application Circuit
Fig. 19: Fixed Output Voltage Version
Typical Application Circuit
Inductor Selection
Inductor ripple current and core saturation
are two factors considered to select the
inductor value.
Eq. 1:
ΔI _L
=
f
1
⋅
L
V_OUT
⎜⎜⎝⎛¹
−
V_OUT
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 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 capacitor is
required to be larger than I_RMS calculated
by:
Eq. 2: I _RMS ≅ I_OMAX
( ) V_OUT V_IN −V_OUT
V_IN
THEORY OF OPERATION
The ESR rating of the capacitor is an
important parameter to select C_OUT. The
output ripple V_OUT is determined by:
Eq. 3:
ΔV_OUT
≅
ΔI _{L ⎜⎜⎝⎛} ESR +
8⋅
f
1
⋅ 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 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 Voltage – Adjustable Version
The adjustable output voltage version is
determined by:
Eq. 4:
V_OUT
=
0.6V
⋅ _{⎜⎜⎝⎛}1 +
R₂
R₁
⎟⎟⎠⎞
Thermal Considerations
Allthough 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 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.
Rev G – 12/02/2008
SP6669 1.5MHz 0.6A Sync. Step Down Converter © 2008 Exar Corporation
7/10