Datasheet

R
SEN
=
V
SENSE
- D(V
SL
+ 'V
SL
)
I
SWPEAK
2
'V
OUT
=
I
OUT
1-D
+
'I
L2
( )
ESR
L2 >
(V
IN
- V
Q
)D
2I
OUT
f
S
L1 >
(V
IN
- V
Q
)(1-D)
2I
OUT
f
S
LM3488
LM3488-Q1
SNVS089M –JULY 2000–REVISED MARCH 2013 www.ti.com
(40)
(41)
maintains the condition I
L
> Δi
L
/2 to ensure constant current mode.
(42)
(43)
Peak current in the inductor, to ensure the inductor does not saturate:
(44)
(45)
I
L1PK
must be lower than the maximum current rating set by the current sense resistor.
The value of L1 can be increased above the minimum recommended to reduce input ripple and output ripple.
However, once D
IL1
is less than 20% of I
L1AVE
, the benefit to output ripple is minimal.
By increasing the value of L2 above the minimum recommended, Δ
IL2
can be reduced, which in turn will reduce
the output ripple voltage:
where
• ESR is the effective series resistance of the output capacitor. (46)
If L1 and L2 are wound on the same core, then L1 = L2 = L. All the equations above will hold true if the
inductance is replaced by 2L. A good choice for transformer with equal turns is Coiltronics CTX series Octopack.
SENSE RESISTOR SELECTION
The peak current through the switch, I
SW(PEAK)
can be adjusted using the current sense resistor, R
SEN
, to provide
a certain output current. Resistor R
SEN
can be selected using the formula:
(47)
Sepic Capacitor Selection
The selection of SEPIC capacitor, CS, depends on the rms current. The rms current of the SEPIC capacitor is
given by:
(48)
The SEPIC capacitor must be rated for a large ACrms current relative to the output power. This property makes
the SEPIC much better suited to lower power applications where the rms current through the capacitor is
relatively small (relative to capacitor technology). The voltage rating of the SEPIC capacitor must be greater than
the maximum input voltage. Tantalum capacitors are the best choice for SMT, having high rms current ratings
relative to size. Ceramic capacitors could be used, but the low C values will tend to cause larger changes in
voltage across the capacitor due to the large currents. High C value ceramics are expensive. Electrolytics work
well for through hole applications where the size required to meet the rms current rating can be accommodated.
There is an energy balance between CS and L1, which can be used to determine the value of the capacitor. The
basic energy balance equation is:
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