LM3488MM National Semiconductor, LM3488MM Datasheet - Page 20

• Manufacturer Part Number: LM3488MM
• Description: IC, N-CH CONTROLLER, MSOP-8
• Manufacturer: National Semiconductor

Specifications of LM3488MM

• Primary Input Voltage: 40V
• No. Of Outputs: 1
• Output Current: 1A
• No. Of Pins: 8
• Operating Temperature Range: -40°C To +125°C
• Termination Type: SMD
• Input Voltage Primary Min: 2.95V
• Output Voltage Min: 1.26V
• Rohs Compliant: No
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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I
the current sense resistor.
The value of L1 can be increased above the minimum rec-
ommended to reduce input ripple and output ripple. However,
once D
is minimal.
By increasing the value of L2 above the minimum recom-
mended, Δ
output ripple voltage:
where ESR is the effective series resistance of the output ca-
pacitor.
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 re-
placed 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
ed using the current sense resistor, R
output current. Resistor R
mula:
Sepic Capacitor Selection
The selection of SEPIC capacitor, CS, depends on the rms
current. The rms current of the SEPIC capacitor is given by:
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 ca-
pacitor 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 cur-
rents. High C value ceramics are expensive. Electrolytics
work well for through hole applications where the size re-
quired to meet the rms current rating can be accommodated.
L1PK
must be lower than the maximum current rating set by
IL1
is less than 20% of I
IL2
can be reduced, which in turn will reduce the
SEN
L1AVE
can be selected using the for-
, the benefit to output ripple
SEN
SW(PEAK)
, to provide a certain
can be adjust-
20
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:
Where
is the ripple voltage across the SEPIC capacitor, and
is the ripple current through the inductor L1. The energy bal-
ance equation can be solved to provide a minimum value for
C
Input Capacitor Selection
Similar to a boost converter, the SEPIC has an inductor at the
input. Hence, the input current waveform is continuous and
triangular. The inductor ensures that the input capacitor sees
fairly low ripple currents. However, as the input capacitor gets
smaller, the input ripple goes up. The rms current in the input
capacitor is given by:
The input capacitor should be capable of handling the rms
current. Although the input capacitor is not as critical in a
boost application, low values can cause impedance interac-
tions. Therefore a good quality capacitor should be chosen in
the range of 100µF to 200µF. If a value lower than 100µF is
used, then problems with impedance interactions or switching
noise can affect the LM3478. To improve performance, es-
pecially with V
20Ω resistor at the input to provide a RC filter. The resistor is
placed in series with the V
attached to the V
ceramic capacitor is necessary in this configuration. The bulk
input capacitor and inductor will connect on the other side of
the resistor with the input power supply.
S
:
IN
IN
below 8 volts, it is recommended to use a
pin directly (see
IN
pin with only a bypass capacitor
Figure
13). A 0.1µF or 1µF