SC1408IMS.TR Ver la hoja de datos (PDF) - Semtech Corporation
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SC1408IMS.TR Datasheet PDF : 12 Pages
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SC1408
POWER MANAGEMENT
Applications Information (Cont.)
COMPONENT SELECTION
SEPIC Converter
RSENSE
Again, with the SEPIC topology, the value of the sense
resistor is the primary determining factor for maximum
output current. The simplest approach to select RSENSE is
to add Vin to Vo and use this value as the output voltage
in the output current curves or in the equation for Boost
converter.
Output Voltage
Output voltage setting works exactly the same in SEPIC
topology as in Boost, including the ability to set to 5V by
connecting the FB pin to GND. Care must be taken to
ensure that the IC supply (pin2; BST) does not exceed its
16.5V rating. In the circuit of Fig.2: This requires maxi-
mum output voltage to be limited to 16.5V-Vin. Higher
output voltages are possible with different IC supply strat-
egies.
Inductor
The SEPIC topology requires a coupled inductor. Again A
good choice for most applications is 22µH. Smaller induc-
tor values result in higher peak currents and increase out-
put ripple, while larger values will result in slower loop re-
sponse.
Transistor selection
The choice of FET can be critical, especially in battery pow-
ered applications where the converter must be able to use
all of the available energy in the battery. This requires that
the converter be capable of starting up from very low
input voltages. For example a two cell alkaline system’s
terminal voltage will drop to 1.8V as it approaches full
discharge. For these demanding applications, a FET with
low VGS(th) is required. A good rule of thumb is that VGS(th)
should be at least 0.5V less than the minimum input volt-
age.
Diode
For most applications, a Schottky diode should be used
as the output rectifier. It will be subjected to reverse volt-
ages of at least VO +VIN and average current will be some-
what less than the Inductor peak current. Industry stan-
dard 1N5817 series or an equivalent surface mount part
would be suitable.
Output Capacitors
Output capacitors should be low ESR to minimize ripple
voltage and maximize efficiency. Low ESR tantalums,
OSCONs or the newer Polymer capacitors should be used.
Input Capacitors
Input capacitors on a SEPIC converter are less critical
than the output capacitors, since there are no fast cur-
rent pulses drawn from the input supply. A 100µF tanta-
lum will be adequate for most applications.
Series Capacitors
The Series capacitor(s) must be capable of handling an
RMS current given by:-
IRMS = IO
VO + 0.5
VIN
2005 Semtech Corp.
6
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