SC1408IMS.TR データシートの表示（PDF） - Semtech Corporation
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SC1408IMS.TR Datasheet PDF : 12 Pages
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SC1408
POWER MANAGEMENT
Applications Information
Theory of Operation
The SC1408 is a modified hysteretic boost converter
controller. The power switch is turned on when the output
voltage falls slightly below it’s setpoint. It remains on for
approximately 16µs, or until the inductor current reaches
limit, whichever occurs first. The power switch is then
turned off for 2.3 µs, or until the output voltage once again
falls below setpoint, whichever occurs last.
The SC1408 is normally powered from the output voltage.
Internal circuitry, such as the bandgap, comparators and
one shots, will not function properly until the BST pin volt-
age reaches 2.5V. To ensure start-up at low input volt-
ages, the normal control circuitry is disabled and a spe-
cial, low voltage start up oscillator generates an approxi-
mate square wave at the GATE pin, initiating boost action.
When the output voltage reaches 2.5V, the normal control
circuitry is enabled and the start up oscillator shuts down.
To conserve power, a SHDN pin is provided which, when
pulled high, shuts down most internal circuitry. The out-
put voltage will then be 1 diode drop below the input.
COMPONENT SELECTION
Boost Converter
RSENSE
The value of the sense resistor is the primary determin-
ing factor for maximum output current. The SC1408 has
a fixed current limit voltage threshold, which is devel-
oped by the peak inductor current flowing through RSENSE.
RSENSE may be determined either from the maximum out-
put current curves or from the equation below:
IO(MAX )
=
VCS
RSENSE
1 −
VO + VF − VIN
VO + VF − VFET
( )( ) −
t off
2L
VIN − VFET VO + VF − VIN
VO + VF − VFET
Where :
VF = Output Diode Forward Voltage Drop
VFET = Voltage across FET, RSENSE and Inductor DCR
In the equation above, the use of 2.3∝s for toff may lead to
slightly optimistic current values for low VO/VIN ratios. The
theoretical curves use the actual value of toff, VF=0.5V,
VFET=0.3V and VCS=0.08V and are generated for L=22µH.
Output Voltage
Output voltage can be set to 5V by connecting the FB pin
to GND, or to any voltage in the 3.0V to 16.5V range using
external divider resistors.
The bottom resistor in the divider chain (R4 in the typical
application circuits) should be 300kΩ or less and the top
resistor (R3 in the application circuits) can be calculated
from
R3
=
R 4
VO
VREF
− 1
Inductor
The SC1408 will work with a wide range of inductor val-
ues. A good choice for most applications is 22µH. Smaller
inductor values result in higher peak currents and in-
crease output ripple, while larger values will result in
slower loop response.
Transistor selection
Normally the power switch will be an N-channel MOSFET,
although in certain circumstances an NPN bipolar may
be substituted.
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 in-
put voltages. For example a two cell alkaline system’s ter-
minal voltage will drop to 1.8V as it approaches full dis-
charge. 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 voltage.
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 V , and average current will be some-
O
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 tantalum or
OSCON capacitors should be used. Ripple voltage will be
approximately:
Input Capacitors
Input capacitors on a boost converter are less critical than
the output capacitors, since there are no fast current pulses
drawn from the input supply. A 100µF tantalum will be
adequate for most applications.
2005 Semtech Corp.
5
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