SC1408IS.TR(2000) データシートの表示（PDF） - Semtech Corporation
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SC1408IS.TR Datasheet PDF : 12 Pages
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LOW VOLTAGE, BOOST DC-DC
CONTROLLER
SC1408
August 22, 2000
THEORY OF OPERATION
pin to GND, or to any voltage in the 3.0V to 16.5V range
using external divider resistors.
The SC1408 is a modified hysteretic boost converter
The bottom resistor in the divider chain (R4) should be
controller. The power switch is turned on when the out- 300kΩ or less and the top resistor (R3 in the application
put voltage falls slightly below it’s setpoint. It remains on circuits) can be calculated from:
for approximately 16µs, or until the inductor current
reaches limit, whichever occurs first. The power switch is
then turned off for 2.3µs, or until the output voltage once
R3
=
R 4ççèæ
VO
VREF
−
ö
1
again falls below setpoint, whichever occurs last.
The SC1408 is normally powered from the output volt- Inductor
age. Internal circuitry, such as the bandgap, compara- The SC1408 will work with a wide range of inductor val-
tors and one shots will not function properly until the BST ues. A good choice for most applications is 22µH.
pin voltage reaches 2.5V. To ensure startup at low input Smaller inductor values result in higher peak currents
voltages, the normal control circuitry is disabled and a and increased output ripple, while larger values will re-
special, low voltage start up oscillator generates an ap- sult in slower loop response.
proximate square wave at the GATE pin, initiating boost
action. When the output voltage reaches 2.5V, the nor- Transistor Selection
mal control circuitry is enabled and the start up oscillator Normally the power switch will be an N-channel MOS-
shuts down.
FET, although in certain circumstances an NPN bipolar
To conserve power, a SHDN pin is provided which,
may be substituted.
when pulled high, shuts down most internal circuitry. The The choice of FET can be critical, especially in battery
output voltage will then be one diode drop below the in- powered applications where the converter must be able
put.
to use all of the available energy in the battery. This re-
COMPONENT SELECTION
quires that the converter be capable of starting up from
very low input voltages. For example a two cell alkaline
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-
system’s terminal voltage will drop to 1.8V as it ap-
proaches full discharge. For these demanding applica-
tions, 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.
oped by the peak inductor current flowing through
RSENSE. RSENSE may be determined either from the maxi- Diode
mum output current curves or from the equation below: For most applications, a Schottky diode should be used
as the output rectifier. It will be subjected to reverse volt-
IO(MAX )
=
VCS
RSENSE
ççèæ 1 −
VO + VF − VIN
VO + VF − VFET
÷÷ö
( )( ) −
t off
2L
ççèæ
VIN − VFET VO + VF − VIN
VO + VF − VFET
ö
ages of at least VO , and average current will be equal to
the output current. Industry standard 1N5817 series or
an equivalent surface mount part would be suitable.
Output Capacitors
Where :
Output capacitors should be low ESR to minimize ripple
VF = Output Diode Forward Voltage Drop
voltage and maximize efficiency. Low ESR tantalums,
OSCONs or the newer Polymer capacitors should be
VFET = Voltage across FET, RSENSE and Inductor DCR used. Ripple voltage will be approximately:
In the equation above, the use of 2.3µs 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
VRIPPLE
=
VCS * RESR
RSENSE
Input Capacitors
Input capacitors on a boost 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.
7
© 2000 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
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