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參數(shù)資料
| 型號(hào)： | SC1408IMS.TR |
| 廠商： | Semtech Corporation |
| 英文描述： | LOW VOLTAGE, BOOST DC-DC CONTROLLER |
| 中文描述： | 低電壓，升壓DC - DC控制器 |
| 文件頁(yè)數(shù)： | 7/12頁(yè) |
| 文件大?。?/td> | 157K |
| 代理商： | SC1408IMS.TR |
2000 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC
CONTROLLER
SC1408
August 22, 2000
7
THEORY OF OPERATION
The SC1408 is a modified hysteretic boost converter
controller. The power switch is turned on when the out-
put voltage falls slightly below it’s setpoint. It remains on
for approximately 16
μ
s, or until the inductor current
reaches limit, whichever occurs first. The power switch is
then turned off for 2.3
μ
s, or until the output voltage once
again falls below setpoint, whichever occurs last.
The SC1408 is normally powered from the output volt-
age. Internal circuitry, such as the bandgap, compara-
tors and one shots will not function properly until the BST
pin voltage reaches 2.5V. To ensure startup at low input
voltages, the normal control circuitry is disabled and a
special, low voltage start up oscillator generates an ap-
proximate square wave at the GATE pin, initiating boost
action. When the output voltage reaches 2.5V, the nor-
mal 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
output voltage will then be one diode drop below the in-
put.
COMPONENT SELECTION
Boost Converter
R
SENSE
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
R
SENSE
. R
SENSE
may be determined either from the maxi-
mum output current curves or from the equation below:
In the equation above, the use of 2.3μs for t
off
may lead
to slightly optimistic current values for low V
/V
IN
ratios.
The theoretical curves use the actual value of t
,
V
F
=0.5V, V
FET
=0.3V and V
CS
=0.08V and are generated
for L=22
μ
H.
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) should be
300k
or less and the top resistor (R3 in the application
circuits) can be calculated from:
Inductor
The SC1408 will work with a wide range of inductor val-
ues. A good choice for most applications is 22
μ
H.
Smaller inductor values result in higher peak currents
and increased output ripple, while larger values will re-
sult in slower loop response.
Transistor Selection
Normally the power switch will be an N-channel MOS-
FET, although in certain circumstances an NPN bipolar
may be substituted.
The choice of FET can be critical, especially in battery
powered applications where the converter must be able
to use all of the available energy in the battery. This re-
quires 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 ap-
proaches full discharge. For these demanding applica-
tions, a FET with low V
is required. A good rule of
thumb is that V
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 equal to
the output current. Industry standard 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. 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 cur-
rent pulses drawn from the input supply. A 100
μ
F tanta-
lum will be adequate for most applications.
(
)(
V
)
DCR
Inductor
and
R
FET,
across
Voltage
V
Drop
Voltage
Forward
Diode
Output
=
V
:
=
Where
V
V
V
V
V
V
V
L
2
t
V
V
V
V
V
V
1
R
V
I
SENSE
FET
F
FET
F
O
IN
F
O
FET
IN
off
FET
F
O
IN
F
O
SENSE
CS
)
MAX
(
O
+
+
+
+
=
=
1
V
V
4
R
3
R
REF
O
SENSE
ESR
CS
R
RIPPLE
V
R
*
V
=
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相關(guān)代理商/技術(shù)參數(shù) |
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|---|---|
| SC1408IMSTRT | 制造商:SEMTECH 制造商全稱(chēng):Semtech Corporation 功能描述:Low Voltage, Boost DC-DC Controller |
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| SC1408IS.TRT | 制造商:Semtech Corporation 功能描述:IC BOOST DC/DC VOLTAGE CONTROLLER 8-SOIC, Primary Input Voltage:16.5V, No. of Ou |
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