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參數(shù)資料
| 型號： | SC1408ISTRT |
| 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 |
| 文件頁數(shù)： | 5/12頁 |
| 文件大?。?/td> | 235K |
| 代理商： | SC1408ISTRT |
5
2005 Semtech Corp.
www.semtech.com
POWER MANAGEMENT
Applications Information
SC1408
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
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 maximum out-
put current curves or from the equation below:
(
)(
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
+
+
+
+
=
In the equation above, the use of 2.3
∝
s 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
=0.5V,
V
FET
=0.3V and V
CS
=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 300k
or less and the top
resistor (R3 in the application circuits) can be calculated
from
=
1
V
V
4
R
3
R
REF
O
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 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
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 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 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 100
μ
F tantalum will be
adequate for most applications.
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相關(guān)代理商/技術(shù)參數(shù) |
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| SC1409 | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:1 A Synchronous Switching Regulator |
| SC1409ISTR | 制造商:SEMTECH 制造商全稱:Semtech Corporation 功能描述:1 A Synchronous Switching Regulator |
| SC140B | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:TRIAC|200V V(DRM)|6A I(T)RMS|TO-220 |
| SC140D | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:TRIAC|400V V(DRM)|6A I(T)RMS|TO-220 |
| SC140M | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:TRIAC|600V V(DRM)|6A I(T)RMS|TO-220 |
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