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参数资料
| 型号： | SC1408CS |
| 元件分类： | 电源模块 |
| 英文描述： | 1-OUTPUT DC-DC REG PWR SUPPLY MODULE |
| 封装： | MS-012AA, SO-8 |
| 文件页数： | 5/7页 |
| 文件大小： | 88K |
| 代理商： | SC1408CS |
1999 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
LOW VOLTAGE, BOOST DC-DC
CONTROLLER
SC1408
PRELIMINARY - September 17, 1999
5
THEORY OF OPERATION
The SC1408 is a modified hysteretic boost converter
controller. The power switch is turned on when the otput
voltage falls slightly below it’s setpoint. It remains on for
approximately 16uS, or until the inductor current reaches
limit, whichever occurs first. The power switch is then
turned off for 2.3uS, 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 approximate 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 oscil-
lator shut down.
To conserve power, a SHDN pin is provided which,
when pulled high, shuts down most internal circuitry. The
output voltage will then be 1 diode drop below the input.
COMPONENT SELECTION
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 maxi-
mum output current curves or from the equation below:
In the equation above, the use of 2.3S for t
off may lead
to slightly optimistic current values for low V
O/VIN ratios.
The theoretical curves use the actual value of t
off,
V
F=0.5V, VFET=0.3V and VCS=0.08V and are generated
for L=22uH.
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 300KOhm or less and the
top resistor (R3 in the application circuits) can be calcu-
lated from:
Inductor
The SC1408 will work with a wide range of inductor val-
ues. A good choice for most applications is 22uH.
Smaller inductor values result in higher peak currents
and increase output ripple, while larger values will result
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
GS(th) is required. A good rule of
thumb is that V
GS(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
O , 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 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 cur-
rent pulses drawn from the input supply. A 100uF tanta-
lum will be adequate for most applications.
()(
)
DCR
Inductor
and
R
FET,
across
Voltage
V
Drop
Voltage
Forward
Diode
Output
V
:
Where
V
L
2
t
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
4
R
3
R
REF
O
SENSE
ESR
CS
RIPPLE
R
*
V
=
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