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NCP1450ASN27T1 Datasheet(PDF) 18 Page - ON Semiconductor

Part # NCP1450ASN27T1
Description  PWM Step?뭫p DC?묭C Controller
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

NCP1450ASN27T1 Datasheet(HTML) 18 Page - ON Semiconductor

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NCP1450A
http://onsemi.com
18
APPLICATION CIRCUIT INFORMATION
Step−up Converter Design Equations
The NCP1450A PWM step−up DC−DC controller is
designed to operate in continuous conduction mode and can
be defined by the following equations. External components
values can be calculated from these equations, however, the
optimized value should obtained through experimental
results.
Calculation
Equation
D
v
VOUT ) VD * VIN
VOUT ) VD * VS
IL
IO
(1
* D)
L
(VOUT ) VD * VIN)(1 * D)2
f
IO
DIR
IPK
IL (1 )
DIR
2
)
DQ
(IL * IO)(1 * D)
f
VPP
[ D
Q
COUT
) (IL * IO) ESR
NOTES:
D
− On−time duty cycle
IL
− Average inductor current
IPK
− Peak inductor current
DIR − Delta inductor current to average inductor current ratio
IO
− Desired dc output current
VIN
− Nominal operating dc input voltage
VOUT − Desired dc output voltage
VD
− Diode forward voltage
VS
− Saturation voltage of the external transistor switch
DQ
− Charge stores in the COUT during charging up
ESR
− Equivalent series resistance of the output capacitor
Design Example
It is supposed that a step−up DC−DC controller with 3.3 V
output delivering a maximum 1000 mA output current with
100 mV output ripple voltage powering from a 2.4 V input
is to be designed.
Design parameters:
VIN = 2.4 V
VOUT = 3.3 V
IO = 1.0 A
Vpp = 100 mV
f = 180 kHZ
DIR = 0.2 (typical for small output ripple voltage)
Assume the diode forward voltage and the transistor
saturation voltage are both 0.3 V. Determine the maximum
steady state duty cycle at VIN = 2.4 V:
D
+
3.3 V
) 0.3V * 2.4V
3.3 V
) 0.3V * 0.3V
+ 0.364
Calculate the maximum inductance value which can
generate the desired current output and the preferred delta
inductor current to average inductor current ratio:
L
v
(3.3 V
) 0.3V * 2.4V)(1 * 0.364)2
180000 Hz
1A
0.2
+ 13.5mH
Determine the average inductor current and peak inductor
current:
IL +
1
1
* 0.364 +
1.57 A
IPK + 1.57A (1 )
0.2
2
)
+ 1.73A
Therefore, a 12
mH inductor with saturation current larger
than 1.73 A can be selected as the initial trial.
Calculate the delta charge stored in the output capacitor
during the charging up period in each switching cycle:
DQ +
(1.57A
* 1A)(1 * 0.364)
18000Hz
+ 2.01mC
Determine the output capacitance value for the desired
output ripple voltage:
Assume the ESR of the output capacitor is 0.15
W,
COUT u
2.01
mC
100mV
* (1.57A * 1A)
0.15
W +
138.6
mF
Therefore, a Tantalum capacitor with value of 150
mF to
220
mF and ESR of 0.15 W can be used as the output
capacitor. However, according to experimental result,
220
mF output capacitor gives better overall operational
stability and smaller ripple voltage.
External Component Selection
Inductor Selection
The NCP1450A is designed to work well with a 6.8 to
12
mH inductors in most applications 10 mH is a sufficiently
low value to allow the use of a small surface mount coil, but
large enough to maintain low ripple. Lower inductance
values supply higher output current, but also increase the
ripple and reduce efficiency.
Higher inductor values reduce ripple and improve
efficiency, but also limit output current.
The inductor should have small DCR, usually less than
1
W, to minimize loss. It is necessary to choose an inductor
with a saturation current greater than the peak current which
the inductor will encounter in the application.


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