NCP1450A Datasheet, PDF (18/24 Pages) ON Semiconductor – PWM Step-up DC-DC Controller

NCP1450A
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.
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
) 0.3 V
180000
*
Hz
2.4 V)(1
1A
* 0.364)2
0.2
+
13.5
mH
Determine the average inductor current and peak inductor
current:
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁCÁÁÁÁÁÁÁÁÁÁalcVDIuÁÁÁPÁÁÁÁÁÁÁDILPLQlKaPtioÁÁÁÁÁÁÁÁÁÁn ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ(V[ÁÁÁOÁÁÁÁÁÁÁvCU(DOTIVÁÁÁLVÁÁÁÁÁÁÁQU)OO*fTILUEUV)ÁÁÁI(ÁÁÁÁÁÁÁTqT(OD1I1uOf)))()**a(IOI1ÁÁÁtÁÁÁÁÁÁÁLViVDV*oD*D2DDInI)RN*DIÁÁÁÁÁÁÁÁÁÁ*IR))O)(1V)V*IÁÁÁSEÁÁÁÁÁÁÁNSDR)ÁÁÁÁÁÁÁÁÁÁ2 ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ
NOTES:
D - On-time duty cycle
I_L - Average inductor current
I_PK - Peak inductor current
DIR - Delta inductor current to average inductor current ratio
I_O - Desired dc output current
V_IN - Nominal operating dc input voltage
V_OUT - Desired dc output voltage
V_D - Diode forward voltage
V_S - Saturation voltage of the external transistor switch
DQ ^{- Charge stores in the C}OUT ^{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:
V_IN = 2.4 V
V_OUT = 3.3 V
I_O = 1.0 A
V_pp = 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 V_IN = 2.4 V:
IL
+
1
*
1
0.364
+
1.57
A
IPK + 1.57A (1 ) ^02.2) + 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 *
18000Hz
0.364)
+
2.01
mC
Determine the output capacitance value for the desired
output ripple voltage:
Assume the ESR of the output capacitor is 0.15 W,
COUT
u
100mV
*
2.01mC
(1.57A * 1A)
0.15W ^{+ 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.
3.3 V ) 0.3 V * 2.4 V
D + _{3.3 V ) 0.3 V * 0.3 V} + 0.364
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