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Rev. 1.12 - 9 -
V
IN
=V
OUT
+(Rp+R
L
)
×
I
OUT
+L
×
I
RP
/ton
Equation 3
When Lx Tr. is OFF:
L
×
I
RP
/toff = V
F
+V
OUT
+R
L
×
I
OUT
Equation 4
Put Equation 4 to Equation 3 and solve for ON duty, ton/(toff+ton)=D
ON
,
D
ON
=(V
OUT+
V
F
+R
L
×
I
OUT
)/(V
IN
+V
F
-Rp
×
I
OUT
)
Equation 5
Ripple Current is as follows;
I
RP
=(V
IN
-V
OUT
-Rp
×
I
OUT
-R
L
×
I
OUT
)
×
D
ON
/f/L
…
Equation 6
Wherein, peak current that flows through L, Lx Tr., and SD is as follows;
ILmax=I
OUT
+I
RP
/2
…
Equation 7
Consider ILmax, condition of input and output and select external components.
#
The above explanation is directed to the calculation in an ideal case in continuous mode.
I
External Components
1. Inductor
Select an inductor that peak
current does not exceed ILmax. If larger current than allowable current flows,
magnetic saturation occurs and make transform efficiency worse.
When the load current is definite, the smaller value of L, the larger the ripple current.
Provided that the allowable current is large in that case and DC current is small, therefore, for large output current,
efficiency is better than using an inductor with a large value of L and vice versa.
2. Diode
Use a diode with low V
F
(Schottky type is recommended.) and high switching speed.
Reverse voltage rating should be more than V
IN
and current rating should be equal or more than ILmax.
3. Capacitors
As for C
IN
, use a capacitor with low ESR (Equivalent Series Resistance) and a capacity of at least 10
μ
F for stable
operation.
C
OUT
can reduce ripple of Output Voltage, therefore 47
μ
F or more value of tantalum type capacitor is
recommended.
4. Lx Transistor
Pch Power MOSFET is required for this IC.
Its breakdown voltage between gate and source should be a few V higher than Input Voltage.
In the case of Input Voltage is low, to turn on MOSFET completely, to use a MOSFET with low threshold voltage is
effective.
If a large load current is necessary for your application and important, choose a MOSFET with low ON resistance
for good efficiency.
If a small load current is mainly necessary for your application, choose a MOSFET with low gate capacity for good
efficiency.
Maximum continuous drain current of MOSFET should be larger than peak current, ILmax.