tonc=T´V
IN
/V
OUT
×××
Equation 2
When ton<tonc, the mode is the discontinuous mode, and when ton=tonc, the mode is the continuous mode.
n
OUTPUT CURRENT AND SELECTION OF EXTERNAL COMPONENTS
When LxTr is ON:
(Wherein, Ripple Current P-P value is described as I
RP
, ON resistance of LXTr is described as Rp the direct current
of the inductor is described as R
L
.)
V
IN
=V
OUT
+(Rp+R
L
)´I
OUT
+L´I
RP
/ton
When LxTr is OFF:
L´I
RP
/toff = V
F
+V
OUT
+R
L
´I
OUT
×××Equation
3
×××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
¼Equation
6
wherein, peak current that flows through L, LxTr, and SD is as follows;
ILmax=I
OUT
+I
RP
/2
¼Equation
7
Consider ILmax, condition of input and output and select external components.
HThe
above explanation is directed to the calculation in an ideal case in continuous mode.
n
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 same, 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. Capacitor
As for C
IN
, use a capacitor with low ESR(Equivalent Series Resistance) and a capacity of at least 10mF for stable
operation.
COUT can reduce ripple of Output Voltage, therefore 47 to 100mF tantalum type is recommended.
4. Lx Transistor
Pch Power MOS FET is required for this IC.
Its breakdown voltage between gate and source should be a few volt higher than Input Voltage.
In the case of Input Voltage is low, to turn on MOS FET completely, select a MOS FET with low threshold voltage.
If a large load current is necessary for your application and important, choose a MOS FET with low ON resistance
for good efficiency.
If a small load current is mainly necessary for your application, choose a MOS FET with low gate capacity for good
efficiency.
Maximum continuous drain current of MOS FET should be larger than peak current, ILmax.
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Rev. 1.11
-9-
RICOH [ RICOH ELECTRONICS DEVICES DIVISION ]
