SC1402ISSTR 查看數據表（PDF） - Semtech Corporation
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SC1402ISSTR
Semtech Corporation
SC1402ISSTR Datasheet PDF : 18 Pages
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Multi-Output, Low-Noise Power Supply
Controller for Notebook Computers
SC1402
December 15, 2000
by using the maximum current limit threshold:
IPEAK
0.120
= R9
0.120
= 0.020
=6
From this value choose an inductor with Isat > 6 Amps,
and for the FET choose a continuous conduction cur-
rent rating greater than 6 Amps.
The same calculation process can be made for the 5V
supply for its Inductor L2 and Rsense resistor, R8.
The results are:
L2 = 13.4 uH at 300KHz, 20uH at 200KHz, For our
EVAL board we choose the transformer with a 11uH
primary at the expense of slightly higher ripple current.
R8 = 0.02 Ω
Co = C18 = 330uF/10V
Calculating output capacitance and ESR for
Stability.
Now that the basic output filter has been designed, it is
time to check if the output filter will allow stable opera-
tion. Since the control loop is internal to the SC1402,
the output filter capacitor and its associated equivilant
series resistance (ESR), will effect stability. It is impor-
tant to choose a capacitor with its ESR for stable
SMPS operation. A seven step procedure for choosing
the output capacitor and ESR ensuring a stable control
loop is shown below for the 3.3V supply. The same
procedure should also be implemented for the 5V sup-
ply.
System Paramaters:
Fs = 300000Hz Vout = 3.3V
Vref = 2.5V Rs = 0.020 Ω
Vinmin = 6V
Where Fs is the switching frequency.
Vout is the output voltage.
Vinmin is the minimum input voltage.
Vref is the SC1402 reference voltage.
Rs is the sense resistor, (R9 on the EVAL board).
Step 1: Determine the Crossover Frequency
Fc
=
3
•
Fs
çæ1+
è
Vout
Vinmin
ö
Fc = 64.516KHz
Step 2: Determine the Maximum & Minimum ESR
Capacitance
ESRmax
=
Vout
Vref
• Rs
ESRmax = 0.026Ω
ESRmin
=
ESRmax
(1.2)2
ESRmin= 0.018Ω
Step 3: Determine the Minimum Output Capacitance
( ) Comin
=
2
•
π
• Fc
•
Vref
Vout • Rs
•
tan
300
Comin = 162uF
Step 4: Determine the Actual Output Capacitance
Co
=
ESRmax
ESRmin
• Comin
Co = 233uF
Step 5: Determine the Actual ESR Value
ESR
=
ESRmax +
2
ESRmin
ESR = 0.022 Ω
Step 6: From Above Calculations Choose:
Co ≥ 233uF
ESR = 0.022 Ω
Step 7: Check the Ripple Current
If Co can handle the ripple current you’re done. Other-
wise increase the output capacitance to handle the rip-
ple current at maximum Vin and recheck the ESR us-
ing the equation for determining the actual output ca-
pacitance.
If you are using tantalum or electrolytic capacitors you
should increase the capacitance to the level of ap-
proaching the calculated ESR.
If you are using poly capacitors a small resistor in se-
ries with the capacitor to bring the ESR to the desired
level may be necessary for stability due to their low
ESR values.
© 2000 SEMTECH CORP.
12
652 MITCHELL ROAD NEWBURY PARK CA 91320
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