sc1402 Semtech Corporation, sc1402 Datasheet - Page 12

• Manufacturer Part Number: sc1402
• Description: Multi-output, Low-noise Power Supply Controller For Notebook Computers
• Manufacturer: Semtech Corporation

Fc
Fc
by using the maximum current limit threshold:
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
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
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
© 2000 SEMTECH CORP.
December 15, 2000
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1
0.120
R9
Fs
Vinmin
KHz
Vout
Rs = 0.020
0.120
0.020
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6
Vinmin = 6V
Multi-Output, Low-Noise Power Supply
Controller for Notebook Computers
652 MITCHELL ROAD
ESRmax
ESRmax
ESRmin
ESRmin
ESR
ESR
ESR
Comin
Comin
Co
Co
Co
Step 2: Determine the Maximum & Minimum ESR
Capacitance
Step 3: Determine the Minimum Output Capacitance
Step 4: Determine the Actual Output Capacitance
Step 5: Determine the Actual ESR Value
Step 6: From Above Calculations Choose:
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.
ESRmax
233uF
ESRmin
ESRmax
233uF
0.022
0.022
0.018
2
ESRmax
0.026
162uF
Vout
Vref
1.2




2
2
Fc
ESRmin
Rs
Comin
Vout
Vref
NEWBURY PARK CA 91320
Rs
tan
30
0
SC1402
12