SC1402(2001) View Datasheet(PDF) - Semtech Corporation
Part Name
Description
Manufacturer
SC1402
(Rev.:2001)
(Rev.:2001)
Semtech Corporation
SC1402 Datasheet PDF : 18 Pages
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SC1402
POWER MANAGEMENT
Application Information (Cont.)
From this calculation we choose L1 to be 10uH.
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 supply.
For a 200kHz operating frequency, L1 calculates to be 14.5uH. System Paramaters:
Fs = 300000Hz Vout = 3.3V
Choosing Inductor & FET Current Rating
Vref = 2.5V Rs = 0.020 Ω
Vinmin = 6V
The current carrying capability of the inductor and MOSFETs
should be sized to handle the maximum current of the supply
set by the current sense resistor, Rsense = R9. Here we use
the minimum current threshold value:
R9
=
80mV
IPEAK
Where Ipeak equals:
IPEAK
= IOUT
+
ΔIO
2
Ipeak = 3.5 Amps
R9
=
0.080
3.5
= 0.023 Ω
Here we chose a sense resistor of 20mΩ. Now we can
determine FET and inductor current carrying capability by using
the maximum current limit threshold:
IPEAK
=
0.120
R9
=
0.120
0.020
= 6 Amps
From this value choose an inductor with Isat > 6 Amps, and for
the FET choose a continuous conduction current 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 operation. 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 important to choose a capacitor with its ESR for
stable SMPS operation. A seven step procedure for choosing
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
1+
Vout
Vinmin
Fc = 64.516KHz
Step 2: Determine the Maximum & Minimum ESR
Capacitance
ESRmax
=
Vout
Vref
Rs
ESRmax= 0.026Ω
ESRm
in=
ESRm
(1.2)2
ax
ESRm in= 0.018Ω
Step 3: Determine the Minimum Output Capacitance
( ) Comin = 2
π
Fc
Vref
Vout Rs
tan 30°
Comin = 162uF
Step 4: Determine the Actual Output Capacitance
Co
=
ESRm ax
ESRm in
Comin
Co = 233uF
Step 5: Determine the Actual ESR Value
ESR
=
ESRm
ax+
2
ESRm
in
ESR = 0.022 Ω
2001 Semtech Corp.
13
www.semtech.com
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