SC1402 datasheet(12/18 Pages) SEMTECH

12 / 18 page

SC1402

652 MITCHELL ROAD

NEWBURY PARK CA 91320

Multi-Output, Low-Noise Power Supply

Controller for Notebook Computers

December 15, 2000

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

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

Vinmin = 6V

Vref = 2.5V

Rs = 0.020

Ω

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

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.

•

Vref

Vout

ESRmax

()2

1.2

ESRmax

ESRmin

0.026Ω

ESRmax

Ω

0.018

ESRmin

()0

tan

Vout

Vref

Comin

•

162uF

Comin

ESRmin

ESRmax

•

233uF

≥

Ω

0.022

ESR

0.020

0.120

PEAK

æ +

•

Vinmin

Vout

KHz

64.516

ESRmin

ESRmax

ESR

Ω

0.022

ESR

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