CS5132 datasheet(12/19 Pages) CHERRY

12 / 19 page

than ±165mV. Repeating step 2a, we select four (4)

1200µF/10V Sanyo GX output capacitors.

Step 3: Duty Cycle, Switching Frequency, TON & TOFF

Duty Cycle

» VOUT / VIN.

D = 2.0V / 5V = 40% for 2V output.

D = 3.3V / 5V = 66% for 3.3V output.

Select 200kHz Switching Frequency (FSW).

Step 3a: Calculate On-Time for 2V Output

TON =

= 2µs

Calculate Off-Time:

TOFF =

= 5µs - 2µs = 3µs.

Select the COFF1 capacitor in order to set the Off-Time:

COFF1 =

= 750pF.

A standard COFF1 capacitance value of 680pF can be used.

The 3980 factor is a characteristic of the CS5132.

Step 3b: Calculate On-Time for 3.3V Output

TON =

= 3.3µs

Calculate Off-Time:

TOFF =

- TON = 5µs Ð 3.3µs = 1.7µs.

Select COFF2 to be 390pF.

Step 4: Output Inductor

The inductor should be selected based on its inductance,

current capability, and DC resistance. Increasing the induc-

tor value will decrease output voltage ripple, but degrade

transient response. There are many factors to consider in

selecting the inductor including: cost, efficiency, EMI and

ease of manufacture. The inductor must be able to handle

the peak current at the switching frequency without satu-

rating, and the copper resistance in the winding should be

kept as low as possible to minimize resistive power loss.

There are a variety of materials and types of magnetic

cores that could be used for this application. Among them

are: ferrites, molypermalloy cores (MPP), amorphous and

powdered iron cores. We will use a powdered iron core.

Iron powdered cores are very suitable due to their high sat-

uration flux density and have low loss at high frequencies,

a distributed gap and exhibit very low EMI.

Calculate Inductor Value:

=1.2µH.

Step 4a: Select 2% Ripple on 2V Output

DVOUT = 2% ´ 2V = 40mV

The maximum allowable Inductor Ripple Current for a 2%

ripple on the 2V output is:

DIL=

= 7.3A,

which corresponds to the following maximum Inductor

Peak and Valley currents:

IL(PEAK) = IOUT +

()= 16A + () = 19.6A,

IL(VALLEY) = IOUT -

() = 16A - () = 12.4A.

The selected 1.2µH inductor yields the following ripple

current:

DIL=

= 5A.

The maximum inductor peak current becomes:

IL(PEAK) = 16A +

= 16A + 2.5A = 18.5A.

The inductor valley current becomes:

IL(VALLEY) = 16A -

= 16A - 2.5A = 13.5A.

The above values are well within the maximum allowable

inductor peak and valley currents for a 2% output voltage

ripple.

Select Toroid Powdered Iron Core, low cost, low core loss-

es at 200kHz, low EMI.

Select XFMRS Inc, XF0016-VO4 1.2µH inductor with RDC =

0.003½ typical, 0.008½ maximum.

Step 4b: Select 2% Ripple on 3.3V Output

Repeating Step 4a for the 3.3V output, we find 3.5µH is a

suitable value for this output.

Step 5: Input Capacitors

These components must be selected and placed carefully to

yield optimal results. Capacitors should be chosen to pro-

vide acceptable ripple on the input supply lines. Key speci-

fications for input capacitors are their ripple rating.

Step 5a: VCC(CORE) Buck Regulator Input Capacitors

The input capacitor CIN should also be able to handle the

(5V - 2V)

´ 0.4

200kHz

´ 1.2µH

(VIN - VOUT) ´ D

FSW ´ L

7.3A

DIL

7.3A

DIL

40mV

5.5m½

DVOUT

Total ESR

´ 6µs

15A

(5V-2V)

´ 6µs

15A

(VIN - VOUT) tTR

FSW

0.66

200kHz

FSW

5µs

´ 0.6

3980

Period

´ (1-D)

3980

- TON

FSW

0.40

200kHz

FSW

Application Information: continued

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