SP6122ACUSLASHTR datasheet(10/19 Pages) SIPEX

10 / 19 page

Rev. 7/16/03

SP6122 Low Voltage, Micro 8, PFET, Buck Controller

Guidelines for Component Selection

GENERAL

The SP6122 is a minimum on-time PFM

controller. This means there is no error amp

controlling the loop. Although an internal

algorithm adjusts the on-time approximate

the performance of a fixed frequency con-

troller, the loop control is generated by

looking at OUTPUT RIPPLE. The peak to

peak value of this output ripple must be no

less than 2% of the DC output voltage in

order to maintain reasonable fixed frequency

operation. In addition, as with all PFM con-

trollers, board layout is critical and careful

attention must be paid to minimize paths

that can generate noise. Fortunately, the

SP6122 is designed for simplicity and minimal

external components, making it easy to de-

sign small, quiet power converters up to 12W.

INDUCTOR SELECTION

In a SP6122 application, the main factors

for choosing an inductor are likely to be

cost, size, saturation current and efficiency.

If you use low inductor values, you get the

smallest size, but you may cause larger

ripple currents and poor efficiency and re-

quire more output capacitance to smooth

the output ripple. Increasing the inductor

value will decrease the output voltage ripple

but degrade the transient response. For a

good compromise between size, losses and

cost, set the inductor ripple current between

20% to 40% of the maximum output current.

The inductor operating point and switching

frequency determine the inductor value as

seen in the following expression:

L = (VOUT + VDIODE)*(VIN – VOUT)/

((VIN + VDIODE)*( FS KR IOUT(max)))

Where FS = switching frequency (see Soft

Start Frequency Specification)

KR = ratio of the ac inductor ripple current to

the maximum output current

VDIODE = forward Schottky diode voltage

For an application with 1.9V out, 4A maxi-

mum IOUT, 3.3V input supply, 400 mV typical

forward diode voltage, 300kHz frequency

and a 30% inductor ripple current, a 2.2µH

inductor was selected (see Table 1 SP6122

Component Selection).

The peak to peak inductor ripple current is:

IPP = (VOUT + VDIODE)*(VIN – VOUT)/

((VIN + VDIODE)*( FS L))

For that same 2.2µH inductor application,

the IPP = 1.32A.

The inductor must be selected to not satu-

rate the core at the peak inductor current:

IPEAK = IOUT(max) + IPP/2

Figure 3. SP6122 Load Regulation with Input

Voltage = 3.3V.

Figure 4. SP6122 Line Regulation with I

LOAD = 2A.

1.897

1.898

1.899

1.900

1.901

1.902

ILOAD (A)

1.899

1.900

1.901

1.902

1.903

1.904

1.905

ILOAD (A)

APPLICATION INFORMATION: Continued

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