PM6680 datasheet(32/49 Pages) STMICROELECTRONICS

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Device description

PM6680

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7.13

Design guidelines

The design of a switching section starts from two parameters:

●

Input voltage range: in notebook applications it varies from the minimum battery

voltage, VINmin to the AC adapter voltage, VINmax.

●

Maximum load current: it is the maximum required output current, ILOAD(max).

7.13.1

Switching frequency

It's possible to set 3 different working frequency ranges for the two sections with FSEL pin

(Table 7 on page 30).

Switching frequency mainly influences two parameters:

●

Inductor size: for a given saturation current and RMS current, greater frequency allows

to use lower inductor values, which means smaller size.

●

Efficiency: switching losses are proportional to frequency. High frequency generally

involves low efficiency.

7.13.2

Inductor selection

Once that switching frequency is defined, inductor selection depends on the desired

inductor ripple current and load transient performance.

Low inductance means great ripple current and could generate great output noise. On the

other hand, low inductor values involve fast load transient response.

A good compromise between the transient response time, the efficiency, the cost and the

size is to choose the inductor value in order to maintain the inductor ripple current

∆I

between 20% and 50% of the maximum output current ILOAD(max). The maximum ∆IL

occurs at the maximum input voltage. With this considerations, the inductor value can be

calculated with the following relationship:

Equation 12

where fSW is the switching frequency, VIN is the input voltage, VOUT is the output voltage and

∆I

L is the selected inductor ripple current.

In order to prevent overtemperature working conditions, inductor must be able to provide an

RMS current greater than the maximum RMS inductor current ILRMS:

Equation 13

Where

∆I

L(max) is the maximum ripple current:

OUT

∆

−

(max))

(

(max))

(

LOAD

LRMS

∆

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