LED7707 datasheet(27/48 Pages) STMICROELECTRONICS

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LED7707

Application information

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Equation 13b

Where VIN,min is the minimum input voltage and IOUT is the overall output current.

Note that, the lower the inductor value (and the higher the switching frequency), the higher

the bandwidth can be achieved. The output capacitor is directly involved in the loop of the

boost converter and must be large enough to avoid excessive output voltage drop in case of

a sudden line transition from the maximum to the minimum input voltages.

However a more significant requirement concerns the output voltage ripple.

The output capacitor should be chosen in accordance with the following expression:

Equation 14

where

ΔV

OUT, max is the maximum acceptable output voltage ripple, IL, peak is the peak

inductor current, TOFF is the off-time of the switching cycle (for an extensive explanation see

Section 6.4.4 on page 34

Once the output capacitor has been chosen, the RCOMP can be calculated as:

Equation 15

Where GM = 2.7 S and gEA = 375 µS

Equation 15 places the loop bandwidth at fU. Then, the CCOMP capacitor is determined to

place the frequency of the compensation zero 5 times lower than the loop bandwidth:

Equation 16

Where fZ = fU/5.

In most of the applications an experimental approach is also very valid to compensate the

circuit. A simple technique to optimize different applications is to choose CCOMP = 4.7 nF

and to replace RCOMP with a 10 kΩ trimmer adjusting its value to properly damp the output

transient response. Insufficient damping will result in excessive ringing at the output and

poor phase margin.

Figure 18

(a and b) give an example of compensation adjustment for a typical application.

OUT

(

)

max

OUT

OFF

OUT

peak

OUT

⋅

−

COMP

⋅

COMP

⋅

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