AN2115 datasheet(12/33 Pages) STMICROELECTRONICS

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Operation description

AN2115

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Doc ID 11165 Rev 5

Since the device can be synchronized at a higher frequency, the inductor value can be

adjusted based on this. In fact, for a given current ripple, the required inductor value is

inversely proportional to the frequency. Finally, the input voltage affects the OFF time slope

as well. This is obvious because, for a given duty cycle, the output voltage (and thus the

OFF time inductor current slope) is directly proportional to the input voltage.

In order to better manage these issues, the amount of slope compensation in the L6928

depends both on the switching frequency and input voltage.

In the table above the minimum inductance values to ensure current loop stability with input

voltage of 3.3 V and 5 V are shown. There is also a maximum inductor value, because if the

inductor is too high the inductor current ripple will be very low (theoretically down to zero)

and will be compared with the slope compensation (a triangular waveform) to generate the

duty cycle. This system is similar to the voltage mode control causing stability problems due

to the LC double pole (the pole splitting effect will not be present).

4.3.2

Voltage loop compensation

After closing the current loop, the pole splitting effect will separate the complex double pole,

due to the inductor and the output capacitor, into 2 separate poles. The pole due to the

inductor will shift outside of the system bandwidth (i.e. the inductor ideally acts like a current

source), while the pole due to the output capacitor will remain within the bandwidth.

Figure 9 shows the equivalent circuit used to study the voltage loop compensation:

Table 2.

Minimum inductor value to ensure loop stability

Vin [V]

Vout [V]

FSW [kHz]

Minimum inductor

value [µH]

3.3

1.8

1000

1.0

2000

1.0

53.3

1000

2.2

2000

2.2

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