LED7706

Operation description - boost section

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Figure 9.

External sync waveforms

6.6

System stability

The boost section of the LED7706 is a Fixed Frequency, Current-Mode converter. During

normal operation, a minimum voltage selection circuit compares all the voltage drops across

the active current generators and provides the minimum one to the error amplifier. The

output voltage of the error amplifier determines the inductor peak current in order to keep its

inverting input equal to the reference voltage (270 mV typ). The compensation network

dynamic performance of the boost converter and is strictly related to the operating

conditions.

6.6.1

Loop compensation

The compensation network can be quickly calculated using equations 6 through 10. Once

order to get the optimal dynamic performance from the application.

The first parameter to be fixed is the switching frequency. Normally, a high switching

frequency allows reducing the size of the inductor but increases the switching losses and

negatively affects the dynamic response of the converter. For most of applications, the fixed

value (660 kHz) represents a good trade-off between power dissipation and dynamic

response, allowing to save an external resistor at the same time. In low-profile applications,

the inductor value is often kept low to reduce the number of turns; an inductor value in the

4.7 µH-15 µH range is a good starting choice.

Even if the loop bandwidth of the boost converter should be chosen as large as possible, it

should be set to 20 % of the switching frequency, taking care not to exceed the CCM-mode

Right Half-Plane Zero (RHPZ).

Slave SYNC pin voltage

300mV threshold

FSW pin voltage (ext. sync)

Slave LX pin voltage

250ns minimum

270

270