LED7706 Datasheet, PDF (17/31 Pages) STMicroelectronics – 6-rows 30 mA LEDs driver with boost regulator for LCD panels backlight

LED7706
Figure 9. External sync waveforms
Operation description - boost section
FSW pin voltage (ext. sync)
237000mV threshold
Slave SYNC pin voltage
227500ns minimum
Slave LX pin voltage
6.6
6.6.1
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
consists of a simple RC series (R_COMP - C_COMP) between the COMP pin and ground.
The calculation of R_COMP and C_COMP is fundamental to achieve optimal loop stability and
dynamic performance of the boost converter and is strictly related to the operating
conditions.
Loop compensation
The compensation network can be quickly calculated using equations 6 through 10. Once
both R_COMP and C_COMP have been determined, a fine-tuning phase may be required in
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).
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