AL1665 datasheet(13/17 Pages) DIODES

13 / 17 page

AL1665

Document number: DS41772 Rev. 3 - 2

13 of 17

www.diodes.com

March 2020

AL1665

100

PWM Duty (%)

PWM Frequency=1kHz

Application Information (continued)

2. PWM Dimming Mode

In PWM dimming mode, the dimming signal is added to NTC/PWM pin. Figure 8 shows the setting circuit. The output current is chopped by the

dimming signal directly. The logic high level of the dimming signal must be higher than 1V while the logic low level is lower than 0.5V. The switch

turns off at logic low level. Figure 9 shows the dimming curve. The dimming range can be 100% to 0.5% with 1KHz frequency of PWM signal.

NTC

/PWM

AL1665

PWM Dimming Signal

Figure 8. PWM Dimming Setting Circuit

Figure 9. PWM Dimming Curve

Operation Parameters Design

1. Setting the Current Sense Resistor RCS

The current sense resistance is calculated using the following equation:

REF

O MEAN







Where:



IO_MEAN is the mean output current



RCS is the current sense resister, which is shown as Figure 1



VREF is the internal reference voltage that is equal to 0.4V



NPS is the turns’ ratio of flyback transformer (NPS=1 for buck-boost)

2. Setting Transformer Selection (T1)

NPS is limited by the electrical stress of the switch MOSFET and can be calculated with the following formula:

)

90%

MOS

BR DS

MAX

D F







 





Where:



VMOS_(BR)DS is the breakdown voltage of the switch MOSFET



VIN_MAX is the max rated input voltage



∆VS is the overshoot voltage clamped by RCD snobbier during OFF time



VO is the output voltage



VD_F is the forward voltage of secondary diode



NPS is the turns’ ratio of flyback transformer (NPS=1 for buck-boost)

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