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SSL2102 데이터시트(PDF) 5 Page - NXP Semiconductors |
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SSL2102 데이터시트(HTML) 5 Page - NXP Semiconductors |
5 / 21 page SSL2102_1 © NXP B.V. 2009. All rights reserved. Product data sheet Rev. 01 — 29 June 2009 5 of 21 NXP Semiconductors SSL2102 SMPS IC for dimmable LED lighting 8. Functional description The SSL2102 is a LED driver IC that operates directly from the rectified mains. The SSL2102 uses on-time mode control and frequency control to control the LED brightness. The BRIGHTNESS and PWMLIMIT input of the IC can be used to control the LED light output in combination with an external dimmer. The PWMLIMIT input can also be used for Thermal Lumen Management (TLM) and for precision LED current control. 8.1 Start-up and UnderVoltage LockOut (UVLO) Initially, the IC is self-supplying from the rectified mains voltage. The IC starts switching as soon as the voltage on pin VCC passes the VCC(startup) level. The supply can be taken over by the auxiliary winding of the transformer as soon as VCC is high enough and the supply from the line is stopped for high efficiency operation. Alternatively the IC can be supplied via a bleeder resistor connected to a high voltage. Remark: The maximum VCC voltage rating of the IC. If for some reason the auxiliary supply is not sufficient, the high-voltage supply can also supply the IC. As soon as the voltage on pin VCC drops below the VCC(UVLO) level, the IC stops switching and will restart from the rectified mains voltage, if the internal current delivered is sufficient. 8.2 Oscillator An internal oscillator inside the IC provides the timing for the switching converter logics. The frequency of the oscillator is set by the external resistors and the capacitor on pin RC and pin RC2. The external capacitor is charged rapidly to the VRC(max) level and, starting from a new primary stroke, it discharges to the VRC(min) level. Because the discharge is exponential, the relative sensitivity of the duty factor to the regulation voltage at low duty factor is almost equal to the sensitivity at high duty factors. This results in a more constant gain over the duty factor range compared to Pulse Width Modulated (PWM) systems with a linear sawtooth oscillator. Stable operation at low duty factors is easily realized. The frequency of the converter when VBRIGHTNESS is high can be estimated using Equation 1: (1) R equals the parallel resistance of both oscillator resistors. C is the capacitor connected at the RC pin (pin 8). The BRIGHTNESS input controls the frequency reduction mode. Figure 3 shows that the oscillator switches over from an RC curve with R1//R2 to R1 only. A low BRIGHTNESS voltage will reduce the switching frequency. RC 1 3.5 ------- 1 f osc ---------- t ch e arg – ⋅ = |
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