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LT1932 Datasheet(PDF) 8 Page - Linear Technology |
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LT1932 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page 8 LT1932 1932f Dimming Using a Filtered PWM Signal While the direct PWM method provides the widest dim- ming range and the purest white light output, it causes the LT1932 to enter into Burst Mode ® operation. This opera- tion may be undesirable for some systems, as it may reflect some noise to the input source at the PWM fre- quency. The solution is to filter the control signal by adding a 10k resistor and a 0.1 µF capacitor as shown in Figure 6, converting the PWM to a DC level before it reaches the RSET pin. The 10k resistor minimizes the capacitance seen by the RSET pin. Dimming Using a Logic Signal For applications that need to adjust the LED brightness in discrete steps, a logic signal can be used as shown in Figure 6. RMIN sets the minimum LED current value (when the NMOS is off): R V I MIN LED MIN = 225 01 • . () RINCR sets how much the LED current is increased when the NMOS is turned on: R V I INCR LED INCREASE = 225 01 • . () Dimming Using a DC Voltage For some applications, the preferred method of brightness control uses a variable DC voltage to adjust the LED current. As the DC voltage is increased, current flows through RADJ into RSET, reducing the current flowing out APPLICATIO S I FOR ATIO Figure 6. Five Methods of LED Dimming of the RSET pin, thus reducing the LED current. Choose the RADJ value as shown below where VMAX is the maximum DC control voltage, ILED(MAX) is the current programmed by RSET, and ILED(MIN) is the minimum value of ILED (when the DC control voltage is at VMAX). R VV II ADJ MAX LED MAX LED MIN = 225 01 • –. – () ( ) Regulating LED Current when VIN > VOUT The LT1932 contains special circuitry that enables it to regulate the LED current even when the input voltage is higher than the output voltage. When VIN is less than VOUT, the internal NPN LED switch (transistor Q2 in Figure 1) is saturated to provide a lower power loss. When VIN is greater than VOUT, the NPN LED switch comes out of saturation to keep the LED current in regulation. Soft-Start/Controlling Inrush Current For many applications, it is necessary to minimize the inrush current at start-up. When first turned on and the LED current is zero, the LT1932 will initially command the maximum switch current of 500mA to 600mA, which may give an inrush current too high for some applications. A soft-start circuit (Figure 7) can be added to significantly reduce the start-up current spike. Figure 8 shows that without soft-start the input current reaches almost 600mA. Figure 9 shows that when the soft-start circuit is added, the input current has only a brief 300mA spike, and on average does not exceed 100mA. LT1932 RSET VDC RSET 4 1932 F06 RADJ LT1932 RSET PWM LT1932 PWM FILTERED PWM DC VOLTAGE LOGIC RSET PWM RSET 4 RPWM LT1932 PWM SHDN PWM 5 RSET 0.1 µF 4 10k RPWM LT1932 RSET RMIN LOGIC SIGNAL 4 RINCR Burst Mode is a registered trademark of Linear Technology Corporation. |
Similar Part No. - LT1932_15 |
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Similar Description - LT1932_15 |
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