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MAX16801A Datasheet(PDF) 10 Page - Maxim Integrated Products |
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MAX16801A Datasheet(HTML) 10 Page - Maxim Integrated Products |
10 / 15 page Off-Line and DC-DC PWM Controllers for High-Brightness LED Drivers 10 ______________________________________________________________________________________ Applications Information Startup Time Considerations for High- Brightness LED Drivers Using MAX16801 The IN bypass capacitor C1 supplies current immedi- ately after wake-up (Figure 5). The size of C1 and the connection configuration of the tertiary winding deter- mine the number of cycles available for startup. Large values of C1 increase the startup time but also supply gate charge for more cycles during initial startup. If the value of C1 is too small, VIN drops below +9.7V because NDRV does not have enough time to switch and build up sufficient voltage across the tertiary wind- ing that powers the device. The device goes back into UVLO and does not start. Use low-leakage capacitors for C1 and C2. Assuming that off-line LED drivers keep typical startup times to less than 500ms even in low-line conditions (85VAC input for universal off-line applications), size the startup resistor R1 to supply both the maximum startup bias of the device (90µA worst case) and the charging current for C1 and C2. The bypass capacitor C2 must charge to +9.5V and C1 to +24V, all within the desired time period of 500ms. Because of the internal 60ms soft-start time of the MAX16801, C1 must store enough charge to deliver current to the device for at least this much time. To cal- culate the approximate amount of capacitance required, use the following formula: where IIN is the MAX16801’s internal supply current after startup (1.4mA), Qgtot is the total gate charge for Q1, fSW is the MAX16801’s switching frequency (262kHz), Vhyst is the bootstrap UVLO hysteresis (11.9V) and tSS is the internal soft-start time (60ms). For example: choose the 15µF standard value. Assuming C1>C2, calculate the value of R1 as follows: where VIN(MIN) is the minimum input supply voltage for the application, VSUVR is the bootstrap UVLO wake-up level (+23.6V max), and ISTART is the IN supply current at startup (90µA, max). For example, for minimum AC input of 85V: Choose the 150k Ω standard value. Choose a higher value for R1 than the one calculated above if longer startup time could be tolerated in order to minimize power loss on this resistor. The above startup method is applicable to a circuit sim- ilar to the one shown in Figure 5. In this circuit, the ter- tiary winding has the same phase as the output windings. Thus, the voltage on the tertiary winding at any given time is proportional to the output voltage and goes through the same soft-start period as the output voltage. The minimum discharge voltage of C1 from +22V to +10V must be greater than the soft-start time of 60ms. Another method of bootstrapping the circuit is to have a separate bias winding than the one used for regulating the output voltage and to connect the bias winding so that it is in phase with the MOSFET ON time (see Figure 9). In this case, the amount of capacitance required is much smaller. However, in this mode, the input voltage range has to be less than 2:1. Another consideration is whether the bias winding is in phase with the output. If so, the LED driver circuit hiccups and soft-starts under output short- circuit conditions. However, this property is lost if the bias winding is in phase with the MOSFET ON time. I VV ms mA R VF mA A k C1 24 12 500 072 1 120 15 072 90 133 4 = () − () () = = ()×µ () +µ () =Ω . . ( ) . I VC ms R VV II C SUVR IN MIN SUVR C START 1 1 1 500 1 = × () = − + () Ig nC kHz mA C mA mA ms V F () ( ) . .. . =× = = + ()×() () =µ 8 262 2 1 1 14 21 60 12 17 5 IQ f C II t V ggtotSW IN g SS hyst =× = + () () 1 |
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