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MAX16801B Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX16801B Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 15 page in applications with higher input DC voltages by imple- menting resistor-Zener bias (Figure 2a) or transistor- Zener-resistor bias (Figure 2b). MAX16801/MAX16802 Undervoltage Lockout The MAX16801/MAX16802 have an input voltage UVLO/EN pin. The threshold of this UVLO is +1.28V. Before any operation can commence, the voltage on this pin has to exceed +1.28V. The UVLO circuit keeps the CPWM comparator, ILIM comparator, oscillator, and output driver in shutdown to reduce current con- sumption (see the Functional Diagram). Use this UVLO function to program the input start voltage. Calculate the divider resistor values, R2 and R3 (Figure 5), by using the following formulas: The value of R3 is calculated to minimize the voltage- drop error across R2 as a result of the input bias cur- rent of the UVLO/EN pin. VULR2 = +1.28V, IUVLO = 50nA (max), VIN is the value of the input-supply voltage where the power supply must start. where IUVLO is the UVLO/EN pin input current, and VULR2 is the UVLO/EN wake-up threshold. MAX16801 Bootstrap Undervoltage Lockout In addition to the externally programmable UVLO func- tion offered in both the MAX16801/MAX16802, the MAX16801 has an additional internal bootstrap UVLO that is very useful when designing high-voltage LED drivers (see the Functional Diagram). This allows the device to bootstrap itself during initial power-up. The MAX16801 attempts to start when VIN exceeds the bootstrap UVLO threshold of +23.6V. During startup, the UVLO circuit keeps the CPWM comparator, ILIM comparator, oscillator, and output driver shut down to reduce current consumption. Once VIN reaches +23.6V, the UVLO circuit turns on both the CPWM and ILIM comparators, as well as the oscillator, and allows the output driver to switch. If VIN drops below +9.7V, the UVLO circuit will shut down the CPWM comparator, ILIM comparator, oscillator, and output driver thereby returning the MAX16801 to the startup mode. MAX16801 Startup Operation In isolated LED driver applications, VIN can be derived from a tertiary winding of a transformer. However, at startup there is no energy delivered through the trans- former. Therefore, a special bootstrap sequence is required. Figure 3 shows the voltages on IN and VCC during startup. Initially, both VIN and VCC are 0V. After the line voltage is applied, C1 charges through the startup resistor R1 to an intermediate voltage. At this point, the internal regulator begins charging C2 (see Figure 5). The MAX16801 uses only 45µA of the current supplied by R1, and the remaining input current charges C1 and C2. The charging of C2 stops when the VCC voltage reaches approximately +9.5V, while the voltage across C1 continues rising until it reaches R VV V R IN ULR ULR 23 2 2 =× − R VV IV V ULR IN UVLO IN ULR 3 500 2 2 ≅ × × () − Off-Line and DC-DC PWM Controllers for High-Brightness LED Drivers 8 _______________________________________________________________________________________ IN VDC R D (a) MAX16802A IN VDC Q R DC (b) MAX16802A Figure 2. (a) Resistor-Zener and (b) Transistor-Zener-Resistor Bias Arrangements |
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