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HV9921N8-G Datasheet(PDF) 5 Page - Supertex, Inc |
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HV9921N8-G Datasheet(HTML) 5 Page - Supertex, Inc |
5 / 10 page 5 NR092005 HV9921/HV9922/HV9923 Functional Description The HV9921/22/23 are PWM peak current controllers for controlling a buck converter topology in continuous conduction mode (CCM). The output current is internally preset at 20mA (HV9921), 50mA (HV9922), or 30mA (HV9923). When the input voltage of 20 to 400V appears at the DRAIN pin, the internal high-voltage linear regulator seeks to maintain a voltage of 7VDC at the V DD pin. Until this voltage exceeds the internally programmed under-voltage threshold, the output switching MOSFET is non-conductive. When the threshold is exceeded, the MOSFET turns on. The input current begins to flow into the DRAIN pin. Hysteresis is provided in the under-voltage comparator to prevent oscillation. When the input current exceeds the internal preset level, a current sense comparator resets an RS flip-flop, and the MOSFET turns off. At the same time, a one-shot circuit is activated that determines the duration of the off-state (10.5 µS typ.). As soon as this time is over, the flip-flop sets again. The new switching cycle begins. A “blanking” delay of 300nS is provided that prevents false triggering of the current sense comparator due to the leading edge spike caused by circuit parasitics. Application Information The HV9921/22/23 is a low-cost off-line buck converter IC specifically designed for driving multi-LED strings. It can be operated from either universal AC line range of 85 to 264VAC, or 20 to 400VDC, and drives up to tens of high brightness LEDs. All LEDs can be run in series, and the HV9921/22/23 regulates at constant current, yielding uniform illumination. The HV9921/22/23 is compatible with triac dimmers. The output current is internally fixed at 20mA for the HV9921, 50mA for the HV9922, and 30mA for HV9923. These parts are available in space saving TO-92 and SOT-89 packages. Selecting L1 and D1 There is a certain trade-off to be considered between optimal sizing of the output inductor L1 and the tolerated output current ripple. The required value of L1 is inversely proportional to the ripple current ∆I O in it. (1) V O is the forward voltage of the LED string. TOFF is the off- time of the HV9921/22/23. The output current in the LED string (I O) is calculated then as: (2) where I TH is the current sense comparator threshold. The ripple current introduces a peak-to-average error in the output current setting that needs to be accounted for. Due to the constant off-time control technique used in the HV9921/22/23, the ripple current is independent of the input AC or DC line voltage variation. Therefore, the output current will remain unaffected by the varying input voltage. Adding a filter capacitor across the LED string can reduce the output current ripple even further, thus permitting a reduced value of L1. However, one must keep in mind that the peak-to-average current error is affected by the variation of T OFF. Therefore, the initial output current accuracy might be sacrificed at large ripple current in L1. Another important aspect of designing an LED driver with the HV9921/22/23 is related to certain parasitic elements of the circuit, including distributed coil capacitance of L1, junction capacitance and reverse recovery of the rectifier diode D1, capacitance of the printed circuit board traces C PCB and output capacitance C DRAIN of the controller itself. These parasitic elements affect the efficiency of the switching converter and could potentially cause false triggering of the current sense comparator if not properly managed. Minimizing these parasitics is essential for efficient and reliable operation of the HV9921/22/23. Coil capacitance of inductors is typically provided in the manufacturer’s data books either directly or in terms of the self-resonant frequency (SRF). where L is the inductance value, and C L is the coil capacitance.) Charging and discharging this capacitance every switching cycle causes high-current spikes in the LED string. Therefore, connecting a small capacitor C O (~10nF) is recommended to bypass these spikes. Using an ultra-fast rectifier diode for D1 is recommended to achieve high efficiency and reduce the risk of false triggering of the current sense comparator. Using diodes with shorter reverse recovery time t rr and lower junction capacitance C J achieves better performance. The reverse voltage rating V R of the diode must be greater than the maximum input voltage of the LED lamp. The total parasitic capacitance present at the DRAIN pin of the HV9921/22/23 can be calculated as: (3) When the switching MOSFET turns on, the capacitance C P is discharged into the DRAIN pin of the IC. The discharge current is limited to about 150mA typically. However, it may become lower at increased junction temperature. The duration of the leading edge current spike can be estimated /( ) L SRF 1 2 L C π =⋅ PDRAIN PCB L J CC C C C =+ + + OOFF O VT L1 I ∆ ⋅ = OTH O 1 II I 2 ∆ =− ⋅ |
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