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Número de pieza	LR745
Descripción	High Input Voltage SMPS Start-up
Fabricantes	Supertex Inc
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No Preview Available ! LR745 High Input Voltage SMPS Start-up Features ► Accepts inputs from 35V to 450V ► Output current limiting ► For PWM ICs with start-up threshold voltage of 13.9 - 18.0V ► Very low power consumption after start-up Applications ► Notebook and laptop computers ► Telecommunication power supplies ► Battery chargers ► Motor controllers General Description The Supertex LR745 is a high input voltage SMPS start- up circuit. The LR745 is ideally suited for use with industry standard low voltage PWM ICs having start thresholds of 13.9 - 18.0V. It allows the PWM ICs to be operated from rectiﬁed 120 or 240VAC lines, and eliminates the use of power resistors often used for this purpose. The internal circuitry of the LR745 allows the PWM ICs to operate at a VCC voltage below their start threshold voltage after start-up. The auxiliary voltage can be less than the start threshold voltage, which allows for improved efﬁciency. Current from the high voltage line is drawn only during the start-up period. After start-up, the internal high voltage line is disconnected from the IC, thereby reducing the continuous power dissipation to a minimum. Ordering Information Device Package Options TO-92 TO-243AA (SOT-89) LR745 LR745N3-G -G indicates package is RoHS compliant (‘Green’) LR745N8-G Pin Conﬁgurations VOUT VIN GND TO-92 (N3) GND VOUT GND VIN TO-243AA (SOT-89) (N8) Package Markings Absolute Maximum Ratings Parameter Value Input voltage 450V Output voltage 25V Operating and storage temperature -55°C to +150OC Soldering temperature* 300OC Absolute Maximum Ratings are those values beyond which damage to the device may occur. Functional operation under these conditions is not implied. Continuous operation of the device at the absolute rating level may affect device reliability. All voltages are referenced to device ground. * Distance of 1.6mm from case for 10 seconds. SiLR 745 YWLL Y = Last Digit of Year Sealed W = Code for Week Sealed L = Lot Number = “Green” Packaging Package may or may not include the following marks: Si or TO-92 (N3) LR7W W = Code for Week Sealed = “Green” Packaging Package may or may not include the following marks: Si or TO-243AA (SOT-89) (N8) ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com 1 page LR745 Stage III At this stage the LR745’s output is turned off and the PWM IC is operating from the VAUX supply. The auxiliary voltage, VAUX, can be designed to vary anywhere between the mini- mum operating VCC voltage of the PWM IC (10V) to the maxi- mum auxiliary voltage rating of the LR745 (22V). Design Considerations Consider for example, a PWM IC with a switching frequency of 100KHz, operating current of 20mA, start threshold of 16V, and a minimum operating voltage of 10V. If 100 clock cycles are required to charge the auxiliary voltage to 10V, the minimum value of C1 is calculated as follows: 1 100KHz x (100) x (20mA) C1 = (16V -10V) I. Calculating the value for C1 Sizing the VCC capacitor, C1, is an important factor. Making C1 too large will cause the SMPS to power up too slowly. However, if too small, C1 will not allow the SMPS to power up due to insufﬁcient charge in the capacitor to power the IC and MOSFET until the auxiliary supply is available. The value of C1 can be approximated by the following equation: 1 f x (N) x (l) C1 = (VSTART - VMIN) where, f = switching frequency N = number of clock cycles required to chargeVAUX to VMIN value I = PWM operating current VSTART = PWM IC start threshold rating VMIN = PWM IC minimum VCC operating voltage C1 = 3.3µF II. SMPS with wide minimum to maximum load An important point is that the LR745’s output voltage, VOUT, must discharge to below the nominal VOFF trip point of 13.25V in order for its output to turn off. If the SMPS requires a wide minimum to maximum output load variation, it will be difﬁcult to guarantee that VCC will fall below 13.25V under minimum load conditions. Consider an SMPS that is required to power small as well as large loads and is also required to power up quickly. Such a SMPS may power up too fast with a small load, not allowing the VCC voltage to fall below 13.25V. For such conditions, the circuit in Figure 3 is recommended. In Figure 3, the VREF pin of the UC3844 is used to bias the ground pin of the LR745. The VREF pin on the UC3844 is a 5.0V reference, which stays at 0V until the VCC voltage reaches the start threshold voltage. Once VCC reaches the start threshold voltage, VREF will switch digitally from 0V to 5.0V. During start-up, the LR745 will be on, and VCC will start to increase up to 16V. Once VCC reaches16V, the UC3844 will start to operate and VREF will increase from 0V to 5.0V. The LR745 will see an effective VOUT voltage of 11V (16V mi- nus 5.0V) because the ground of the LR745 is now at 5.0V. The LR745 will immediately turn off its output, VOUT, with- out having to wait for the VCC voltage to decrease. The VREF switching from 0 to 5.0V during start is a common feature in most PWM ICs. Figure 3: Using VREF for the LR745 Ground Voltage VIN LR7 GND VOUT VCC VREF C1 PWM IC UC3844 ● 1235 Bordeaux Drive, Sunnyvale, CA 94089 ● Tel: 408-222-8888 ● www.supertex.com 5 5 Page

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