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LTC3721EGN-1 Datasheet(PDF) 10 Page - Linear Technology |
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LTC3721EGN-1 Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  LTC3721-1 10 sn37211 37211fs OPERATIO VCC should be bypassed with a 0.1µF to 1µF multilayer ceramic capacitor to decouple the fast transient currents demanded by the output drivers and a bulk tantalum or electrolytic capacitor to hold up the VCC supply before the bootstrap winding, or an auxiliary regulator circuit takes over. CHOLDUP = (ICC + IDRIVE) • tDELAY/3.8V (minimum UVLO hysteresis) Regulated bias supplies as low as 7V can be utilized to provide bias to the LTC3721-1. Refer to Figure 2 for various bias supply configurations. Programming Undervoltage Lockout The LTC3721-1 provides undervoltage lockout (UVLO) control for the input DC voltage feed to the power con- verter in addition to the VCCUVLO function described in the preceding section. Input DC feed UVLO is provided with the UVLO pin. A comparator on UVLO compares a divided down input DC feed voltage to the 5V precision reference. When the 5V level is exceeded on UVLO, the SS pin is released and output switching commences. At the same time a 10µA current is enabled which flows out of UVLO into the voltage divider connected to UVLO. The amount of DC feed hysteresis provided by this current is: 10µA • RTOP, (Figure 3). The system UVLO threshold is: 5V • {(RTOP + RBOTTOM)/RBOTTOM}. If the voltage applied to UVLO is present and greater than 5V prior to the VCC UVLO circuitry activation, then the internal UVLO logic will prevent output switching until the following three condi- tions are met: (1) VCC UVLO is enabled, (2) VREF is in regulation and (3) UVLO pin is greater than 5V. UVLO can also be used to enable and disable the power converter. An open drain transistor connected to UVLO as shown in Figure 3 provides this capability. Off-Line Bias Supply Generation If a regulated bias supply is not available to provide VCC voltage to the LTC3721-1 and supporting circuitry, one must be generated. Since the power requirement is small, approximately 1W, and the regulation is not critical, a simple open-loop method is usually the easiest and lowest cost approach. One method that works well is to add a winding to the main power transformer, and post regulate the resultant square wave with an L-C filter (see Figure 4a). The advantage of this approach is that it maintains decent regulation as the supply voltage varies, and it does not require full safety isolation from the input winding of the transformer. Some manufacturers include a primary wind- ing for this purpose in their standard product offerings as well. A different approach is to add a winding to the output inductor and peak detect and filter the square wave signal (see Figure 4b). The polarity of this winding is designed so Figure 2. Bias Configurations 37211 F02 12V ±10% 1.5k VCC VIN VCC CHOLD 1N5226 3V 1µF 1µF VBIAS < VUVLO RSTART 1N914 + Figure 3. System UVLO Setup ON OFF RBOTTOM RTOP UVLO 37211 F03 Figure 4a. Auxiliary Winding Bias Supply Figure 4b. Output Inductor Bias Supply 19211 F04a + VCC VIN CHOLD 1µF RSTART 2k 15V* *OPTIONAL 19211 F04b VCC VOUT VIN CHOLD RSTART + 1µF LOUT ISO BARRIER |
Similar Part No. - LTC3721EGN-1 |
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Similar Description - LTC3721EGN-1 |
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