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LM5041AMTC Datasheet(PDF) 10 Page - National Semiconductor (TI) |
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LM5041AMTC Datasheet(HTML) 10 Page - National Semiconductor (TI) |
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10 / 16 page  Detailed Operating Description The LM5041A PWM controller contains all of the features necessary to implement either current-fed or voltage-fed push-pull or bridge power converters. These “Cascaded” topologies are well suited for multiple output and higher power applications. The LM5041A’s four control outputs in- clude: the buck stage controls (HD and LD) and the push- pull control outputs (PUSH and PULL). Push-pull outputs are driven at 50% nominal duty cycle at one half of the switching frequency of the buck stage and can be configured for either a guaranteed overlap time (for current-fed applications) or a guaranteed both-off time (for voltage-fed applications). Push-pull stage MOSFETs can be driven directly from the internal gate drivers while the buck stage requires an exter- nal driver such as the LM5102. The LM5041A includes a high-voltage start-up regulator that operates over a wide input range of 15V to 100V. The PWM controller is designed for high-speed capability including an oscillator frequency range up to 1 MHz and total propagation delays of less than 100ns. Additional features include: line Under-Voltage Lock- out (UVLO), soft-start, an error amplifier, precision voltage reference, and thermal shutdown. High Voltage Start-Up Regulator The LM5041A contains an internal high-voltage start-up regulator, thus the input pin (Vin) can be connected directly to the line voltage. The regulator output is internally current limited to 15mA. When power is applied, the regulator is enabled and sources current into an external capacitor con- nected to the Vcc pin. The recommended capacitance range for the Vcc regulator is 0.1uF to 100uF. When the voltage on the Vcc pin reaches the regulation point of 9V and the internal voltage reference (REF) reaches its regulation point of 5V, the controller outputs are enabled. The Buck stage outputs will remain enabled until Vcc falls below 7V or the line Under-Voltage Lockout detector indicates that Vin is out of range. The push-pull outputs continue switching until the REF pin voltage falls below approximately 3V. In typical applications, an auxiliary transformer winding is connected through a diode to the Vcc pin. This winding must raise the Vcc voltage above 9.3V to shut off the internal start-up regulator. Powering V CC from an auxiliary winding improves efficiency while reducing the controller’s power dissipation. The recommended capacitance range for the Vref regulator output is 0.1uF to 10uF. The external V CC capacitor must be sized such that the capacitor maintains a V CC voltage greater than 7V during the initial start-up. During a fault mode when the converter aux- iliary winding is inactive, external current draw on the V CC line should be limited so the power dissipated in the start-up regulator does not exceed the maximum power dissipation of the controller. An external start-up or other bias rail can be used instead of the internal start-up regulator by connecting the V CC and the V IN pins together and feeding the external bias voltage into the two pins. Line Under-Voltage Detector The LM5041A contains a line Under-Voltage Lockout (UVLO) circuit. An external set-point resistor divider from V IN to ground sets the operational range of the converter. The divider must be designed such that the voltage at the UVLO pin will be greater than 2.5V when V IN is in the desired operating range. If the Under-Voltage threshold is not met, all functions of the controller are disabled and the controller will enter a low-power state with input current <300µA. ULVO hysteresis is accomplished with an internal 20µA cur- rent source that is switched on or off into the impedance of the set-point divider. When the UVLO threshold is exceeded, the current source is activated to instantly raise the voltage at the UVLO pin. When the UVLO pin falls below the 2.5V threshold, the current source is turned off causing the volt- age at the UVLO pin to fall. The UVLO pin can also be used to implement a remote enable / disable function. By shorting the UVLO pin to ground, the converter can be disabled. Buck Stage Control Outputs The LM5041A Buck switch maximum duty cycle clamp en- sures that there will be sufficient off time each cycle to recharge the bootstrap capacitor used in the high side gate driver. The Buck switch is guaranteed to be off, and the sync switch on, for at least 250ns per switching cycle. The Buck stage control outputs (LD and HD) are CMOS buffers with logic levels of 0 to 5V. During any fault state or Under-Voltage off state, the buck stage control outputs will default to HD low and LD high. Push-Pull Outputs The push pull outputs operate continuously at a nominal 50% duty cycle. A distinguishing feature of the LM5041A is the ability to accurately configure either dead time (both-off) or overlap time (both-on) on the complementary push-pull outputs. The overlap/dead time magnitude is controlled by a resistor connected to the TIME pin on the controller. The TIME pin holds one end of the resistor at 2.5V and the other end of the resistor should be connected to either REF for dead time control setting or to GND for overlap control. The polarity of the current in the TIME is detected by the LM5041A The magnitude of the overlap/dead time can be calculated as follows: Overlap Time (ns) = (3.66 x R SET)+7 Overlap Time in ns, R SET connected to GND, RSET in k Ω Dead Time (ns) = (3.69 x R SET)+21 Dead Time in ns, R SET connected to REF, RSET in k Ω Recommended R SET programming range: 10k Ω to 100kΩ Current-fed designs require a period of overlap to insure there is a continuous path for the buck inductor current. Voltage-fed designs require a period of dead time to insure there is no time when the push-pull transformer acts as a shorted turn to the low impedance sourcing node. The push- pull outputs alternate continuously under all conditions pro- vided REF the voltage is greater than 3V. www.national.com 10 |
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