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LM5025AMTCX Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM5025AMTCX Datasheet(HTML) 11 Page - National Semiconductor (TI) |
11 / 16 page PWM Outputs (Continued) Compound Gate Drivers The LM5025A contains two unique compound gate drivers, which parallel both MOS and Bipolar devices to provide high drive current throughout the entire switching event. The Bi- polar device provides most of the drive current capability and provides a relatively constant sink current which is ideal for driving large power MOSFETs. As the switching event nears conclusion and the Bipolar device saturates, the internal MOS device continues to provide a low impedance to com- pete the switching event. During turn-off at the Miller plateau region, typically around 2V - 3V, is where gate driver current capability is needed most. The resistive characteristics of all MOS gate drivers are adequate for turn-on since the supply to output voltage differential is fairly large at the Miller region. During turn-off however, the voltage differential is small and the current source characteristic of the Bipolar gate driver is beneficial to provide fast drive capability. 20107413 PWM Comparator The PWM comparator compares the ramp signal (RAMP) to the loop error signal (COMP). This comparator is optimized for speed in order to achieve minimum controllable duty cycles. The internal 5k Ω pull-up resistor, connected between the internal 5V reference and COMP, can be used as the pull-up for an optocoupler. The comparator polarity is such that 0V on the COMP pin will produce a zero duty cycle on both gate driver outputs. Volt Second Clamp The Volt x Second Clamp comparator compares the ramp signal (RAMP) to a fixed 2.5V reference. By proper selection of RFF and CFF, the maximum ON time of the main switch can be set to the desired duration. The ON time set by Volt x Second Clamp varies inversely with the line voltage be- cause the RAMP capacitor is charged by a resistor con- nected to Vin while the threshold of the clamp is a fixed voltage (2.5V). An example will illustrate the use of the Volt x Second Clamp comparator to achieve a 50% duty cycle limit, at 200KHz, at a 48V line input: A 50% duty cycle at a 200KHz requires a 2.5µs of ON time. At 48V input the Volt x Second product is 120V x µs (48V x 2.5µs). To achieve this clamp level: R FF xCFF =VIN xTON / 2.5V 48 x 2.5µ / 2.5 = 48µ Select C FF = 470pF R FF = 102k Ω The recommended capacitor value range for CFF is 100pF to 1000pF. The C FF ramp capacitor is discharged at the conclusion of every cycle by an internal discharge switch controlled by either the internal clock or by theVxS Clamp comparator, whichever event occurs first. Current Limit The LM5025A contains two modes of over-current protec- tion. If the sense voltage at the CS1 input exceeds 0.5V the present power cycle is terminated (cycle-by-cycle current limit). If the sense voltage at the CS2 input exceeds 0.5V, the controller will terminate the present cycle, discharge the softstart capacitor and reduce the softstart current source to 1µA. The softstart (SS) capacitor is released after being fully discharged and slowly charges with a 1µA current source. When the voltage at the SS pin reaches approximately 1V, 20107412 FIGURE 1. www.national.com 11 |
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