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Tygole When applying power to the LM evaluation board a. The printed circuit board consists of 2 datawheet of 2 ounce. If more than a. When applying power to the LM evaluation board cer. The LM includes a high-voltage startup regulator that operates over a wide input range to V.

The PWM controller is designed for high speed capa- bility including an oscillator frequency range to 1MHz and total propagation delays less than ns. Additional features include an error amplifier, precision reference, line under- voltage lockout, cycle-by-cycle current limit, slope compen- sation, softstart, oscillator sync capability and thermal shut- down.

The functional block diagram of the LM is shown in Figure 1. The LM is designed for current-mode con- trol power converters, which require a single drive output, such as Flyback and Forward topologies. The LM pro- vides all of the advantages of current-mode control including line feed-forward, cycle-by-cycle current limiting and simpli- fied loop compensation.

The LM contains an internal high voltage startup regu- lator, that allows the input pin Vin to be connected directly to line voltages as high as V. The regulator output is internally current limited to 15mA.

When power is applied, the regulator is enabled and sources current into an external capacitor connected to the V CC pin. The recommended ca- pacitance range for the Vcc regulator is 0. When the voltage on the V CC pin reaches the regulation level of 7. The controller will remain enabled until V CC falls below 6V.

In typical applications, a transformer auxiliary winding is connected through a diode to the V CC pin. This winding CC voltage above 8V to shut off the internal startup regulator. Powering V CC from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller. During a fault condition when the con- verter auxiliary winding is inactive, external current draw on the V CC line should be limited such that the power dissipated in the start-up regulator does not exceed the maximum power dissipation capability 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 Vin pins together and feeding the external bias voltage 8- 15V to the two pins. An external set-point voltage divider from Vin to GND sets the operational range of the converter. The resistor divider must be designed such that the voltage at the UVLO pin is greater than 1.

If the under voltage threshold is not met, all functions of the controller are disabled and the controller remains in a low power standby state. When the UVLO pin voltage falls below the 1. If an external transistor pulls the UVLO pin below the 1.

In non-isolated applications, the power converter output is connected to the FB pin via volt- age scaling resistors. For most iso- lated applications the error amplifier function is implemented on the secondary side of the converter and the internal error amplifier is not used. The internal error amplifier is config- ured as an open drain output and can be disabled by con- necting the FB pin to ground.

An internal 5K pull-up resistor between a 5V reference and COMP can be used as the pull-up for an optocoupler in isolated applications. Current limit is accomplished by an internal current sense comparator. If the voltage at current sense comparator input exceeds 0. A small RC filter, located near the control- ler, is recommended to filter noise from the current sense signal.

The discharge device remains on an additional 50ns after the beginning of the new cycle to attenuate the leading edge spike on the current sense signal.

The LM current sense and PWM comparators are very fast, and may respond to short duration noise pulses. Layout considerations are critical for the current sense filter and sense resistor. The capacitor associated with the CS filter must be located very close to the device and connected directly to the pins of the controller CS and GND. If a current sense transformer is used, both leads of the trans- former secondary should be routed to the sense resistor and the current sense filter network.

A sense resistor located in the source of the primary power MOSFET may be used for current sensing, but a low inductance resistor is required.

When designing with a current sense resistor all of the noise sensitive low power ground connections should be con- nected together local to the controller and a single connec- tion should be made to the high current power ground sense resistor ground point.

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100V Current Mode PWM Controller

Check for Samples: LM Synchronization Capability. The LM high voltage pulse-width-modulation. PWM controller contains all of the features needed. Output voltage regulation is based on.


National Semiconductor



Datasheet Texas Instruments LM5020


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