Table 1. Design Parameters

8.2.2.1 V_CC

While the LM5030 internally generates a voltage at VCC (7.7 V), the internal regulator is used mainly during the start-up sequence. Once the load current begins flowing through L2, which is both an inductor for the output filter and a transformer, a voltage is generated at the secondary of L2, which powers the VCC pin. When the externally applied voltage exceeds the internal value (7.7 V), the internal regulator shuts off, thereby reducing internal power dissipation in the LM5030. L2 is constructed such that the voltage supplied to V_CC ranges from approximately 10.6 V to approximately 11.3 V, depending on the load current (see Figure 9).

Figure 9. V_CC Voltage vs Load Current

8.2.2.2 Current Sense

Monitoring the input current provides a good indication of the operation of the circuit. If an overload condition should exist at the output (a partial overload or a short circuit), the input current would rise above the nominal value shown in Figure 12. Transformer T2, in conjunction with D3, R9, R12 and C10, provides a voltage to pin 8 on the LM5030 (CS) which is representative of the input current flowing through its primary. The average voltage seen at pin 8 is plotted in Figure 10. If the voltage at the first current sense comparator exceeds 0.5 V, the LM5030 disables its outputs, and the circuit enters a cycle-by-cycle current limit mode. If the second level threshold (0.625 V) is exceeded due to a severe overload and transformer saturation, the LM5030 will disable its outputs and initiate a softstart sequence. However, the very short propagation delay of the cycle-by-cycle current limiter (CS1), the design of the CS filter (R9, R12, and C10), and the conservative design of the output inductor (L2), may prevent the second level current threshold from being realized on this evaluation board.

Figure 10. Average Voltage at the CS Pin vs Input Current

8.2.2.3 Shutdown

The Shutdown pad (SD) on the board connects to the SoftStart pin on the LM5030 (pin 10), and permits on/off control of the converter by an external switch. SD should be pulled below 0.45 V, with an open collector or open drain device, to shut down the LM5030 outputs and the VCC regulator. If the voltage at the SD pad is between 1.0 and 1.5 V, a partial-on condition results, which could be disruptive to the system. Therefore, the voltage at the SD pad should transition quickly between its open circuit voltage (4.9 V) and ground.

8.2.2.4 External Sync

Although the LM5030 includes an internal oscillator, its operating frequency can be synchronized to an external signal if desired. The external source frequency must be higher than the internal frequency set with the RT resistor (262 kHz with R_T = 20 kΩ). The sync input pulse width must be between 15 and 150 ns, and have an amplitude of 1.5 to 3.0 V at the Sync pad on the board. The pulses are coupled to the LM5030 through a 100-pF capacitor (C16) as specified in the data sheet.

Table 2. Bill of Materials

Table 3. Test Data