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LM5010AHMH Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM5010AHMH Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 17 page  Control Circuit Overview (Continued) The LM5010A requires a minimum of 25 mV of ripple voltage at the FB pin for stable fixed-frequency operation. If the output capacitor’s ESR is insufficient additional series resis- tance may be required (R3 in the Block Diagram). The LM5010A operates in continuous conduction mode at heavy load currents, and discontinuous conduction mode at light load currents. In continuous conduction mode current always flows through the inductor, never decaying to zero during the off-time. In this mode the operating frequency remains relatively constant with load and line variations. The minimum load current for continuous conduction mode is one-half the inductor’s ripple current amplitude. The operat- ing frequency in the continuous conduction mode is calcu- lated as follows: (2) The buck switch duty cycle is equal to: (3) Under light load conditions, the LM5010A operates in dis- continuous conduction mode, with zero current flowing through the inductor for a portion of the off-time. The oper- ating frequency is always lower than that of the continuous conduction mode, and the switching frequency varies with load current. Conversion efficiency is maintained at a rela- tively high level at light loads since the switching losses diminish as the power delivered to the load is reduced. The discontinuous mode operating frequency is approximately: (4) where R L = the load resistance. Start-Up Bias Regulator (V CC) A high voltage bias regulator is integrated within the LM5010A. The input pin (VIN) can be connected directly to line voltages between 6V and 75V. Referring to the block diagram and the graph of V CC vs. VIN, when VIN is between 6V and the bypass threshold (nominally 8.9V), the bypass switch (Q2) is on, and V CC tracks VIN within 100 mV to 150 mV. The bypass switch on-resistance is approximately 50 Ω, with inherent current limiting at approximately 100 mA. When VIN is above the bypass threshold, Q2 is turned off, and V CC is regulated at 7V. The VCC regulator output current is limited at approximately 15 mA. When the LM5010A is shutdown using the RON/SD pin, the V CC bypass switch is shut off, regardless of the voltage at VIN. When V IN exceeds the bypass threshold, the time required for Q2 to shut off is approximately2-3µs. The capacitor at VCC (C3) must be a minimum of 0.47 µF to prevent the voltage at VCC from rising above its absolute maximum rating in response to a step input applied at VIN. C3 must be located as close as possible to the LM5010A pins. In applications with a relatively high input voltage, power dissipation in the bias regulator is a concern. An auxiliary voltage of between 7.5V and 14V can be diode connected to the VCC pin (D2 in Figure 2) to shut off the VCC regulator, reducing internal power dissipation. The current required into the VCC pin is shown in the Typical Performance Character- istics. Internally a diode connects VCC to VIN requiring that the auxiliary voltage be less than V IN. The turn-on sequence is shown in Figure 1. When VCC exceeds the under-voltage lock-out threshold (UVLO) of 5.25V (t1 in Figure 1), the buck switch is enabled, and the SS pin is released to allow the soft-start capacitor (C6) to charge up. The output voltage V OUT is regulated at a reduced level which increases to the desired value as the soft-start voltage increases (t2 in Figure 1). 20153816 FIGURE 2. Self Biased Configuration www.national.com 9 |
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