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LM2694SD датащи(PDF) 10 Page - National Semiconductor (TI) |
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LM2694SD датащи(HTML) 10 Page - National Semiconductor (TI) |
10 / 19 page Functional Description The LM2694 Step Down Switching Regulator features all the functions needed to implement a low cost, efficient buck bias power converter capable of supplying at least 0.6A to the load. This high voltage regulator contains a 30V N-Channel buck switch, is easy to implement, and is available in the TSSOP-14 and the thermally enhanced LLP-10 packages. The regulator’s operation is based on a constant on-time control scheme, where the on-time is determined by V IN. This feature allows the operating frequency to remain rela- tively constant with load and input voltage variations. The feedback control requires no loop compensation resulting in very fast load transient response. The valley current limit detection circuit, internally set at 0.62A, holds the buck switch off until the high current level subsides. This scheme protects against excessively high currents if the output is short-circuited when V IN is high. The functional block dia- gram is shown in Figure 7. The LM2694 can be applied in numerous applications to efficiently regulate down higher voltages. Additional features include: Thermal shutdown, V CC under-voltage lockout, gate drive under-voltage lockout, and maximum duty cycle limiter. Control Circuit Overview The LM2694 buck DC-DC regulator employs a control scheme based on a comparator and a one-shot on-timer, with the output voltage feedback (FB) compared to an inter- nal reference (2.5V). If the FB voltage is below the reference the buck switch is turned on for a time period determined by the input voltage and a programming resistor (R ON). Follow- ing the on-time the switch remains off for a minimum of 265 ns, and until the FB voltage falls below the reference. The buck switch then turns on for another on-time period. Typi- cally, during start-up, or when the load current increases suddenly, the off-times are at the minimum of 265 ns. Once regulation is established, the off-times are longer. When in regulation, the LM2694 operates in continuous conduction mode at heavy load currents and discontinuous conduction mode at light load currents. In continuous con- duction mode current always flows through the inductor, never reaching 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 operating frequency is approximately: (1) The buck switch duty cycle is equal to: (2) In discontinuous conduction mode current through the induc- tor ramps up from zero to a peak during the on-time, then ramps back to zero before the end of the off-time. The next on-time period starts when the voltage at FB falls below the reference - until then the inductor current remains zero, and the load current is supplied by the output capacitor (C2). In this mode the operating frequency is lower than in continu- ous conduction mode, and varies with load current. Conver- sion efficiency is maintained at light loads since the switch- ing losses reduce with the reduction in load and frequency. The approximate discontinuous operating frequency can be calculated as follows: (3) where R L = the load resistance. The output voltage is set by two external resistors (R1, R2). The regulated output voltage is calculated as follows: V OUT =2.5x(R1+R2)/R2 Output voltage regulation is based on ripple voltage at the feedback input, requiring a minimum amount of ESR for the output capacitor C2. The LM2694 requires a minimum of 25 mV of ripple voltage at the FB pin. In cases where the capacitor’s ESR is insufficient additional series resistance may be required (R3 in Figure 7). Start-Up Regulator, V CC The start-up regulator is integral to the LM2694. The input pin (VIN) can be connected directly to line voltage up to 30V, with transient capability to 33V. The V CC output regulates at 7.0V, and is current limited at 9 mA. Upon power up, the regulator sources current into the external capacitor at VCC (C3). When the voltage on the VCC pin reaches the under- voltage lockout threshold of 5.7V, the buck switch is enabled and the Softstart pin is released to allow the Softstart capaci- tor (C6) to charge up. The minimum input voltage is determined by the regulator’s dropout voltage, the V CC UVLO falling threshold ( )5.5V), and the frequency. When V CC falls below the falling thresh- old the V CC UVLO activates to shut off the output. If VCC is externally loaded, the minimum input voltage increases. To reduce power dissipation in the start-up regulator, an auxiliary voltage can be diode connected to the V CC pin. Setting the auxiliary voltage to between 8V and 14V shuts off the internal regulator, reducing internal power dissipation. The sum of the auxiliary voltage and the input voltage (V CC +V IN) cannot exceed 47V. Internally, a diode connects VCC to VIN. See Figure 9. www.national.com 10 |
Аналогичный номер детали - LM2694SD |
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Аналогичное описание - LM2694SD |
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