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LM25007MM Arkusz danych(PDF) 7 Page - National Semiconductor (TI) |
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LM25007MM Arkusz danych(HTML) 7 Page - National Semiconductor (TI) |
7 / 12 page Detailed Operating Description The LM25007 is a monolithic step-down switching regulator featuring all of the functions needed to implement low cost, efficient, power supply. Capable of driving a 0.5Amp load over a 9-42V input voltage range, this device is easy to apply and is provided in the small MSOP-8 and the thermally enhanced LLP-8 packages. Ultra-fast transient response is achieved through the use of a constant on-time architecture with Vin feed forward. This feature provides and almost constant operating frequency over load and input voltage variations. The operating frequency may be adjusted up to 800kHz depending on input and output voltages. This archi- tecture is easy to use and tolerant of component selection. An intelligent current limit is implemented in the LM25007 with forced off time that is inversely proportional to Vout to ensure recovery from fault conditions. The output voltage may be set from 2.5V to >30V. To improve efficiency in light load applications, the Vcc pin can be tied to an external voltage source thus eliminating the drop through the inte- grated internal regulator. Additional protection features in- clude: thermal shutdown, Vcc under-voltage lockout, gate drive under-voltage lockout, and max duty cycle limiter. Hysteretic Control Circuit Overview The LM25007 is a Buck DC-DC regulator that uses a con- stant on-time control scheme. The on-time is programmed by an external resistor and varies inversely with line input voltage (Vin). The core regulation elements of the LM25007 are the feedback comparator and the on-time one-shot. The regulator output voltage is sensed at the feedback pin (FB) and is compared to an internal reference voltage (2.5V). If the FB signal is below the reference voltage, the buck switch is turned on for a fixed time pulse determined by the line voltage and a programming resistor (R ON). Following the on period the switch will remain off for at least the minimum off timer period of 300ns. If the FB pin voltage is still below the reference after the 300ns off-time, the switch will turn on again for another on time period. This switching behavior will continue until the FB pin voltage reaches the reference voltage level. The LM25007 operates in discontinuous conduction mode at light load currents or continuous conduction mode at heavier load currents. In discontinuous conduction mode, current through the output inductor starts at zero and ramps up to a peak value during the buck switch on-time and then back to zero during the off-time. The inductor current remains at zero until the next on-time period starts when FB falls below the internal reference. In discontinuous mode the operating fre- quency is relatively low and will vary with load. Therefore at light loads the conversion efficiency is maintained, since the switching losses decrease with the reduction in load current and switching frequency. The approximate discontinuous mode operating frequency can be calculated as follows: In continuous conduction mode, current flows continuously through the inductor and never ramps down to zero. In this mode the operating frequency is greater than the discontinu- ous mode frequency and remains relatively constant with load and line variations. The approximate continuous mode operating frequency can be calculated as follows: The output voltage (Vout) can be programmed by two exter- nal resistors as shown in Figure 1. The regulated voltage is calculated as follows: V OUT =2.5x(R3+R4)/R4 The feedback comparator in hysteretic regulators depends on the output ripple voltage to switch the output transistor on and off at regular intervals. In order for the internal compara- tor to respond quickly to changes in output voltage, propor- tional to inductor current, a minimum amount of capacitor Equivalent Series Resistance (ESR) is required. A ripple voltage of 25mV is recommended at the feedback pin (FB) for stable operation. In cases where the intrinsic capacitor ESR is too small, additional series resistance may be added. For applications where lower output voltage ripple is re- quired the load can be connected directly to the low ESR output capacitor, as shown in Figure 1. The series resistor (R5) will degrade the load regulation. Another technique for enhancing the ripple voltage at the FB pin is to place a capacitor in parallel with the feedback divider resistor R3. The addition of the capacitor reduces the attenuation of the ripple voltage from the feedback divider. High Voltage Bias Regulator (VCC) The LM25007 contains an internal high voltage bias regula- tor. The input pin (Vin) can be connected directly to line voltages from 9 to 42 Volts. To avoid supply voltage tran- sients due to long lead inductances on the input pin (VIN), it is always recommended to connect a low ESR ceramic chip capacitor () 0.1µF) between VIN and RTN pins located close to the LM25007. The regulator is internally current limited to 11mA. Upon power up, the regulator is enabled and sources current into an external capacitor connected to the VCC pin. When the voltage on the VCC pin reaches its UVLO level (6.3V), the controller output is enabled. An external auxiliary supply voltage can be diode connected to the Vcc pin. If the auxiliary voltage is greater than 7 Volts the internal regulator will shutoff, thus reducing internal power dissipation. Do not exceed 14V at VCC. See Figure 2. www.national.com 7 |
Podobny numer części - LM25007MM |
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Podobny opis - LM25007MM |
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