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LM22677TJE-5.0 bảng dữ liệu(PDF) 7 Page - National Semiconductor (TI) |
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LM22677TJE-5.0 bảng dữ liệu(HTML) 7 Page - National Semiconductor (TI) |
7 / 16 page Detailed Operating Description The LM22677 switching regulator features all of the functions necessary to implement an efficient high voltage buck regu- lator using a minimum of external components. This easy to use regulator integrates a 42V N-Channel switch with an out- put current capability of 5A. The regulator control method is based on voltage mode control with input voltage feed for- ward. The loop compensation is integrated into the LM22677 so that no external compensation components need to be se- lected or utilized. Voltage mode control offers short minimum on-times allowing short duty-cycles necessary in high input voltage applications. The operating frequency is fixed at 500kHz to allow for small external components while avoiding excessive switching losses. The switching frequency can be adjusted with an external resistor from 200 kHz to 1 MHz or it can be synchronized to an external clock up to 1 MHz. The output voltage can be set as low as 1.285V with the -ADJ device. Fault protection features include current limiting, ther- mal shutdown and remote shutdown capability. The device is available in the TO-263 THIN package featuring an exposed pad to aid thermal dissipation. The functional block diagram with typical application of the LM22677 are shown in Figure 1. The internal compensation of the -ADJ option of the LM22677 is optimized for output voltages up to 5V. If an output voltage of 5V or higher is needed, the -5.0 fixed output voltage option with an additional external resistive feedback voltage divider may also be used. Precision Enable The precision enable pin (EN) can be used to shut down the power supply. Connecting this pin to ground or to a voltage less than typical 1.6V will completely turn off the regulator. The current drain from the input supply when off is typically 25 µA with 12V input voltage. The power consumed during this off state is mostly defined by an internal 2 M Ω resistor to VIN. The enable pin has an internal pull-up current source of approximately 6 µA. When driving the enable pin, the high voltage level for the on condition should not exceed the 6V absolute maximum limit. When enable control is not required, the EN pin should be left floating. The precision feature en- ables simple sequencing of multiple power supplies with a resistor divider from another power supply. The EN pin can also be used as an external UVLO to disable the part when input voltage falls below a lower boundary of operation. This is often used to prevent excessive battery dis- charge. It can also be used to prevent early turn-on as Vin is rising which can cause undesirable on-off toggling if Vin droops below 4.5V during startup. Using EN as en external UVLO is also recommended to prevent abnormal device op- eration in applications where the input voltage falls below the minimum operating voltage of 4.5V, during power down for example. Maximum Duty-Cycle / Dropout Voltage The typical maximum duty-cycle is 90% at 500 kHz switching frequency. This corresponds to a typical minimum off-time of 200 ns. When operating at switching frequencies higher than 500 kHz, the 200 ns minimum off-time results in a lower max- imum duty-cycle limit than 90%. This forced off-time is impor- tant to provide enough time for the Cboot capacitor to charge during each cycle. The lowest input voltage required to maintain operation is: Where V D is the forward voltage drop across the re-circulating Schottky diode and V Q is the voltage drop across the internal power N-FET of the LM22677. The R DS(ON) of the FET is specified in the electrical characteristics section of this datasheet to calculate V Q according to the FET current. F is the switching frequency. Minimum Duty-Cycle Besides a minimum off-time, there is also a minimum on-time which will take effect when the output voltage is adjusted very low and the input voltage is very high. Should the operation require an on-time shorter than minimum, individual switching pulses will be skipped. Pulse skipping is a normal mode of operation which appears as a decrease in switching frequency. It has no effect on op- eration or regulation except for an increase in output ripple voltage. The pulse skipping function is required to maintain proper regulation and overcurrent protection under the full range of operating conditions. The specified typical minimum on time of 100 ns is based on the blanking time during current limit operation. During normal operation, the minimum on-time will also include the effect of propagation delay. Assume approximately 150 ns as a typical operating minimum on time. where D is the duty-cycle. Current Limit When the power switch turns on, the slight capacitance load- ing of the Schottky diode, D1, causes a leading-edge current spike with an extended ringing period. This spike can cause the current limit comparator to trip prematurely. A leading edge blanking time (T BLK) of 100 ns (typical) is used to avoid sampling the spike. When the switch current reaches the current limit threshold the switch is immediately turned off. If T ON is larger than the minimum (100 ns typical) the switcher will hold the output current flat at the set current limit value. But if T ON is at or decreases to the minimum T ON (100 ns typical) the switching frequency decreases to 1/5 the typical frequency. This effec- tively causes the output current to fold back to a lower and safe value. When the current limit condition is removed the switching frequency is restored to nominal. This 5X frequency fold back will result in a lower duty cycle pulse of the power switch to minimize the overall fault condition power dissipa- tion. 7 www.national.com |
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