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LM22677TJ-ADJ Scheda tecnica(PDF) 8 Page - National Semiconductor (TI) |
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LM22677TJ-ADJ Scheda tecnica(HTML) 8 Page - National Semiconductor (TI) |
8 / 16 page 30074150 FIGURE 2. Output Current in Foldback vs. Nominal Duty Cycle The percentage of output current limit fold back is affected by duty cycle, inductance, and switching frequency. See Figure 2 for details. The current limit will only protect the inductor from a runaway condition if the LM22677 is operating in its safe operating area. A runaway condition of the inductor is potentially catas- trophic to the application. For every design, the safe operating area needs to be calculated. Factors in determining the safe operating area are the switching frequency, input voltage, output voltage, minimum on-time and feedback voltage dur- ing an over current condition. As a first pass check, if the following equation holds true, a given design is considered in a safe operating area and the current limit will protect the circuit: V IN x TBLK x F < VOUT x 0.724 If the equation above does not hold true, the following sec- ondary equation will need to hold true to be in safe operating area: If both equations do not hold true, a particular design will not have an effective current limit function which might damage the circuit during startup, over current conditions, or steady state over current and short circuit condition. Oftentimes a reduction of the maximum input voltage or a reduction of the switching frequency will bring a design into the safe operating area. When synchronized to an external sync pulse, the LM22677 will not fold back the switching frequency in an over current condition. Soft-Start The soft-start feature allows the regulator to gradually reach the initial steady state operating point, thus reducing start-up stresses and surges. The soft-start is fixed to 500 µs (typical) start-up time and cannot be modified. Switching Frequency Setting and Synchronization There are three different modes for the RT/SYNC pin. It can be left floating for a 500 kHz switching frequency. A resistor from the RT/SYNC pin to ground can be used to adjust the switching frequency between 200 kHz and 1 MHz. An external synchronization pulse can be applied to the RT/ SYNC pin for switching frequencies up to 1 MHz. The LM22677 internally sets the RT/SYNC mode during start up. Many applications are limited to lower switching frequencies due to minimum on-time and minimum off-time requirements as well as increased switching losses at switching frequen- cies higher than 500 kHz. In case of synchronizing to an external sync pulse, the clock pulse must be in a valid low or high state when the LM22677 starts up and can be applied after device regulation. The internal oscillator will synchronize to the external sync pulse rising edge. If the external sync pulse is disconnected the LM22677 switching frequency will return to default 500 kHz (typ.). When operating at frequencies above 500 kHz, external com- ponents such as the inductor and the output capacitors can- not be reduced dramatically. This is due to the fixed internal compensation and stability requirements. Running the LM22677 at frequencies higher than 500 kHz is intended for applications with EMI or beat frequency concerns. The flexi- bility of increasing the switching frequency above 500 kHz can be used to operate outside a critical signal frequency band of an application. This can also be used to set multiple switchers in an application to the same frequency to reduce beat fre- quencies and simplify filtering. See the current limit section of this datasheet for information about the safe operating area. When synchronizing to an ex- ternal synchronization pulse, the LM22677 will not fold back the switching frequency in an over current condition. The typical curve below shows adjusted switching frequen- cies with different frequency set resistors from the RT/SYNC pin to ground. 30074113 FIGURE 3. Switching Frequency vs RT/SYNC Resistor Self Synchronize It is also possible to self-synchronize multiple LM22677 reg- ulators to share the same switching frequency. This can be done by attaching the RT/SYNC pins together and putting a www.national.com 8 |
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