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LM76002-Q1 Datasheet(PDF) 19 Page - Texas Instruments |
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LM76002-Q1 Datasheet(HTML) 19 Page - Texas Instruments |
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19 / 54 page  SYNC RTERM EXT CLOCK 19 LM76002-Q1, LM76003-Q1 www.ti.com SNVSAU3 – DECEMBER 2017 Product Folder Links: LM76002-Q1 LM76003-Q1 Submit Documentation Feedback Copyright © 2017, Texas Instruments Incorporated The LM76002-Q1/LM76003-Q1 switching action can also be synchronized to an external clock from 300 kHz to 2.2 MHz. TI recommends connecting an external clock to the SYNC pin with appropriate termination resistor. Ground the SYNC pin if not used. Figure 18. Frequency Synchronization The recommendations for the external clock include high level no lower than 2 V, low level no higher than 0.4 V, duty cycle between 10% and 90%, and both positive and negative pulse width no shorter than 80 ns. When the external clock fails at logic high or low, the LM76002-Q1/LM76003-Q1 switches at the frequency programmed by the RT resistor after a time-out period. TI recommends connecting a resistor RT to the RT pin so that the internal oscillator frequency is the same as the target clock frequency when the LM76002-Q1/LM76003-Q1 is synchronized to an external clock. This allows the regulator to continue operating at approximately the same switching frequency if the external clock fails. The choice of switching frequency is usually a compromise between conversion efficiency and the size of the circuit. Lower switching frequency implies reduced switching losses (including gate charge losses, switch transition losses, etc.) and usually results in higher overall efficiency. However, higher switching frequency allows use of smaller LC output filters and hence a more compact design. Lower inductance also helps transient response (higher large signal slew rate of inductor current), and reduces the DCR loss. The optimal switching frequency is usually a trade-off in a given application and thus needs to be determined on a case-by-case basis. It is related to the input voltage, output voltage, most frequent load current level(s), external component choices, and circuit size requirement. The choice of switching frequency may also be limited if an operating condition triggers tON-MIN or tOFF-MIN. 7.3.8 Minimum On-Time, Minimum Off-Time, and Frequency Foldback at Dropout Conditions Minimum on-time, tON-MIN, is the smallest duration of time that the HS switch can be on. tON-MIN is typically 65 ns in the LM76002-Q1/LM76003-Q1. Minimum off-time, tOFF-MIN, is the smallest duration that the HS switch can be off. tOFF-MIN is typically 95 ns in the LM76002-Q1/LM76003-Q1. In CCM operation, tON-MIN and tOFF-MIN limits the voltage conversion range given a selected switching frequency. The minimum duty cycle allowed is: DMIN = tON-MIN × fSW (7) And the maximum duty cycle allowed is: DMAX = 1 – tOFF-MIN × fSW (8) Given fixed tON-MIN and tOFF-MIN, the higher the switching frequency the narrower the range of the allowed duty cycle. In the LM76002-Q1/LM76003-Q1, frequency foldback scheme is employed to extend the maximum duty cycle when tOFF-MIN is reached. The switching frequency decreases once longer duty cycle is needed under low VIN conditions. The switching frequency can be decreased to approximately 1/10 of the programmed frequency by RT or the synchronization clock. Such a wide range of frequency foldback allows the LM76002-Q1/LM76003- Q1 output voltage to stay in regulation with a much lower supply voltage VIN. This leads to a lower effective dropout voltage. See Typical Characteristics for more details. Given an output voltage, the choice of the switching frequency affects the allowed input voltage range, solution size and efficiency. The maximum operational supply voltage can be found by: VIN_MAX = VOUT / (fSW × tON-MIN) (9) At lower supply voltage, the switching frequency decreases once tOFF-MIN is tripped. The minimum VIN without frequency foldback can be approximated by: VIN_MIN = VOUT / (fSW × tOFF-MIN) (10) Considering power losses in the system with heavy load operation, VIN-MIN is higher than the result calculated in Equation 10. With frequency foldback, VIN-MIN is lowered by decreased fSW. When the device is operating in auto mode at voltages near maximum rated input voltage and light load conditions, an increased output voltage ripple during load transient may be observed. For this reason TI recommends that device operating point be calculated with sufficient operational margin so that minimum on-time condition is not triggered. |
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