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UCC1570 Datasheet(PDF) 5 Page - Texas Instruments |
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UCC1570 Datasheet(HTML) 5 Page - Texas Instruments |
5 / 9 page 5 UCC1570 UCC2570 UCC3570 Latched Shutdown If CURLIM rises above 0.6V, or COUNT rises to 4V, the shutdown latch will be set. This will force OUT low, dis- charge SOFTST and COUNT, and reduce IVCC to ap- proximately 1mA. When, and if, VCC falls below 9V, the shutdown latch will reset and IVCC will fall to 85µA, allow- ing the circuit to restart. If VCC remains above 9V, an al- ternate restart will occur if VFWD is momentarily reduced below 1V. External shutdown commands from any source may be added into either the COUNT or CURLIM pins. Deadtime Control The voltage waveform on RAMP has independently con- trolled rising and falling edges. At the start of the clock period, RAMP is at 1V and rises to 4V. It then discharges back to 1V and awaits the next clock period. OUT can only be high during the rising part of the waveform, while it is positively blanked off during the falling portion. Set- ting the –dV/dt slope by R4 from ISET to GND estab- lishes a minimum deadtime as: td R CR =• • 03 4 . Choose R4 between 20k and 200k and CR greater than 50pF. In order to have a pulse at OUT in the next clock period, RAMP must fall to 1V prior to the end of the cur- rent period. If it does not, OUT will remain low for the en- tire next clock period. Voltage Feedforward The +dV/dt on RAMP is made proportional to line volt- age. The slope is: () dV dt VFWD RCR =• • 10 3 where VFWD is line voltage scaled by R1 and R2. There- fore, a changing line voltage will accomplish an immedi- ate proportionate pulse width change without any action from the feedback amplifier. This will result in constant volt-second drive to the power transformer providing both international voltage operation, and excellent dynamic line regulation. VFWD is intended to operate over a 4:1 range (1V to 4V) with undervoltage and overvoltage sen- sors designed to drive OUT low if this range is exceeded. Choose R3 between 20k and 200k. APPLICATION INFORMATION (cont.) Figure 1. UCC1570 typical application. |
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