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UC2841 数据表(PDF) 8 Page - Texas Instruments |
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UC2841 数据表(HTML) 8 Page - Texas Instruments |
8 / 9 page Since Pin 10 is a direct input to the PWM comparator, this point can also serve as a current sense port for current mode control. In this application, current sensing is ground refer- enced through RCS. Resistor R1 sets a 400mV offset across R2 (assuming R2 > RCS) so that both the Error Amplifier and Fault Shutdown can force the current completely to zero. R2 is also used along with CF as a small filter to attenuate leading- edge spikes on the load current waveform. In this mode, current limiting can be accomplished by divider R3/R4 which forms a clamp overriding the output of the Error Amplifier. In this circuit, R1 is used in conjunction with CR (not shown) to establish a minimum ramp charging current such that the ramp voltage reaches 4.2V at the required maximum output pulse width. The purpose of Q1 is to provide an increasing ramp current above a threshold established by R2 and R3 such that the duty cycle is further reduced with increasing VIN. The minimum ramp current is: lR (MIN) = VREF − VIN SENSE R1 ≈ 4 V R1 The threshold where VIN begins to add extra ramp current is: VIN ≈ 5.6V R2 + R3 R3 Above the threshold, the ramp current will be: lR (VARIAB) ≈ 4 R1 + VIN − 5.6 R2 − 5.6 R3 UC1841 UC2841 UC3841 ERROR LATCH INTERNAL CIRCUITRY PROGRAMMABLE SOFT START AND RESTART DELAY CIRCUIT The Error Latch consists of Q5 and Q6 which, when both on, turns off the PWM Output and pulls the Slow-Start pin low. This latch is set by either the Over-Voltage or Current Shutdown comparators, or by a high signal on Pin 4. Reset is accom- plished by either the Reset comparator or a low signal on Pin 4. An activation time delay can be provided with an external capacitor on Pin 4 in conjunction with the ≈ 100µA collector current from Q4. UNITRODE INTEGRATED CIRCUITS 7 CONTINENTAL BLVD. • MERRIMACK, NH 03054 TEL. (603) 424-2410 • FAX (603) 424-3460 VOLTAGE FEED-FORWARD COMBINED WITH MAXIMUM DUTY-CYCLE CLAMP CURRENT MODE CONTROL 8 |
类似零件编号 - UC2841 |
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类似说明 - UC2841 |
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