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LTC3723-1 Datasheet(PDF) 9 Page - Linear Technology |
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LTC3723-1 Datasheet(HTML) 9 Page - Linear Technology |
9 / 26 page LT4430 9 4430fb preventing overshoot. A capacitor, connected from the OC pin to GND and charged by internal 8.5μA current source IOC, sets the ramp rate. On start-up, Q1 actively holds the OC capacitor low until VIN of the LT4430 reaches its typi- cal undervoltage lockout threshold of 2.2V. Q1 then turns off and the OC capacitor charges linearly. Q2 and Q3 OR the OC pin voltage and the 600mV reference voltage at the noninverting terminal of the error amplifier. The OC pin voltage is the reference voltage for the error amplifier until it increases above 600mV. If the feedback loop is in control, the FB pin voltage follows and regulates to the OC pin voltage. As the OC pin voltage increases past 600mV, the reference voltage takes control of the error amplifier and the FB pin regulates to 600mV. The OC pin voltage increases until it is internally clamped by R2, Q6 and V1. The OC pin’s typical clamp voltage of 0.93V ensures that Q3 turns off. All of I1’s current flows in Q2, matching I2’s current in Q4. In a short-circuit condition, the output voltage decreases to something well below the regulated level. The error ampli- fier reacts by increasing the COMP pin voltage, thereby decreasing the drive to the opto-coupler. The decreased opto-coupler bias signals the primary-side controller to increase the amount of power it delivers in an attempt to raise the output voltage back to its regulated value. As long as the fault persists, the output voltage remains low. The error amplifier’s COMP pin voltage increases until it reaches a clamp level set by Q7 and V2. Q7’s resultant collector current drives internal logic that closes normally open switch S1. This action activates the overshoot control amplifier which employs a unity-gain follower configura- tion. The overshoot control amplifier monitors the FB pin voltage and, on S1’s closing, pulls the OC pin voltage down to the FB pin voltage plus a built-in offset voltage of typically 48mV. The built-in offset voltage serves two purposes. First, the offset voltage prevents the overshoot control amplifier from interfering with normal transient operating conditions. Second, the offset voltage biases the feedback loop so that if the short-circuit condition ends, the feedback loop immediately starts to increase the output voltage to its regulated value. Block Diagram Operation A precision voltage reference, a high-bandwidth error amplifier, an inverting opto-coupler driver and an overshoot control amplifier comprise the LT4430. Referring to the block diagram, a start-up circuit establishes all internal current and voltage biasing for the IC. A precision-trimmed bandgap generates the 600mV reference voltage and a 1.1V bias voltage for the opto-coupler driver. Room tem- perature reference voltage accuracy is specified at ±0.75% and operating temperature range tolerance is specified at ±1.25%. The 600mV reference ties to the noninverting input of the error amplifier. The LT4430 error amplifier senses the output voltage through an external resistor divider and regulates the FB pin to 600mV. The FB pin ties to the inverting input of the error amplifier. The error amplifier’s open loop DC gain is 80dB and its unity-gain crossover frequency of 9MHz provides negligible phase shift at typical feedback loop crossover frequencies. The error amplifier is a true voltage-mode amplifier and frequency compensation con- nects around the amplifier. Typical LT4430 compensation schemes use series R-C in parallel with C networks from the COMP pin to the FB pin. The opto-coupler driver amplifies the voltage difference between the COMP pin and the 1.1V bias potential applied to its noninverting terminal with an inverting gain of 6. This signal drives the opto-coupler referenced to GND. Driving the opto-coupler referenced to GND accommodates low output voltages and simplifies loop frequency compen- sation as the secondary feedback path with a traditional “431” topology is eliminated. A resistor in series with the opto-coupler sets the opto-coupler’s DC bias current. The opto driver amplifier sources a guaranteed maximum of 10mA, sinks 350μA typically and is short-circuit protected. The opto-coupler driver amplifier’s typical –3dB bandwidth is 600kHz. The opto-coupler’s output crosses the galvanic isolation barrier and closes the feedback loop to the pri- mary-side controller. The LT4430 incorporates a unique overshoot control function that allows the user to ramp the output voltage on start-up and recovery from short-circuit conditions, APPLICATIONS INFORMATION |
Similar Part No. - LTC3723-1 |
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Similar Description - LTC3723-1 |
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