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UCC3895PWTRG4 Datasheet(PDF) 6 Page - Texas Instruments |
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UCC3895PWTRG4 Datasheet(HTML) 6 Page - Texas Instruments |
6 / 14 page Programming DELAB, DELCD, and the Adaptive Delay Set The UCC3895 allows the user to set the delay between switch commands within each leg of the full bridge power circuit according to the following formula from the data sheet: t R V n DELAY DEL DEL = •• + − () sec 25 10 25 12 For this equation VDEL is determined in conjunction with the desire to utilize (or not utilize) the adaptive delay set feature from the following formula: () [] VV V V DEL CS ADS =• − + 075 0 5 .. The following diagram illustrates the resistors needed to program the delay periods and the adaptive delay set function. APPLICATION INFORMATION 6 UCC1895 UCC2895 UCC3895 RAMP: The Inverting Input of the PWM Comparator. This pin receives either the CT waveform in voltage and aver- age current mode controls, or the current signal (plus slope compensation) in peak current mode control. An in- ternal discharge transistor is provided on RAMP, which is triggered during the oscillator dead time. RT: Oscillator Timing Resistor. (Refer to Fig. 1, Oscillator Block Diagram) The oscillator in the UCC3895 operates by charging an external timing capacitor, CT, with a fixed current programmed by RT. RT current is calculated as follows: I V R RT T = 30 . where RT is in Ohms and IRT is in Amperes. RT can range from 40k W to 120kW Soft start charging and dis- charging current are also programmed by IRT . SS/DISB: Soft Start/Disable. This pin combines the two independent functions. : A rapid shutdown of the chip is accomplished by any one of the following: externally forcing SS/DISB below 0.5V, externally forcing REF below 4V, VDD dropping below the UNLO threshold, or an overcurrent fault is sensed (CS = 2.5V). In the case of REF being pulled below 4V or an UVLO condition, SS/DISB is actively pulled to ground via an internal MOSFET switch. If an overcurrent is sensed, SS/DISB will sink a current of (10 • IRT) until SS/DISB falls below 0.5V. Note that if SS/DISB is externally forced below 0.5V the pin will start to source current equal to IRT. Also note that the only time the part switches into the low IDD current mode is when the part is in undervoltage lockout. After a fault or disable condition has passed, VDD is above the start threshold, and/or SS/DISB falls below 0.5V during a soft stop, SS/DISB will switch to a soft start mode. The pin will now source current, equal to IRT. A user-selected capacitor on SS/DISB determines the soft start (and soft-start) time. In addition, a resistor in parallel with the capacitor may be used, limiting the maximum voltage on SS/DISB. Note that SS/DISB will actively clamp the EAOUT pin voltage to approximately the SS/DISB pin voltage during both soft start, soft stop, and disable conditions. SYNC: Oscillator Synchronization. (Refer to Fig. 1, Oscil- lator Block Diagram) This pin is bidirectional. When used as an output, SYNC can be used as a clock, which is the same as the chip’s internal clock. When used as an in- put, SYNC will override the chip’s internal oscillator and act as it’s clock signal. This bidirectional feature allows synchronization of multiple power supplies. The SYNC signal will also internally discharge the CT capacitor and any filter capacitors that are present on the RAMP pin. The internal SYNC circuitry is level sensitive, with an in- put low threshold of 1.9V, and an input high threshold of 2.1V. A resistor as small as 3.9k W may be tied between SYNC and GND to reduce the sync pulse width. VDD: Power Supply. VDD must be bypassed with a mini- mum of a 1.0 mF low ESR, low ESL capacitor to ground. REF: 5V, ±1.2% voltage reference. The reference supplies power to internal circuitry, and can also supply up to 5mA to external loads. The reference is shut down during undervoltage lock-out but is operational during all other disable modes. For best performance, bypass with a 0.1 mF low ESR, low ESL capacitor to ground. PIN DESCRIPTIONS (cont.) 9 11 10 12 DELCD DELAB ADS CS R DELCD R DELAB UCC3895 |
Similar Part No. - UCC3895PWTRG4 |
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Similar Description - UCC3895PWTRG4 |
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