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TC18C43MJA Arkusz danych(PDF) 4 Page - TelCom Semiconductor, Inc |
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TC18C43MJA Arkusz danych(HTML) 4 Page - TelCom Semiconductor, Inc |
4 / 7 page 4-96 TELCOM SEMICONDUCTOR, INC. TC18C43 TC28C43 TC38C43 BiCMOS CURRENT MODE PWM CONTROLLERS Pin No. Pin No Pin No 8-Pin 14-Pin 16-Pin Symbol Description PIN DESCRIPTION (Cont.) 6 7 NC No Connection 13 8 NC No Connection 9 NC No Connection 8 10 POWER GROUND Ground return of output driver. 5 9 11 ANALOG GND For all the low level analog signal returns. 6 10 12 OUTPUT Output to drive switching transistor gate input. 11 13 VDD Supply power input terminal for the output drivers. 712 14 VIN Voltage bias supply of all PWM Controller circuit functions. 814 15 VREF Reference: 5.0 volt output. 16 NC No Connection. FREQUENCY OF OPERATION The frequency of oscillation for the TC38C43 is con- trolled by a resistor to VREF (RT) and a capacitor to ground (CT). VREF supplies current through the resistor and charges the capacitor until its voltage reaches the threshold of the upper comparator ( ≈2.5V). A 2.5mA current is then applied to the capacitor to discharge it to near ground ( ≈0.15V). The discharge current is then shut off and the cycle repeats. An approximate equation for the frequency of operation is: 7 VREF 14 PIN NUMBERS FOR 14-PIN DIP 9 RT/CT GND (5V) RT CT REFERENCE SECTION The reference is a zener-based design with a buffer amplifier to drive the output. It is unstable with capacitances between 0.01 µF and 3.3µF. In a normal application a 4.7µF is used. In some lower noise layouts the capacitor can be eliminated entirely. The reference is active as soon as the 38C4x has power supplied. This is different than its bipolar counter- parts, in that the bipolar reference comes on only after the IC has come out of its under voltage mode. Thus, on the 38C4x, the reference pin can not be used as a reset function such as on a soft start circuit. OSCILLATOR SECTION The oscillator frequency is set by the combination of a resistor from the reference to the RT/CT pin and by a capacitor from this pin to ground. The oscillator is designed to have ramp amplitude from 0.15 to 2.5 volts. This is approximate, as over shoot on the oscillator comparator causes the ramp amplitude to increase with frequency due to comparator delay. Minimum values for CT and RT are 33pF and 1k Ω respectively. Maximum values are depen- dent on leakage currents in the capacitor, not on the input currents to the RT/CT pin. fO ≈ 1 (RT in Ohms and CT in Farads) RT CT The value of RT affects the discharge current and the upper and lower comparators each have delays. As RT gets smaller and as the frequency of operation gets higher, the above equation is no longer valid. |
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