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TK75005MCMG Datasheet(PDF) 5 Page - TOKO, Inc |
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TK75005MCMG Datasheet(HTML) 5 Page - TOKO, Inc |
5 / 9 page January 1999 TOKO, Inc. Page 5 TK75005 PIN DESCRIPTIONS DRIVE PIN (DRV) This pin drives the external MOSFET with a totem pole output stage capable of sinking or sourcing a peak current of about 1 A. In standby mode, the DRV pin can sink about 5 mA while keeping the drive pin pulled down to about 1 V. This ensures that the external MOSFET can not be inadvertently turned on by leakage currents. The maximum duty cycle of the output signal is typically 89%. GROUND PIN (GND) This pin provides ground return for the IC. OVERVOLTAGE PROTECTION INPUT PIN (OVP) This pin provides a means of turning off the external transistor drive output independent of the PWM loop. This pin is normally used for overvoltage protection, but can also be used to provide a drive disabled function. The pin is the input to comparator with its other input referenced to 2.6 V, which tracks V ref of error amp over temperature. and its output controlling the output driver of the IC. Therefore, if a voltage appears at this pin over 2.6 V, the voltage at the DRV pin drops to zero. TIMING CAPACITOR PIN (C T) The external timing capacitor is connected to the C T pin. That capacitor is the only component needed for setting the clock frequency. The frequency measured at the C T pin is the same frequency as measured at the DRV pin. As the frequency of operation increases above 200 kHz, the maximum duty cycle decreases from a typical 89% at 200 kHz to 82% at 1.6 MHz. The maximum recommended clock frequency of the device is 1.6 MHz. At normal operation, during the rising section of the timing-capacitor voltage, a trimmed internal current of 175 µA flows out from the C T pin and charges the capacitor. During the falling section of the timing-capacitor voltage, an internal current of about 1.8 mA discharges the capacitor. FEEDBACK INPUT PIN (FB) The feedback pin normally receives the sum of three signals: the switch current signal, the error signal (from the internal error amplifier and the GM stage), and a voltage ramp (from an internal sawtooth-shaped current with a peak value of about 205 µA) generated across the external terminating resistance. The switch current signal is needed in current-mode controlled converters and in converters with cycle-by-cycle overload protection. The error signal is needed for stabilizing the output voltage or current. The voltage ramp is needed for slope compensation (necessary for avoiding subharmonic instability in constant-frequency peak-current controlled current-mode converters above 50% duty ratio), or for Pulse Width Modulation (PWM) (in voltage-mode controlled converters). At higher clock frequencies, the bandwidth limitation of the internally-generated sawtooth-shaped current source becomes more apparent. The degree to which ramp bandwidth is tolerable depends on performance requirements at narrow pulse widths. A low impedance at the feedback pin can effectively eliminate the internally- generated ramp effects and an external ramp can be readily created to attain higher performance at high frequencies, if desired. ERROR AMPLIFIER COMPENSATION INPUT PIN (EA IN) This pin is the inverting input of an operational amplifier which has its non-inverting input connected to 2.5 V. This is called the error amp because it amplifies the error between this pin’s voltage and 2.5 V reference, which should reflect the error in the power supply’s output regulation. The error amp provides a high gain stage so that the voltage loop gain can be high enough to provide good output voltage regulation. ERROR AMPLIFIER COMPENSATION OUTPUT PIN (EA OUT) This pin is the output of the operational amplifier mentioned in the EA IN pin description. By picking the proper resistor and capacitor network connected between pins 6 and 7, the gain and frequency response of the error amp block of the voltage loop can be set, thus providing gain and frequency compensation into the PWM voltage loop as needed. This pin also acts as the input to the GM stage of the voltage control loop. SUPPLY VOLTAGE PIN (V CC) This pin is connected to the supply voltage. The IC is in a low-current (250 µA typ.) standby mode before the supply voltage exceeds 10 V (typ.), which is the upper threshold of the undervoltage lockout circuit. The IC switches back to standby mode when the supply voltage drops below 8 V (typ.). |
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