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MC44605P Datasheet(PDF) 9 Page - ON Semiconductor |
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MC44605P Datasheet(HTML) 9 Page - ON Semiconductor |
9 / 20 page MC44605 http://onsemi.com 9 The Error Amp Output (Pin 13) is provided for external loop compensation. The output voltage is offset by two diodes drops ( [1.4 V) and divided by three before it connects to the inverting input of the Current Sense Comparator. This guarantees that no drive pulses appear at the Source Output (Pin 3) when Pin 13 is at its lowest state (VOL). This occurs when the power supply is operating and the load is removed, or at the beginning of a soft–start interval. The Error Amp minimum feedback resistance is limited by the amplifier’s minimum source current (0.2 mA) and the required output voltage (VOH) to reach the current sense comparator’s 1.0 V clamp level: R1(min) + (3 1V) ) 1.4 V 0.2 mA + 22 kΩ Current Sense Comparator and PWM Latch The MC44605 operates as a current mode controller. The circuit uses a current sense comparator to compare the inductor current to the threshold level established by the Error Amplifier output (Pin 13). When the current reaches the threshold, the current sense comparator terminates the output switch conduction that has been initiated by the oscillator, by resetting the PWM Latch. Thus the error signal controls the peak inductor current on a cycle–by–cycle basis. This configuration ensures that only one single pulse appears at the Source Output during the appropriate oscillator cycle. Figure 2. Output Totem Pole R S Q UVLO Disout Vdemag out Thermal Protection PWM Latch Current Sense Comparator Substrate Current Sense 14 3 7 C RS R R3 Q1 Vin VC R2 VS The inductor current is converted to a voltage by inserting the ground referenced sense resistor RS in series with the power switch Q1. This voltage is monitored by the Current Sense Input (Pin 7) and compared to a level derived from the Error Amp output. The peak inductor current under normal operating conditions is controlled by the voltage at Pin 13 where: I pk [ V (pin13) * 1.4 V 3 R S The Current Sense Comparator threshold is internally clamped to 1.0 V. Therefore the maximum peak switch current is: I pk(max) + 1V R S Undervoltage Lockout Section As depicted in Figure 3, an undervoltage lockout has been incorporated to guarantee that the IC is fully functional before allowing the system working. In effect, the VCC is connected to the non inverting input of a comparator that has an upper threshold equal to 14,5 V (typical Vstup–th) and a lower one equal to 7.5 V (typical Vdisable 2). This hysteresis comparator enables or disables the reference block that generates the voltage and current sources required by the system. This block particularly, produces Vref (pin 16 voltage) and Iref that is determined by the resistor Rrefconnectedbetween pin 16 and the ground: I ref + V ref R ref where V ref + 2.5 V (typically) Figure 3. VCC Management Reference Block: Voltage and Current Sources Generator (Vref, Iref, ...) Vref enable CSTART–UP 10 1 0 Vdisable 7.5 V START–UP 14.5 V UVLO1 (to SOFTSTART) Vdisable1 9.0 V CUVLO1 Rref Pin 16 MC44605 VCC (Pin 1) In addition to this, VCC is compared to a second threshold level that is nearly equal to 9 V (Vdisable1) so that a signal UVLO1 is generated to reset the soft start block and so, to disable the output stage (refer to the Soft–Start §) as soon as VCC becomes lower than Vdisable 1. In this way, the circuit is reset and made ready for a next start–up, before the reference block is disabled (refer to Figure 3). Thus, finally the upper limit for the minimum normal operating voltage |
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