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MRA4005T1 Datasheet(PDF) 2 Page - ON Semiconductor |
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MRA4005T1 Datasheet(HTML) 2 Page - ON Semiconductor |
2 / 10 page AND8098/D http://onsemi.com 2 through the output and a low-frequency ripple will be found in the output voltage. Hence, the value of C2 is needed to be small enough to increase this charging frequency fVCC in order to reduce output voltage ripple because some efficiency is lost due to this low-frequency ripple. Input Output D3 C3 D L C1 C Z1 C2 VCC FB DS D2 Z2 D1 R 1 (a) Buck Input Output D3 C3 D L C1 C Z1 C2 VCC FB DS D2 Z2 D1 R1 (b) Buck-boost Istart Istart Figure 2. Charging Current of C2 In Figure 2b it is noted that in the buck-boost topology the charging current path is blocked by diode D and hence the charging of C2 does not affect the output voltage directly. However, it still affects the output voltage indirectly and slightly by adding some low-frequency noise on the inductor. Hence, small value of C2 is also wanted. C1 Vout R1 (a) Buck D1 C1 Vout R1 (b) Buck-boost D1 Figure 3. Output Voltage Couples to C1 with a Charging Current The function of diode D1, capacitor C1 and resistor R1 are to transfer the magnitude of output voltage to a voltage across C1 so that the IC can regulate the output voltage. In Figure 3, when the main switch inside the IC is opened and the diode D is closed. In buck, the potential of the IC reference ground (pin S) becomes almost 0 V in this moment. In buck-boost, the potential of the IC reference ground (pin S) becomes -Vout in this moment. The voltage in C1 will be charged to the output voltage. On the other hand, when main switch is closed and the diode D is opened, diode D1 is reverse biased by a voltage with magnitude Vin and Vin+Vout respectively. Hence, D1 does not affect the normal operation of the buck and buck-boost converter. It is noted that the instantaneous voltage in C1 can be possibly greater than the output voltage especially when output current or output ripple is too large. It directly affects the load regulation of the circuit since the IC regulates the output voltage based on the voltage in C1. In order to solve it, larger values of L and R1 can help to slow down the charging speed of C1. It reduces the maximum instantaneous voltage in C1 so that output voltage at high output current can be pulled up and a good regulation is made. Larger value of L can help the load regulation but it usually unwanted because it is bulky. Hence, resistor R1 is recommended. Larger value of R1 makes higher output voltage. Hence, it is called as a “pull-up resistor” and it can help to pull up the output voltage slightly. The voltage in C1 representing the output voltage is feedback to the feedback (FB) pin of the NCP1052 through a diode D2 and zener diode Z2. When output voltage is too high, there will be a greater-than-50 mA current inserting into the feedback pin of the NCP1052. The NCP1052 will stop switching when it happens. When output voltage is not high enough, the current inserting into the feedback is smaller than 50 mA. The NCP1052 enables switching and power is delivered to the output until the output voltage is too high again. The purpose of the diode D2 is to ensure the current is inserting into the feedback pin because the switching of NCP1052 can also be stopped when there is a greater-than-50 mA current sinking from the FB pin. The purpose of the zener diode Z2 is to set the output voltage threshold. The FB pin of NCP1052 with a condition of 50 mA sourcing current is about 4.3 V. The volt-drop of the diode D2 is loosely about 0.7 V at 50 mA. Hence, the output voltage can be loosely set as follows: Vout + zener ) 4.3 V ) 0.7 V (eq. 1) + zener ) 5V According to (1), the possible minimum output voltage of the circuit is 5.0 V when there is no zener diode Z2. If there is no load, the IC will automatically minimize its duty cycle to the minimum value but the output voltage is still possible to be very high because there is no passive component in the circuit try to absorb the energy. As a result, |
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