MIC2182 Current-Mode Synchronous Buck Regulator

The input voltage range of current-mode buck controllers provided by several semiconductor manufacturers is 26V to 36V. However, the output voltage range is only from the reference voltage to about 6V. This output voltage limitation is caused by the common-mode voltage limit of the current detection amplifier. In practical applications, the required power supply should be able to generate higher output voltages for printers, servers, routers, network equipment, and test equipment. It is a problem to use conventional buck regulators to provide higher voltages.

The circuit shown in Figure 1 can solve this problem. Using an external operational amplifier, a small signal pnp transistor and a voltage step-down regulator IC1, a 20V output voltage can be provided with a 27V input power supply when the load current is as high as 2.5A. As long as the resistance value of the detection resistor R4 is adjusted, a larger load current can be provided. IC1 in Figure 1 is MIC2182. Operational amplifier IC2 is MIC6211. Resistors R3 and R6 determine the output voltage according to the following formula: Vout = Vfb (1 + R3/R6) = 20V.

The buck regulator's Csh (pin ⑧) and Vout (pin ⑨) are usually connected across the sense resistor R4. The regulator determines the current limit when the voltage between these two pins approaches 100mv. In the case of Vout = 20v, the op amp and Q3 will reduce the voltage drop across R2 from 20v to 5V, which puts the buck regulator's internal current sense amplifier within the input common mode range. To understand the working principle of the circuit more clearly, assume that the voltage drop across R2 is 40mv to obtain the specified load current. The current flowing through R8 is 400μA ((20.04-20)/100), and the current flowing through R9 and R4 (via Q3) is also 400uA. This current produces a voltage drop of 40mv (400uA×100Ω) across R4. The controller's Vout pin is connected to the internal 5V regulator (VDD) and R4, and the other end of R4 is connected to pin Csh. The voltage on the Vout pin of IC1 is 5v, and the voltage on the Csh pin is 5.04v. The voltage difference between these two pins is exactly the voltage drop on R2. The simple circuit shown in Figure 1 enables a conventional low output voltage current type buck regulator to achieve an efficiency greater than 95% when the load current is as high as 2.5A and Vout = 20v.