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Design Article

# Here's how to convert a positive 10 -- 50V input to a minus 10V output

## 3/19/2006 4:26 PM EST

Buck regulators are commonly used to efficiently step down a higher level, unregulated, positive input voltage to a regulated positive output voltage. Shown in Figure 1 is a simplified Buck regulator operating in continuous conduction (the inductor current always remains positive) mode. The output voltage (Vout) is equal to D x Vin, where D is the duty cycle ratio of the Buck switch Q1 and Vin is the input voltage. The duty cycle D is equal to Ton / Ts, where Ton is the on-time of Q1 and Ts is the switching frequency period (1/Fs).

Figure 1. Buck Regulator Operation

A Buck regulator can be reconfigured into a Buck-Boost regulator to convert a positive input voltage to a negative output voltage. Shown in Figure 2 is a simplified Buck-Boost regulator. The basic component configuration of a Buck regulator and a Buck-Boost regulator are very similar, with the inductor and the rectifier diode transposed. Since the main switch (Q1) is in the same location for each configuration, a Buck regulator IC can be used for either topology. For the Buck-Boost topology, the minimum switch rating (Q1) of the regulator is the difference between Vin and Vout. When Q1 is turned on, the input voltage (Vin) is applied across the power inductor (L1). Current in the inductor ramps up during the on-time. When Q1 is turned off the inductor current continues to flow through C1, the load resistance and D1, establishing a negative output voltage. During the next Q1 on-time the load is supported by the output capacitor. The magnitude of the output voltage for a Buck-Boost regulator is Vout = D x Vin / 1- D

Figure 2. Buck-Boost Regulator Operation

An interesting implementation of a Buck-Boost regulator utilizes a Buck regulator IC that operates with the Emulated Current Mode (ECM) principal. Using current mode control in higher voltage applications, the challenge is the measurement and scaling of the switch (Q1) current. This signal is used as the Pulse Width Modulator (PWM) modulating ramp. Propagation delays and switching transients make it almost impossible to use current mode control for high voltage applications where very small on-times are required. Even with the best design practices, the current sense and level shift circuits will add significant propagation delay.