Generate Negative Voltages with Buck Regulators

Sep 1, 2008 12:00 PM
By Zach Zhang, Staff Applications Engineer, Alpha & Omega Semiconductor, Sunnyvale, Calif.

Negative voltage sources are widely used in various applications such as computing systems, liquid-crystal display (LCD) source drivers and audio amplifiers. Using the additional secondary winding of an isolation transformer is the widely used approach to generate the negative voltage source for computing systems. But the isolation transformer may not be otherwise required in the application, plus regulation is poor.

The LCD source driver usually employs a charge pump to generate a negative voltage source due to its lowest cost. However, there are obvious drawbacks including limited power delivery and very bad regulation. In addition, a charge pump can only generate an output that is some integral times the input voltage.

In some high-end audio amplifier applications, the textbook method — buck boost — is applied. The lack of dedicated commercial buck-boost controllers and the difficulty in controlling the buck-boost (right-plane zero) prevent the method from being widely used. Thus, designing a high-quality negative voltage source is always a challenge in system applications.

However, integrated buck converters offer an easily implemented alternative. With power MOSFETs, a controller and even a diode integrated on the same die (or sometimes copackaged), an integrated buck converter needs only a few external passive components to form a complete dc-dc converter. Since current-mode control is employed, the compensation is faired easy, and various protection features have already been integrated on chip. These benefits ease design effort, testing and board-space requirements while improving reliability.

Buck-Boost-Type Converter

The classic solution is to configure an integrated buck converter as a buck-boost-type converter. Fig. 1 illustrates this configuration using the EZBuck IC as an example of the integrated buck converter. In this circuit, an extra error amplifier is needed to generate the positive feedback voltage. This error amplifier may not be very cheap because it requires low offset voltage and large dc gain.

Another issue with this implementation is the additional electrostatic discharge (ESD) diode between the switching node (L_X) and ground. To prevent damage due to the switching node caused by ESD during shipping or assembly, a tiny ESD diode is usually connected between the switching node and ground. With this ESD diode, when the switching node has a large negative potential caused by ESD, the additional electronics can be discharged to the ground of the IC, avoiding damage to the switching node.

When the EZBuck is configured as a buck converter, the ESD diode does not function because the switching node switches between V_IN and ground. When the EZBuck is configured as a buck-boost converter, the ESD diode is actually forward biased due to the negative output voltage and may be damaged by a large forward bias current.

Buck-Type Converter

An alternative to the buck-boost approach is to use the buck converter configured as a buck to generate a negative voltage. Basically, the IC uses the output voltage as its virtual ground and regulates the system ground to generate a negative voltage (Fig. 2). The basic operation is exactly the same as a positive-output buck converter.

The IC monitors the system ground, and the voltage difference between the system ground and the output voltage (the virtual ground) is amplified and compensated as the control voltage. At the beginning of each cycle, the PMOS switch is turned on by the clock and the inductor current is charged up by V_IN. The inductor current during PMOS turn-on is equal to the PMOS current and this information is sensed and amplified to compare with the control voltage.

When the current information is larger than the control voltage, the PMOS switch turns off and waits for the next cycle to turn on. During the PMOS turn-off, the current in the inductor freewheels through the diode. According to the inductor voltage and second balance, it can be obtained that:

V_IN 3 D 3 T = V_OUT 3 (1-D) 3 T.

Rearranging Eq. 1, the relationship between the input voltage and the output voltage is:

V_OUT = -V_IN [ D / (D - 1) ]

This formula is the same as for the buck-boost converter; so, why is it essentially a buck-type converter? Think of this circuit another way: This buck converter steps down a voltage equal to V_IN - V_OUT (the output voltage is a virtual ground) to -V_OUT. Relating the buck converter's input and output voltages:

(V_IN - V_OUT)D = -V_OUT.

Rearranging Eq. 3 shows the relationship between the input and output voltages and that it's the same as in Eq. 2.

Since this circuit is a buck-type converter and the IC ground (virtual ground) is equal to the output voltage, which is the most negative voltage, there is no need for an additional amplifier to generate the positive voltage and there is no potential damage to the ESD diode as with the buck-boost-type converter. And because this is a pure buck-converter operation, an integrated synchronous buck converter can be used as well if the duty cycle is small and high efficiency is sought.

Fig. 3 shows a practice example using the Alpha & Omega Semiconductor AOZ1016 to generate a -3.3-V power source with 2-A sourcing capability. Fig. 4 shows the startup waveform for a 2-A resistive load for the circuit in Fig. 3.

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