Normally the output voltage of a switching power supply is higher than the error amplifier reference, and so a simple resistive divider between VOUT and ground at the non-inverting input of the PWM error amplifier is all that’s required to set the regulation voltage. However, when VOUT is less than the error amplifier reference voltage, the feedback voltage must be divided up instead of down. A simple technique for feeding back an output voltage is lower than the PWM internal error amplifier reference voltage is shown. Practical design equations are developed and applied to a simple application schematic.
For low-voltage, high-current power supply applications, the gate drive requirements of a switching power supply become especially critical. As several MOSFET devices are often placed in parallel to meet the high current specifications of a particular design, the convenience of a single device controller and driver solution is no longer a viable option. MOSFETs are placed in parallel to lower the overall drain-to-source on resistance, resulting in lower conduction losses. However, as more devices are placed in parallel, the gate charge requirements quickly add up. Since the internal impedance of the MOSFET is much lower than the internal impedance of the driver stage, most of the power losses associated with driving parallel combinations of MOSFETs seen in the form of dissipated heat within the controller device. As such, the driver stages of many single chip solutions simply are not adequate for efficiently driving higher gate charges resulting from parallel combinations of MOSFETs. In response, the industry has recently seen an increase in advanced MOSFET driver product offerings. Many of these new drivers include drive current capability much higher than otherwise available in a single chip solution. With the driver device placed closer to the MOSFET gates, higher drive current means more parallel MOSFETs can be driven more efficiently. In addition to increased drive current, many of today’s advanced MOSFET drivers also use sophisticated control techniques to precisely control the timing between two switches, such as those found in a synchronous buck application.
Using an external MOSFET driver along with a separate pulse width modulator (PWM) controller allows power supply designers the flexibility needed to meet the high performance gate drive demands of these types of low voltage, high current power converters. The combinations of features that can arise from this approach are seemingly endless given the variety of currently available PWM controllers and drivers. As output voltages approach the sub 1-V level, power supply control manufacturers have responded by recently introducing products that include appropriate internal low-voltage references. But what if a designer wishes to use a high performance driver along with a PWM that includes an internal reference higher than the feedback voltage? In other words, to regulate an output voltage of 1 V would normally require a voltage reference of 1 V or less, available at the non-inverting input of the PWM internal error amplifier.
The application circuit shown in Figure 1 proposes an alternate method for feeding back an output voltage lower than the PWM internal error amplifier reference voltage. Normally the output voltage is higher than the error amplifier reference, and so a simple resistive divider between VOUT and ground sets the regulated voltage at the non-inverting input of the PWM error amplifier. However, when VOUT is less than the error amplifier reference voltage, the feedback voltage must be divided up instead of down. Dividing up implies that some additional voltage must be added to the feedback from another regulated voltage source.
Figure 1;
