Most common dc-dc power application circuits are designed for a positive buck
or a boost circuit. However, certain designs may require a negative buck or
boost topology. The problem usually arises because most board designers aren't
accustomed to thinking in terms of negative voltages.
The transfer function for a positive buck is VOUT/VIN
= D, and interestingly, the transfer function for a negative buck is –VOUT/
–VIN = D. Similarly, the transfer function for a positive boost
is VOUT/VIN = 1/(1 D), and for a negative boost it's –VOUT/
–VIN = 1/(1–D).
A negative buck regulator can easily be configured by starting with a standard
positive buck-regulator circuit and flipping the polarity of all the voltage
rails and power switches (Fig. 1). The
circuit in Figure 2 is an actual negative
buck regulator that converts a 7-V input to a –5.2-V output at 500 mA.
The purpose of the small-signal transistor (Q1) is to level-shift the feedback
voltage so that the output is regulated at –5.2 V. The MIC2288 is a 1.2-MHz
pulse-width-modulation (PWM) boost regulator in an SOT23-5 package with an internal
1.2-A peak current switch between the SW and the GND pins. So, there's no need
to connect an external npn power transistor Q2 as shown in Figure
1b.
The chip GND is sitting on –7 V (VIN), and feedback on the
MIC2288 will be 1.24-V higher than GND, so R3 will program 100 µA in the
collector lead. This current will produce approximately –4.6 V on the
emitter, and the base will go down to approximately –5.2 V (assuming Q1
s VBE = 0.6 V). R1 and C1 program an enable delay on the MIC2288
to make sure that input supply is fully stable before the negative buck converter
starts switching.
The maximum load current is limited by the switch handling capability on the
MIC2288. For higher load current applications, one can certainly employ a boost
controller that uses an external MOSFET or a bipolar switch. This circuit's
efficiency at 500 mA was just over 87%.
A negative boost converter can easily be configured by following the same steps
as for a negative buck converter. Figures 3a,
3b, and 4 show the positive boost, negative
boost after source and switch transformation, and an actual circuit for a –5.2-VIN
to –7-VOUT, 500-mA application. The MIC4690 is a 500-kHz PWM
non-synchronous buck regulator in an SO-8 package, with an internal 1.3-A peak
current switch between the VIN and the SW pins on the regulator.
Thus, there s no need to connect an external npn power transistor Q3 as shown
in Figure 3b.
The output voltage is programmed when 124 µA (1.24 V/10 kΩ) flows
through R1 and R2. Therefore, VOUT = –1.24 V + (46.4 kΩ
X 124 µA) = –7 V. The MIC4690 is enabled by pulling the SHDN pin
to the lowest potential voltage.
Again, the maximum load current is limited by the switch handling capability
of the MIC4690. For higher load-current applications, one can use a buck controller
that uses an external MOSFET or a bipolar switch. The efficiency of this circuit
at 500 mA was about 75%.
Ed. note: Ajmal Godil has moved on from Micrel Corp. as of publication. All queries can be sent to Marty Galinski at [email protected].
