Linear μModule regulators are perfect for point-of-load supplies because their easy-to-drop-in, compact, all-in-one design. They fit tight spaces with minimal engineering effort—only a few components are required in addition to the µModule package itself. Any step-down µModule regulator can can also be used to effortlessly produce a negative voltage solution, while retaining the usual easy design and low parts count advantages inherent to the µModule regulator.
Three Steps to Negative Voltage Output
The three simple design steps below convert a buck topology to an inverting buck-boost topology, yielding a negative output rail.
- Connect the μModule regulator's VOUT pins to the system ground (PGND). This creates the grounded inductor configuration required for the inverting buck-boost converter.
- Power the converter by connecting the input supply between the μModule regulator's VIN pins and PGND.
- The μModule regulator's GND pins now become the negative output rail (–VOUT). The load is connected between this negative output and PGND.
As shown in the standard buck configuration of Figure 1(a), just a few input and output capacitors are required to produce a fully functioning step-down DC/DC converter using μModule regulator, such as the LTM4601. In this configuration, the μModule regulator's output pads are connected to their corresponding voltages, i.e., VOUT pads are connected to the external VOUT, and the GND pads are connected to external GND. If the external connections are changed to the configuration shown in Figure 1(b), a buck μModule regulator becomes an inverting buck-boost converter. Here the μModule regulator's VOUT pads are connected to the external GND, while the GND pads become the negative VOUT. Thus the μModule regulator GND refers to –VOUT.
Level-Shifting the Control Signals
Configuring the control circuitry requires a little more consideration than the power stage because of the difference in ground levels (PGND and GND). The µModule regulator references pin voltages with respect to its own GND pins, but the output rail voltage it provides is with respect to the system ground (PGND). In this configuration, communicating any PGND-referenced external control signals (such as RUN, TRACK, PGOOD, etc.) to the µModule regulator, requires translating the signals to the µModule regulator's ground reference, GND (–VOUT).
The circuit examples below show how to level shift system ground-referenced signals for RUN, PGOOD and TRACK pin compatibility.
RUN Level Shift
The RUN pin input allows a µModule regulator to be turned on and off by applying an Enable signal to the pin. In many cases the module can be enabled by the presence of VIN with a pull-up resistor between the RUN pin and VIN, but if the RUN pin is to be controlled via external PGND referenced signal, extra consideration is needed.
Imagine first what would happen if we were to apply a PGND referenced Enable signal (VEN) directly to the RUN pin of our negative converter without the use of a level shifting circuit. Since the voltage the µModule regulator sees on its RUN pin is VEN + |–VOUT|, then (depending on the set output voltage) we could potentially exceed the pin’s abs. max. voltage rating (and cause damage to the part). Also, once the µModule regulator is turned on, the Enable signal may not be sufficiently low to turn the µModule regulator off. Therefore, we need a circuit to translate the PGND referenced Enable signal to levels appropriate for the µModule regulator's RUN pin.
The simple circuit in Figure 3 takes a PGND referenced Enable signal and level shifts it down to a voltage level appropriate for RUN pin. In the circuit when the Enable signal is high, PNP transistor Q1 is turned on generating a bias current with Rb, Rc to bring the RUN pin voltage above its threshold and turn the µModule regulator on. The RUN pin max voltage is clamped by D1 to prevent exceeding the pin’s abs. max rating. When the Enable signal is low, Q1 is off and resistor R3 discharges the RUN pin below its threshold to turn the µModule regulator off. Note resistor R3 and/or Zener D1 may already be included internal to the µModule regulator package, see datasheet for details.
Figure 4 illustrates the setup of the RUN pin level shifting circuit used with the LTM4609 in a negative output configuration. VIN = 10V, –VOUT = –12V @ 2A.
PGOOD Level Shift
The PGOOD output pin indicates whether the µModule regulator output voltage is within regulation (PGOOD High) or not (PGOOD Low). Internal to the module PGOOD pin is an open drain mosfet, so a pull up resistor to a bias voltage is required. If the PGOOD function is to be used in a negative output configuration the PGOOD signal seen by the µModule regulator must be appropriately level shifted up to a PGND-referenced signal for further use in the system.
Imagine first what would happen if we were to use the module’s PGOOD output signal directly without any level shifting circuit. With a PGND referenced pull up supply (VS) on PGOOD, since the max voltage the module sees on its PGOOD pin is VS + |–VOUT|, then (depending on the set output voltage) the pin’s abs. max rating can be exceeded. Also in this setup, when module PGOOD pin signals low, the PGOOD signal with respect to PGND may not be suitable for use in the system (i.e. PGOOD low signal level < system ground potential). We need a circuit to translate the module’s PGOOD signal to a PGND referenced signal for further use in the system.
The simple circuit in Figure 5 takes the µModule regulator's GND pin referenced PGOOD signal and level shifts it up to PGND referenced signal PGOOD2 which is appropriate for further use in the system. In the circuit, when the module outputs a PGOOD high signal, PMOS M4 is turned off and R2 pulls up PGOOD2 high to supply voltage Vs. Zener diode D1 protects the PGOOD pin from exceeding its abs. max. rating. When the module outputs a PGOOD low signal, M4 is turned on and pulls PGOOD2 low to system ground. Note that zener diode D1 may be omitted if VS + |–VOUT| < PGOOD abs. max. rating.
The following illustrates the setup of the PGOOD pin level shifting circuit used with the LTM4618 in a negative output configuration. VIN = 12V, –VOUT = –5V @ 3A.
TRACK Level Shift
The TRACK pin input allows a µModule regulator to track the output voltage ramp of another supply rail by feeding a tracking signal (i.e., a divided-down version of the master µModule regulator’s output voltage rail) to the slave µModule regulator’s TRACK pin. The TRACK pin voltage seen by each µModule regulator’s internal controller is with respect to its µModule GND pins, but in applications where two negative output converters are to be configured for tracking, the µModule regulator’s GND pins can be at different potentials. The difference in master and slave GND pin potentials means that the track signal provided by the master cannot be directly applied to the slave’s track pin. In these types of applications, extra consideration is needed to make sure the slave phase sees a properly-referenced tracking signal from the master phase.
In the simple circuit in Figure 7 the master µModule’s GND pin referenced tracking signal (Vo1Div – negVo1) is provided to the input of a differential amplifier. The differential amplifier is powered by a supply that is referenced to the slave µModule regulator’s GND pins (in this case it is powered by the slave’s own INTVCC regulator). Because the amplifier is referenced to the slave µModule regulator’s GND pins, its output signal is a level-shifted version of the input tracking signal, properly referenced to the slave’s GND pins. The amplifier’s output signal is then applied to the slave’s TRACK pin allowing the slave to track the movement of the master’s output rail voltage.
Figure 8 illustrates the tracking performance of two LTM4618 µModule regulators configured in negative output configurations using the track pin level shifting circuit. VIN = 12V, –VOUT1 = –5V @ 2A, –VOUT2 = –2V @ 2A.
μModule regulators allow designers to produce negative output applications nearly as effortlessly as positive ones. In fact, any standard step-down μModule regulator demonstration board can be easily configured for negative output applications. This article summarizes control circuitry design considerations that arise from the difference in relative ground levels between power ground and the µModule GND (–VOUT).