Abstract: The flexibility of a battery input combined with a MAX761 step-up DC-DC regulator allow this circuit to
generate ±5V from -2V to -12V input. The 2V input suits two-cell battery applications. The advantages are
the positive output voltage tracks the negative output voltage, the input voltage may range above and
below the positive output voltage, and there is no leakage path from input to output during shutdown.
Configured as in Figure 1, the step-up dc-dc converter IC1 and associated components produce ±5V from input voltages ranging from 2V to 12V. Input voltages are negative with respect to the output ground terminal. Transistor Q1 shifts the feedback voltage to a level compatible with the IC, which is about 1.5V relative to the chip's GND pin.
Figure 1. This regulator circuit produces ±5V from just two battery cells, whose terminal voltage may range above and below the positive output level.
By taking V+ from the highest voltage in the circuit (VIN + VOUT), the chip minimizes internal loss by maximizing the gate drive to its internal switching MOSFET. When this MOSFET (between LX and GND) turns off, the energy stored in T1's primary flows to the V+ output, generating a voltage across the primary equal to V+ plus a diode drop.
The -5V output is generated similarly by the additional winding plus D2 and C6. Regulation is via T1's 1:1 winding ratio, which causes the -5V output magnitude to track that of the 5V output. This negative-output generation isn't possible with the standard step-up topology (Figure 2) because neither winding would see a VOUT-proportional voltage.
Figure 2. The leakage path (dotted line) in this conventional step-up switching regulator prevents regulation when VIN exceeds VOUT.
Figure 1 offers two other advantages over the Figure 2 configuration. First, it remains in regulation when VIN rises above the nominal output level. In Figure 2, the inductor-diode leakage path forces VOUT to track VIN for this condition. In Figure 1, VIN is limited by IC1's absolute-maximum voltage rating: V+ to BATT- must not exceed 17V, so for VOUT = 5V the input range is 2V to 12V. Second, the Figure 1 circuit has no leakage path from input to output during shutdown. With 50kΩ output loads and R1 = 100kΩ, the total shutdown current is only 26µA.
The efficiency in Figure 1 is about 70%-a little lower than that of a standard step-up circuit (Figure 3). This efficiency data is based on VIN = 2.5V, representing two AA cells at 50% discharge. The circuit can start with 50Ω loads and a 2.0V input, but it can't quite regulate with that combination of input and load-the V+/V- outputs will sag to 3.88V/-3.68V.
Figure 3. Conversion efficiency for the Figure 1 circuit is about 70%, depending on the input voltage and the output loads.
Output noise (mostly fast spikes) is nominally 200mVP-P for a wide range of output loads. In addition, IC1's current-limited PFM (pulse-frequency modulation) control causes a variation in the frequency of output ripple and noise. If this is undesirable, IC1 can be replaced with the MAX752 dc-dc converter, whose current-mode PWM (pulse-width modulation) control produces a constant switching frequency (and somewhat lower efficiency due to higher quiescent current).
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