Boost converter works with wide-range negative-input supply

EL7515

Mike Wong, Intersil

EDN

Assume that a design requires positive voltage, but only a negative-voltage power source is available. Using a standard boost-converter IC in the circuit of Figure 1, you can efficiently generate a positive voltage from a negative source. The boost converter generates an output voltage that’s higher than the input voltage. Because the output voltage – 5 V in this example – is higher than the negative-input-voltage ground level, the circuit does not violate the boost-converter principle.


Figure 1.	By using its ground terminals as the negative-voltage input, a boost converter can efficiently generate a positive output voltage.

The circuit in Figure 1 uses the EL7515, a standard boost converter. The ground pins of the converter IC connect to a negative-voltage input source. Ground becomes the “positive” input source. V_OUT is as follows:

The Q₁ and Q₂ pnp transistors form a translator that scales the 5 V output voltage (referred to ground) to a feedback voltage referred to the negative input. The transistor pair also eliminates temperature-change and voltage-drop effects. As the negative input voltage decreases, Q₂ runs at an increasingly higher current than Q₁, causing additional transistor-offset mismatch.


Figure 2.	*The line regulation is within ±40 mV over the full range of negative inputs.*

For optimal line regulation, you should set Q₁ and Q₂ to operate at the same currents with the nominal input-voltage applied. Figure 2 shows the line-regulation results. The maximum output-to-input voltage difference must be within the boost converter’s internal power FET drain-to-source breakdown voltage (V_DS).


Figure 3.	*The output voltage varies by less than 14 mV over the full range of output currents.*

For the EL7515, the maximum V_DS is 18 V. For the 5 V output, the minimum (most negative) input voltage is –12 V. A 1 V safety margin compensates for the D₁ diode drop and any voltage spikes on the drain of the power FET. Figure 3 shows the load-regulation test results.


Figure 4.	*The efficiency of the circuit peaks at 81% for medium output current (200 mA).*

The maximum output current is a function of the input-to-output voltage ratio and current- limit setting of the boost converter. As Figure 4 shows, the circuit yields greater than 80% efficiency at 200-mA output.