Cascode MOSFET increases boost regulator's input- and output-voltage ranges

Targeting use in portable-system applications that require raising a battery's voltage to a higher level, IC boost regulators often include output transistors that can drive storage inductors. However, most boost regulators' absolute-maximum input-voltage rating typically doesn't exceed 6V, an adequate level for battery operation. In addition, breakdown voltage of the regulator's output transistor limits the regulator's absolute-maximum output voltage to 25 to 30V, which may be too low for some applications.

You can extend a boost regulator's output-voltage range by adding an external transistor that has a higher breakdown voltage than the regulator. However, the internal design of a typical boost regulator's control circuitry often prevents direct drive of an external transistor's base or gate. As an alternative, you can add an external higher voltage transistor by connecting it in a cascode configuration.

Most boost regulators feature a peak-current-control method that reduces the number of external components and thus shrinks the overall pc-board area of the converter circuit. Figure 1 shows a boost regulator based on a TPS61040 boost controller, IC₁, which uses peak-current control.

Applying input voltage V_IN to IC₁'s V_CC pin and to one leg of inductor L₁turns on IC₁'s internal MOSFET switch, Q₁, allowing a gradually increasing amount of current to flow from V_IN through L₁, Q₁, and internal current-sense resistor R₁. The circuit's internal controller monitors the voltage across sense resistor R₁ and, upon reaching a predetermined current limit, turns off Q₁.

Interrupting the current through L₁ raises the voltage across the inductor and applies forward bias to diode D₁, which conducts and charges output capacitor C₁ to a higher voltage than would be available from the input voltage alone. The input voltage, L₁'s inductance, and the preset peak current through R₁ all affect Q₁'s on-time, and the output voltage sensed by IC₁'s FB (feedback) pin and its external components determines Q₁'s off-time. To maintain operation and set Q₁'s off- time, IC₁'s internal controller must monitor current through L₁ using Q₁ and R₁.

You can add a higher voltage MOSFET transistor, Q₂ (Figure 2), for applications that require an output voltage higher than the internal transistor's breakdown voltage. To maintain the circuit's current-flow path through L₁ and IC₁'s SW pin, you connect the external transistor in a cascode, or common-gate, configuration.

Q₂ comprises a low-on-resistance, low-gate-voltage-threshold MOSFET with the addition of diode D₂ between Q₂'s gate and source. To ensure the circuit's proper operation, V_CC—5V in this example—must exceed Q₂'s gate-threshold turn-on voltage. In operation, IC₁'s internal control circuit turns on Q₁, which pulls Q₂'s source close to ground level and turns on Q₂ with almost 5V of gate-to-source potential.

Current flows through inductor L₁, external transistor Q₂, internal transistor Q₁, and sense resistor R₁, and IC₁'s control circuit "sees" no difference with the installation of Q₂. Once the inductor current reaches its preset limit, Q₁ turns off, leaving Q₂ with no path for current to flow from its source. The voltage on Q₂'s drain rises rapidly to the desired output voltage plus the voltage drop across D₁. As the drain voltage rises, Q₂'s drain-to-source capacitance attempts to pull the MOSFET's floating source above 5V, which forward-biases D₂, connects IC₁'s SW pin voltage to 5V plus one diode drop, and clamps Q₂'s source to the same voltage.

A boost converter delivers a 180V output at 4 mA (V_OUT) to bias a laser circuit from a 9V power supply (V+). In this application, the 5V input supply need provide only enough current—typically, a few milliamperes—to drive IC₁'s internal logic and the gate of cascode MOSFET Q₂. You can use a dropping resistor and zener-diode voltage regulator (not shown) to supply the 5V requirement from the 9V supply. You can drive the inductor and IC₁ from a common power supply or from a separate source that's within Q₂'s breakdown-voltage rating. The cascode circuit also can produce any output voltage that's within Q₂'s drain-to-source breakdown-voltage rating. Specify other components with an appropriate voltage rating—for example, breakdown-voltage ratings of inductor L₁ and capacitor C₁ should safely exceed the desired output voltage.