When you need to quickly connect a negative power supply under logic control, the negative power-side switch in Figure 1 can help. Although originally intended for driving the gates of high-current MOSFETs, the MIC4451 can assume a different role. It provides complementary, low-on-resistance MOSFET switches to connect a system power-supply rail to a negative input voltage or to ground, enabled by a digital signal. The MIC4451 comprises an input buffer with a small amount of hysteresis and several logic inverter/buffers that ultimately drive a high-current output stage. Figure 2 shows a block diagram of the MIC4451. The on-resistance of the n- and p-channel devices at the output is approximately 1Ω. So, the output can connect a 100-mA load to the negative input voltage with less than 100-mV voltage drop. A noninverting version, the MIC4452, simplifies inversion of logic control as needed. Figure 1 shows details of the interface of the MIC4451 to TTL levels, using a common-base pnp transistor for level translation. The emitter current of Q₁ is approximately: I_E=(V_TTLH–V_BE)/R₁≈(2.4–0.65)/R₁, where V_TTLH is the TTL-high level, 2.4V. I_E should be ≤400 µA in accordance with TTL specs, so I_E=(2.4–0.65)/R₁≤400 µA.

Solving for R₁, you obtain R₁≥1.8V/400 µA=4.5 kΩ. The V_IH (lowest permissible high input) logic-level specification of the MIC4451 is 2.4V. Ignoring base-current errors, I_C≈I_E, so R₂I_C≥2.4V. Note that the MIC-4451's input voltage, V_IH, is specified with respect to the ground pin of the part. To determine R₂:R₂=2.4V/I_E=2.4V/0.4 mA=6 kΩ minimum.

You can comfortably choose real values for R₁ and R₂ somewhat higher than the worst-case limits calculated above, so choose R₁=5.1 kΩ and R₂=7.5 kΩ. Use 1% resistors to ensure worst-case logic levels are satisfied over temperature. Figures 3a and 3b show power-switching times when driven from TTL. The output bypass capacitor and on-resistance of the MIC4451 determine the rise and fall times. Figure 4 shows a simple circuit for sensing the level of a positive supply. The detection threshold, V⁺, is a function of the breakdown voltage of zener diode D₁, V_BE of Q₁, resistor values, and the input threshold of the MIC4451. Referring to Figure 4 and ignoring base-current errors, the collector current of Q₁ is approximately: I_C=I_A–I_B; I_C=(V⁺–V_Z–V_BE)/R_A–V_BE/R_B.

Choosing V⁺=7V and using a 5.6V zener diode with component values from Figure 4 allows you to solve for I_C: I_C=(7–5.6–0.65)V/1.8 kΩ–0.65V/3 kΩ=(0.416–0.216) mA=0.200 mA. Because the input threshold of the MIC4451 is typically 1.5V, this level detector trips when R_CI_C=1.5V, so rearranging: R_C=1.5V/0.2 mA=7.5 kΩ.

Figure 5 shows details of the operation of the negative power switch with positive-supply sensing. To sum up, a circuit intended for driving high-speed-MOSFET gates finds new use as a negative-power-supply switch. You can easily interface the MIC445x to logic-level control signals. You can use a simple circuit to detect the level of a positive supply voltage and connect a negative supply when the positive voltage has risen above a certain threshold level.

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