Zero IQ IC Protects Against Reverse Polarity Inputs

The LM74610-Q1 from Texas Instruments combines with an external NFET to configure a reverse polarity protection circuit. This configuration acts as an ideal “smart diode” that allows positive voltage inputs, but prevents negative inputs from reaching a load, as shown in Fig. 1. This basic smart diode circuit is not referenced to ground, so it has zero I_Q (quiescent current). In addition, it has low reverse leakage current and low power dissipation. This solution has increased complexity compared to other approaches, but exhibits lower power dissipation that eliminates the need for heatsinks or other heat reducing methods.

This new IC replaces two different techniques that have been employed in automotive and industrial applications requiring reverse polarity protection. One technique uses Schottky diodes (Fig. 2) and the other uses PFETs (Fig. 3), to protect the load in case a negative polarity input is received. In automotive systems this can occur due to wrong jump starts or inductive spikes.

Although Fig. 2 is a simple circuit, it has the disadvantage of high forward voltage (V_F) when the diode conducts. For example, the typical V_F of a Schottky diode is ~0.4V and can result in relatively high operating temperatures if conducting for a long time. Operating at 10 A, the diode power dissipation can reach 5 to 10 W, which could affect its reliability and require removal of heat to a safe level. Another disadvantage is the diode’s relatively high reverse leakage current.

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2. Schottky diode employed for reverse polarity protection allows positive inputs and rejects negative inputs from being applied to the load.

PFET solutions are inefficient for high load current and low input voltage. Plus, the PFET has higher I_Q and higher system cost. Using an NFET with a controller IC is a more effective and efficient substitute for the PFET in reverse polarity protection circuitry. The NFET’s lower R_DS(ON)results in a lower on state forward voltage power loss than a PFET.

The LM74610-Q1 and an external N-channel MOSFET can replace a diode or PFET reverse polarity solution in power systems. The voltage across the MOSFET source and drain is constantly monitored by the LM74610-Q1 anode and cathode pins. An internal charge pump provides the gate drive for the external MOSFET. Forward conduction is through the MOSFET 98% of the time and through its body diode 2% of time when energy is stored in an external charge pump capacitor Vcap (Fig. 4). This stored energy is used to drive the gate of the MOSFET. The voltage drop depends on the R_DS(ON) of the MOSFET. The LM74610-Q1 has no ground reference so it resembles a diode.

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3. PFET employed for reverse polarity protection allows positive inputs and rejects negative inputs from being applied to the load.

When power is initially applied, the load current (I_D) flows through the MOSFET body diode and produces a voltage drop (V_F) during t0, as shown in Fig. 5. This forward voltage drop across the body diode of the MOSFET is used to charge the charge pump capacitor, Vcap. During t0, the charge pump capacitor Vcap is charged to a 6.3V threshold (typical).

Once the voltage on the capacitor reaches 6.3V, the charge pump is disabled and the MOSFET turns ON. The energy stored in the capacitor is used to provide the gate drive for the MOSFET (t1 in Fig. 6). When the MOSFET is ON, it provides a low resistive path for the drain current to flow and minimizes the power dissipation associated with forward conduction. The power losses during the MOSFET ON state depend primarily on its R_DS(ON) and load current. When the capacitor voltage reaches its lower threshold, VcapL, 5.15V (typical), the MOSFET gate turns OFF. The drain current, I_D, will then begin to flow through the body diode of the MOSFET, causing the MOSFET body diode voltage drop to appear across the anode and cathode pins. Then, the charge pump circuitry is re-activated and begins charging Vcap.

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4. The LM74610-Q1 allows positive inputs to reach the load but prevents negative inputs from doing the same. Transient voltage suppressor TVS+ prevents the input from exceeding the 45V maximum voltage rating of the load. TVS- prevents the input from exceeding the -45V rating of the LM74610-Q1.

LM74610-Q1 operation keeps the MOSFET ON approximately 98% duty cycle (typical) regardless of the external charge pump capacitor value. This is the key factor to minimizing the power losses. The forward voltage drop during this time is limited by the MOSFET’s R_DS(ON).

Once the MOSFET is turned ON, the Anode and Cathode pins constantly sense the voltage difference across the MOSFET to determine the magnitude and polarity of the voltage across it. When the MOSFET is on, the voltage difference across Anode and Cathode pins depends on the R_DS(ON) and load current. If the voltage difference across source and drain of the external MOSFET becomes negative, it is sensed as a fault condition by Anode and Cathode pins and gate is turned off by Gate Pull Down pin (Fig. 4). The consistent sensing of voltage polarity across the MOSFET enables the LM74610-Q1 to provide a fast response to the power source failure and limit the amount and duration of the reverse current flow.

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5. Output voltage and VGS operation at 1A output current.

VcapH and VcapL are high and low voltage thresholds respectively that the LM74610-Q1 uses to detect when to turn the charge pump circuitry ON and OFF. The capacitor charging and discharging time can be correlated to the duty cycle of the MOSFET gate. Figure 6 shows the voltage behavior across Vcap. During the time period t0, the capacitor is storing energy from the charge pump. The MOSFET is turned off and current flow is only through the body diode during. The conduction though the MOSFET’s body diode is for a short time (2% typical), which rules out the chances of overheating the MOSFET, regardless of the output current. Once the capacitor voltage reaches its high threshold, the MOSFET turns off and charge pump circuity is deactivated until the Vcap reaches its low voltage threshold at t1.

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6. Vcap charging and discarding by the charge pump.

Place a ceramic capacitor between VcapL and VcapH. A capacitor value of 220nF to 4.7uF with X7R/COG characteristic and 16V rating or higher is recommended for this application. A higher value capacitor sets longer MOSFET ON time and OFF time; however, the duty cycle remains at ~98% for MOSFET ON time irrespective of capacitor value.

Important parameters in the selection of the external MOSFET are:

Maximum continuous Drain current I_D (must exceed the maximum continuous load current)
Maximum drain-to-source voltage V_DS(MAX) (must be high enough to withstand the highest differential voltage seen in the application)
Gate-to-source threshold voltage V_GS(TH) (≤3V)
Drain-to-source On resistance R_DS(ON)

Although there are no positive V_DS limitations, it is recommended to use MOSFETS with voltage rating up to 45V for automotive applications, since the LM74610-Q1 has a reverse voltage limit of -45V.

Features of LM74610-Q1 include: