Reverse Current, the Body Diode, and Output Discharge Features in LDOs

Reverse Current, the Body Diode, and Output Discharge Features in LDOs

ersaelectronics.com

Most LDOs are built around a pass transistor that is inherently vulnerable to current flowing in the wrong direction. When the input voltage collapses but the output is held up by a large capacitor, by another regulator, or by a battery, current can flow backward from output to input through the pass element or its parasitic body diode. This reverse current can discharge the output unnecessarily, stress the pass element, back-feed downstream circuits, and even damage the part or the upstream supply. A related set of features concerns what happens to the output when the regulator is disabled: a charged output capacitor must be discharged cleanly before the next startup. This article explains reverse-current paths, protection strategies, OR-ing outputs, and active output discharge.

1. Where Reverse Current Comes From

In a typical LDO the pass element is a PMOS or NMOS transistor fabricated on the die. A MOSFET has an intrinsic parasitic diode, the body diode, between its source and drain. In normal operation this diode is reverse biased and does not conduct. But if the output voltage rises above the input voltage by more than one diode drop, the body diode becomes forward biased and current flows from output to input regardless of the control loop. This situation arises in several common cases. The input source may be disconnected while the output capacitor remains charged. Another regulator sharing the output may keep the rail up. A battery on the output side may feed back through the rail. An input capacitor may discharge through the regulator into the output. In each case the body diode is the first path, and the loop cannot stop it because it controls the gate, not the diode.

1. The Consequences of Reverse Current

A small reverse current through the body diode is not always destructive, but it causes problems. It discharges the output capacitor faster than expected, which can matter in systems that rely on the rail to stay up briefly after input loss. It back-feeds the input rail, potentially powering circuits that were supposed to be off, which is a safety issue in medical or industrial equipment. It can lift the input voltage above its rated maximum if the output is held at a higher voltage. In OR-ing applications, reverse current from one output to another fights the other regulator and causes oscillation or excess dissipation. Over time, repeated reverse conduction can stress the pass element and shorten life.

1. Reverse-Current Protection Structures

Many modern LDOs include internal reverse-current protection. The common approach is to place a second MOSFET in series with the pass element, oriented so that its body diode is in the opposite direction, and to turn it off whenever the output exceeds the input. Some parts use an ideal-diode-like structure that blocks reverse conduction until the loop detects the fault. The datasheet specifies the reverse-current blocking capability, often as a maximum current that can be blocked without damage. Parts without this feature require external protection, typically a Schottky diode in series with the input or output, or an external ideal-diode controller. The tradeoff is that an external protection diode adds its own forward voltage drop, which erodes dropout margin, so a low-forward-voltage Schottky or an active ideal diode is preferred.

1. OR-ing LDO Outputs

OR-ing two regulator outputs to drive one load is sometimes used for redundancy or for combining different sources. Because each regulator sees the other's output through the load, reverse-current blocking is essential. A simple OR of two LDO outputs with external Schottky diodes works, but the diodes drop voltage and reduce efficiency. Active OR-ing with ideal-diode controllers avoids the drop but adds complexity. The important design rule is that each LDO must either include internal reverse-current blocking or be protected externally, and the ground returns must be arranged so that current cannot circulate between the two regulators through the ground path.

1. Output Discharge When Disabled

When an LDO is disabled, its output usually falls slowly because the output capacitor must discharge through the load or through a high internal leakage path. In many systems a slow output fall is harmless, but in others it is not. If a processor requires its core supply to fall quickly to a defined state before the next startup, a slow decay can cause a false power-up or violate sequencing. If the output drives a sensor or a switch that must be off within a defined time, a slow decay leaves the circuit partially energized. To address these cases, many LDOs include an internal active discharge switch: a small MOSFET from output to ground that turns on when the regulator is disabled, pulling the output down rapidly. The discharge current is limited, typically to tens or hundreds of milliamps, sized to discharge the recommended output capacitor within a specified time.

1. Choosing Between Active Discharge and Floating Output

Active discharge is useful, but it is not always desirable. If the output must remain held up by a backup source when the regulator is disabled, an active discharge switch would fight that source and discharge the backup. In such applications the designer selects a part without active discharge, or one where the discharge function can be disabled, so that the output floats when the regulator is off. The choice depends on the system: sequencing and fault-clearing applications benefit from active discharge, battery-hold and backup applications require a floating output.

1. Practical Checks

When designing with reverse-current and discharge behavior in mind, several checks prevent surprises. First, identify every way the output can be held above the input: large capacitors, backup batteries, other regulators, and charged upstream nodes. Second, confirm whether the chosen LDO blocks reverse current internally, or add external protection. Third, decide whether the output must fall quickly when disabled, and select active discharge accordingly. Fourth, calculate the discharge time with the actual output capacitor and the rated discharge current, and confirm it meets the system requirement. Fifth, check the ground return path, because reverse current can also flow through shared ground traces even when the pass element is blocked.

Summary

• The intrinsic body diode of the pass transistor conducts reverse current when the output rises above the input, regardless of the control loop.
• Reverse current discharges output capacitors, back-feeds inputs, and can stress the pass element; it must be blocked in many systems.
• Choose an LDO with internal reverse-current protection, or add an external ideal diode or low-forward-voltage Schottky.
• OR-ing LDO outputs requires reverse-current blocking on each regulator and careful ground-return design.
• Active discharge switches pull the output down quickly when the regulator is disabled, which is useful for sequencing and fault clearing.
• If the output must be held up by a backup source when disabled, choose a part without active discharge or with the function disabled.
• Identify all reverse-current paths, including through ground, before finalizing the design.