PB137ACV

The PB137ACV from STMicroelectronics is a specialized positive voltage regulator designed specifically for lead-acid battery charger applications. It provides a fixed 13.7V output at up to 1.5A, which corresponds to the correct float charge voltage for a 12V lead-acid battery. The device is housed in a 3-pin TO-220 through-hole package and incorporates comprehensive protection features including internal current limiting, thermal shutdown, and safe area protection.

The 13.7V output voltage is the key specification that defines this device. For a 12V lead-acid battery (which typically consists of six 2V cells in series), the float charge voltage at 25C is approximately 13.5-13.8V. The PB137’s 13.7V output with plus/minus 1% tolerance (13.56-13.84V at 25C) falls precisely within this range, making it an ideal single-component solution for maintaining a lead-acid battery at full charge without overcharging.

The most distinctive feature of the PB137 is its extremely low reverse leakage current. When the input voltage is floating (i.e., the charger is unplugged) and the output is connected to the battery at 13.7V, the reverse leakage current is less than 10uA over the 0-40C temperature range. This is critical because any reverse leakage would slowly discharge the battery through the regulator when the charger is disconnected. Standard linear regulators can have reverse leakage currents of several milliamps, which would drain a lead-acid battery over time. The PB137’s sub-10uA reverse leakage ensures negligible battery drain.

The internal current limiting is set to approximately 2.2A (typical at VI-VO=5V, TJ=25C), which protects both the regulator and the battery from excessive charging current. When the battery is deeply discharged and its terminal voltage is low, the voltage difference across the regulator (VI-VO) is large, and the current would be very high without limiting. The PB137 automatically limits the current to a safe level, and as the battery charges and its voltage increases toward 13.7V, the voltage drop across the regulator decreases and the charging current naturally tapers off.

The thermal shutdown and safe area protection work together to prevent damage under fault conditions. If the junction temperature exceeds the thermal shutdown threshold (typically around 150C), the output current is reduced, limiting the power dissipation. The safe area protection ensures that the device operates within its safe operating area (SOA) under all combinations of voltage and current, preventing secondary breakdown. These protections make the device essentially indestructible under normal operating conditions.

The dropout voltage of 2.1V typical at 1A means the input voltage must be at least 15.8V (13.7V + 2.1V) to maintain regulation at 1A output. For the full 1.5A output, the dropout voltage increases, requiring an input voltage of at least 16-17V. The maximum input voltage of 40V provides adequate margin for unregulated transformer/rectifier supplies that may have significant ripple and voltage variation.

The PB137ACV is the plus/minus 1% tolerance version (AC suffix). The standard PB137CV version has a wider tolerance. The device has been marked as Obsolete by STMicroelectronics, meaning it is no longer in active production. Designers should consider alternative solutions for new designs, although the PB137ACV may still be available from distributor stock or specialized suppliers.

For lead-acid battery charger applications, the PB137ACV provides the simplest possible design: connect the input to a rectified and filtered AC supply (16-28V typical), add an input capacitor (1uF minimum) and an output capacitor (10uF recommended), and the battery is charged at the correct float voltage with built-in protection. No external current limiting, no voltage setting, and no temperature compensation are required for basic float charge applications.

The PB137ACV operates as a series-pass linear voltage regulator optimized for lead-acid battery float charging.

Series-Pass Regulator Architecture: The PB137 uses a series-pass element (an internal PNP transistor in a Darlington or compound configuration) between the input (VI) and output (VO). The pass element acts as a variable resistor controlled by a feedback loop to maintain the output voltage at 13.7V regardless of input voltage variations and load current changes. The difference between the input and output voltages (VI – VO) appears across the pass element, and the power dissipated is P = (VI – VO) x IO. This power dissipation is why the device is packaged in a TO-220 package and requires a heat sink for currents above approximately 500mA.

Voltage Reference and Error Amplifier: An internal bandgap voltage reference provides a stable reference voltage. The output voltage is divided down by an internal resistor divider and compared to the reference by an error amplifier. The error amplifier drives the pass element to maintain the output voltage at 13.7V. The reference and divider are trimmed during manufacturing to achieve the plus/minus 1% output voltage accuracy. The 13.7V target is specifically set for lead-acid battery float charging.

Current Limiting: An internal current sense circuit monitors the output current. When the current exceeds approximately 2.2A, the current limiting circuit reduces the drive to the pass element, limiting the current to a safe level. The current limit is designed to protect both the regulator and the battery. The foldback characteristic of the current limiting means the current is further reduced under short-circuit conditions, limiting the power dissipation in the pass element.

Thermal Shutdown: A temperature sensor on the die monitors the junction temperature. When TJ exceeds the thermal shutdown threshold (approximately 150C), the pass element is turned off or its drive is significantly reduced, limiting the output current and preventing further temperature rise. The thermal shutdown has hysteresis, so the device does not restart until TJ drops below a lower threshold (typically 10-20C below the shutdown point). This prevents thermal oscillation.

Safe Area Protection: The safe area protection circuit limits the power dissipation in the pass element as a function of the voltage across it (VI – VO). At higher voltage drops, the maximum allowed current is reduced, keeping the operating point within the safe operating area (SOA) of the pass transistor. This prevents secondary breakdown, which can occur in power transistors when high voltage and high current are present simultaneously.

Reverse Leakage Suppression: The key differentiating feature of the PB137 is its extremely low reverse leakage current when the input is floating and the output is held at 13.7V by the battery. In a standard linear regulator, reverse current can flow from the output back through the pass element or the internal bias circuitry to the input when the input voltage is lower than the output. The PB137 incorporates a special circuit architecture that minimizes this reverse path, achieving less than 10uA reverse leakage at TJ=0-40C. This is typically achieved by using a PNP pass element that naturally blocks reverse current (the collector-base junction is reverse-biased when VO > VI), combined with careful design of the bias circuitry to prevent sneak current paths.

Dropout Voltage: The dropout voltage (2.1V typical at 1A) is the minimum voltage difference between VI and VO required for the regulator to maintain regulation. This voltage represents the saturation voltage of the pass element plus the headroom needed by the control circuitry. The relatively high dropout voltage (compared to modern LDO regulators) is acceptable in battery charger applications because the input is typically derived from a transformer/rectifier supply with ample voltage headroom.