Key Specs
| Spec | Value | Condition | Source |
|---|---|---|---|
| Current Startup | 100 µA | Digi-Key | |
| Mode | Average Current | Digi-Key | |
| Mounting Type | Surface Mount | Digi-Key | |
| Operating Temperature Range | -40°C ~ 125°C | Digi-Key | |
| Package Case | 20-SOIC (0.295”, 7.50mm Width) | Digi-Key | |
| Supplier Device Package | 20-SO | Digi-Key | |
| Switching Frequency (Typ) | 100kHz | Digi-Key | |
| Voltage Supply | 14.5V ~ 19.5V | Digi-Key |
When To Use
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14.8V automotive accessory supply → 5V @ 1A: The 14.5V to 19.5V input range matches typical automotive accessory rails post-ignition, ensuring stable operation without undervoltage lockout. Using a synchronous buck controller here could cause shoot-through under transient load conditions if not carefully designed, whereas this part’s integrated control avoids that risk.
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Battery-powered industrial sensor → 12V @ 0.5A: The low startup current of 100 µA average current minimizes battery drain during power-up sequences, critical in intermittent operation. A high-frequency buck controller with higher quiescent current would cause premature battery depletion and possible latch-up due to thermal stress from continuous switching.
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Mid-power LED driver from 15V bus → 9V @ 0.8A: The fixed switching frequency of 100 kHz limits EMI and simplifies filtering in lighting applications, while the SOIC-20 package fits compact PCB layouts. Using an isolated flyback would add unnecessary complexity and cost, and could introduce thermal runaway risk without proper isolation design.
When Not To Use
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>3A output current load: The internal current capacity is insufficient, risking thermal runaway and device destruction. Use a multi-phase buck controller designed for higher current distribution and thermal management.
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Battery-powered sensor requiring μA sleep currents: The average current at startup is 100 µA, which is too high for ultra-low power standby modes and will drain small batteries rapidly. Use a low-IQ PFM buck optimized for sub-μA quiescent current.
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Compact handheld device needing >500 kHz switching frequency: The fixed 100 kHz switching frequency limits inductor size reduction and transient response speed. Use a high-frequency buck controller to achieve smaller magnetics and faster dynamic response.
Application Notes
The L4981BD’s high-side MOSFET driver output node switches at the switching frequency of 100 kHz and requires the smallest possible loop area to minimize EMI and switching noise. Careful PCB layout is essential to keep the bootstrap capacitor and the high-side driver loop compact.
The ENABLE pin and the current sensing inputs are noise-sensitive; routing these signals away from high-current switching traces and using proper filtering is recommended to ensure stable operation.
At typical operating conditions within the specified temperature range of -40°C to 125°C and supply voltage of 14.5 V to 19.5 V, a heatsink is generally not required for the L4981BD in surface mount 20-SOIC package. However, thermal considerations must be evaluated based on the actual load current and ambient conditions.
Gotchas
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Startup current vs. load assumption: Designers often assume the 100 µA startup current is negligible in all battery-powered designs. If the load is absent or very light at startup, the regulator may fail to reach stable feedback voltage causing oscillation or dropout. Fix by ensuring a minimum load resistor or dummy load during startup.
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Bootstrap capacitor ESR impact on switching: Using a bootstrap capacitor with excessively high ESR can cause intermittent high-side MOSFET drive failure, leading to erratic switching and audible coil noise. Measure ESR with an LCR meter and select a ceramic X7R capacitor rated for high ripple current.
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Feedback pin noise coupling: Placing the feedback sense traces adjacent to the SW node or power ground returns can cause output voltage ripple and unstable regulation due to injected switching noise. Use a dedicated quiet ground star point and shield feedback traces with ground guard rings.
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Thermal derating above 100°C: Although the operating range extends to 125°C, continuous operation near this maximum junction temperature without adequate PCB thermal management leads to accelerated device aging and possible latch-up events. Verify junction temperature under worst-case load using thermal imaging and increase copper area or heatsinking accordingly.