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36-V step-down switching regulator and integrated LDO is ideal for automotive load dump applications

May 13, 2011 // Paul Buckley

36-V step-down switching regulator and integrated LDO is ideal for automotive load dump applications

Linear Technology Corporation has introduced a 500 mA (IOUT), 36 V step-down switching regulator and integrated LDO packaged in a 3 mm x 3 mm QFN or MSOP-12E. The LT3645 operates from a VIN range of 3.6 V to 36 V with transient protection to 55 V, making it ideal for load dump and cold-crank conditions found in automotive applications.

The device’s internal 0.75 A internal switch can deliver up to 500 mA of continuous output current to voltages as low as 0.8 V. The LT3645 uses a constant switching frequency of 750 kHz to minimize noise while optimizing efficiency. In addition to the primary switcher channel, the integrated LDO can be used for an additional low noise output of up to 200 mA of continuous current at voltages as low as 0.8 V.

Powering the LDO from the primary switching output offers both high efficiency and low noise for applications such as microprocessor core voltages. The LT3645 provides a compact, thermally efficient dual output solution, with its 3 mm x 3 mm QFN-16 package (or thermally enhanced MSOP-12E) and high switching frequency, which keeps external capacitors and inductor small.

The LT3645’s main switcher utilizes a high efficiency 750 mA, 450 mVCESAT switch, with the necessary oscillator, control, logic circuitry and LDO integrated into a single die. Special design techniques enable high efficiency over a wide input voltage range while its current mode topology enables fast transient response and excellent loop stability.

A current-limited DA pin offers an additional level of protection to improve overall reliability. The linear regulator operates from the VCC2 pin at voltages ranging from 1.2 V to 16 V with only a 310 mV dropout voltage. Other features of the LT3645 include a power good flag, soft-start and thermal shutdown.

The LT3645EUD is offered in a 3 mm x 3 mm 16-lead QFN package and is priced at $2.25 each. The LT3645EMSE is offered in a 12-lead, thermally enhanced MSOP package and is priced at $2.35 each. Industrial grade versions, namely the LT3645IUD and the LT3645IMSE, are tested and guaranteed to operate from a -40 to 125°C operating junction temperature and are priced at $2.48 and $2.59 each, respectively. The LT3645HMSE is tested and guaranteed to operate from a -40 to 150°C operating junction temperature and is priced at $2.84 each. All prices are for 1,000-piece quantities and all versions are available from stock.

More information about the LT3645 at

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