Regulating in cold crank conditions
18 November 2009
Linear Technology’s LT3689 is a 700 mA, 36 V input step-down switching regulator.
Its Burst Mode operation keeps quiescent current under 85 uA in no load standby conditions. The LT3689’s 3.6 V to 36 V input voltage range, with 60 V transient ride through, makes it ideal for load dump and cold crank conditions commonly found in automotive applications.
Its 1.2 A internal switch can deliver up to 700 mA of continuous output current at voltages as low as 0.8 V. The LT3689’s Burst Mode operation provides ultralow quiescent current, well suited for applications such as automotive or telecom systems, which demand always-on operation and optimum battery life. Switching frequency is user programmable from 350 kHz to 2.2 MHz, enabling the designer to optimise efficiency while avoiding critical noise-sensitive frequency bands. The LT3689’s 3mm x 3mm QFN-14 (or thermally enhanced MSOP-16) package and high switching frequency keeps external inductors and capacitors small, providing a very compact, thermally efficient footprint.
The integrated microprocessor supervisor functions support high reliability applications such as automotive electronic control units. The reset and watchdog timeout periods are independently adjustable using external capacitors. Tight accuracy specifications and glitch immunity ensure reliable reset operation of a system without false triggering. The open collector RST will pull down if output voltage drops 10% below the programmed value. The watchdog timer is pin-selectable for window or timeout modes. In timeout mode, WDO pulls low if too long a period passes before a watchdog transition is detected. In window mode, the LT3689 monitors for WDI falling edges grouped too close together or too far apart.
The LT3689 utiliSes a high efficiency 1.2A, 450 mV switch, with an integrated boost diode, oscillator, control and logic circuitry integrated into a single die. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping output ripple below 15 mVPK-PK. Special design techniques and a new high voltage process enable high efficiency over a wide input voltage range, and current mode topology enables fast transient response and excellent loop stability. Other features include external synchronization (from 350 kHz to 2.2 MHz), a power good flag and soft-start capability.
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