highpowerZONE Products for the week of July 23, 2012
Linear Technology Says…
LT3975/LT3976: 42 V, 2.5 A & 5 A, 2 MHz Buck Dc-Dc Converters
Only 2.7 µA Quiescent Current
Linear Technology Corporation announced the LT3975, a 42V step-down switching regulator that can deliver 2.5A of continuous output current and requires only 2.7µA of quiescent current. Similarly, the LT3976 can operate from a 40V input, delivers up to 5A of output current and requires only 3.3µA of quiescent current. Both devices offer a 4.2V to a nominal 40V input voltage range, making them ideal for automotive and industrial applications. Their internal high 75mOhm switches deliver efficiencies as high as 90%. The LT3975’s and LT3976’s Burst Mode operation offers ultralow quiescent current, a feature that is also well suited for automotive and industrial systems applications that demand always-on operation and optimum battery life. The devices’ unique design maintains a minimum dropout voltage of only 500mV when the output voltage drops below the programmed output voltage, a functional necessity in applications subject to automotive cold-crank conditions. Switching frequency is user-programmable from 200kHz to 2MHz, and synchronizable from 250kHz to 2MHz, enabling the designer to optimize efficiency while avoiding critical noise-sensitive frequency bands. The combination of their 16-lead thermally enhanced MSOP package and high switching frequency keeps external inductors and capacitors small, providing a compact, thermally efficient footprint.
The LT3975 and LT3976 use a high efficiency 75mOhm switch, with the 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 15mVPK-PK. Special design techniques and a new high voltage process deliver high efficiency over a wide input voltage range, and their current-mode topology enables fast transient response and excellent loop stability. Other features include a power good flag, soft-start capability and thermal protection.
These are more products from Linear focusing on the automotive market. The last that we looked at was the quad 1 A buck regulator, the LT3504, which is in the same vein as these new parts. Both the LT3975 and the LT3976 are 200 kHz to 2 MHz (synchronizable from 250 kHz to 2 MHz) constant frequency buck dc-dc regulators with the former offering an input voltage range from 4.3 V to 42 V and the latter up to 40 V. The LT3975 is a 2.5 A part with a no-load quiescent of 2.7 µA (12 V input to 3.3 V output) while the LT3976 is a 5 A part with a no-load quiescent of 3.3 µA. Both have a shutdown current of about 700 nA and a nominal 1.2 V feedback (also the internal reference voltage) topology with a dropout voltage of 500 mV when the input voltage falls below the programmed output voltage – during cold crank, for example. The output ripple is less than 15 mVpp, minimized by the burst mode operation at lower loads. An enable pin shuts down the parts if it falls below 1.02 V.
Both parts have a power-good flag (at 91.6% of programmed output voltage), soft-start capability (external capacitor set), thermal shutdown protection, and current limit foldback (with soft-start override). The on-chip npn bipolar switch is powered by either the input rail or the boost voltage (from an external capacitor) and exhibits an Rds(ON) of 75 mΩ and there is also a built-in Schottky switch diode. The control circuitry in the IC is powered by an internal regulator.
Efficiency, as ever, depends on the difference between input and output voltages, as well as load current. For the LT3976 the typical efficiency for a 12 V to 3.3 V conversion peaks a little above 85% for loads between 500 mA and 2.5 A, falling to about 82% at full load. Good loop stability and fast transient response is ensured by an internally-compensated current-mode architecture and the LT3976 exhibits a typical turn-on time of 120 ns and turn-off time of 150 ns.
The LT3976 will, in particular, find homes in automotive battery regulators and in avionic applications, as well as industrial power supplies; it will sell extremely well. Apart from the MSOP noted in the data sheet the part is also available, apparently, in thermally-enhanced QFN-24.
The LT3975 and LT3976 are in production in thermally-enhanced MSOP-16 with pricing at $3.41 and $4.13, respectively, for industrial temperature range (-40ºC to +125ºC) versions, and $3.66 and $4.38 for extended -40ºC to +150ºC versions.
Data Sheet (LT3976)