lowpowerZONE Products for the week of July 9, 2012
Linear Technology Says…
LT3504: 40 V in, Quad 1 A Output Buck Switching Regulator with 100% Duty Cycle Operation
Linear Technology Corporation announced the LT3504, a quad 1A output, 40V step-down switching regulator with 100% duty cycle operation. The LT3504’s 3.2V to 40V input voltage range makes it ideal for automotive and industrial applications that require multiple outputs. Its internal 1.5A switches can deliver up to 1A of continuous output current per channel to voltages as low as 0.8V. The LT3504’s unique design includes an onboard boost regulator, enabling each channel to operate at 100% duty cycle, even with input voltage as low as 3.2V. This architecture makes the LT3504 well suited for applications requiring continuous output regulation, such as automotive cold-crank and stop-start scenarios. Switching frequency is user-programmable from 250kHz to 2.2MHz and synchronizable throughout the range, enabling the designer to optimize efficiency while avoiding critical noise-sensitive frequency bands. The combination of its 28-lead 4mm x 5mm QFN and high switching frequency keeps external inductors and capacitors small, providing a compact, thermally efficient footprint for multiple output applications.
The LT3504 utilizes four internal 1.5A switches for the step-down conversion and a 220mA switch for the internal boost circuit, with the necessary oscillator, control and logic circuitry integrated into a single die. Special design techniques and a new high voltage process deliver efficiencies up to 88% over a wide input voltage range, while the LT3504’s current mode topology enables fast transient response and excellent loop stability. Anti-phase switching reduces input ripple and robust short-circuit protection enhances system reliability. Other features include output voltage tracking, programmable undervoltage lockout and a power good pin to indicate when all outputs are in regulation.
Linear has been a leader in producing power IC products with a more than decent input voltage range, making them ideal for applications like those in automotives. The LT3504 (also, unfortunately, the part number for a successful four-channel LTU from Dae Instrument Corp) is a quad 40 V input 1 A (per channel) buck switching regulator with a 100% duty cycle of operation.
The actual input range is from 3.2 V to 40 V and each of the four outputs (from 1.75 A switches) can be set individually – with the usual external potential divider to the feedback pins – down to 0.8 V (the feedback and reference voltage). Switching is prevented when the input falls below 2.9 V or the die temperature is too high (>175ºC). With at least a 3.2 V input voltage and a temperature below 175ºC the part starts switching when the enable pin is set above 1.44 V, and the internal boost regulator (see later) charges an external SKY capacitor (typically 1 µF) 5 V above the input voltage. The output load capacitors can be ceramic and can typically be up to 22 µF for the lower output voltage settings. Each channel requires, of course, its own switch inductor.
Apart from the four output channels there is an additional fifth channel which acts as a boost regulator with its own 0.4 A switch and power Schottky together with the necessary drive logic.
The internal oscillator – which can be externally synchronized – is set by an external resistor to a frequency between a low of 200 kHz and a high of 2.4 MHz with an input sync range from about 250 kHz to 2.2 MHz. The clock produces two antiphase signals, each running at 50% duty cycles, with channels 1, 3, and 5 running in antiphase to channels 2 and 4 to reduce the output ripple on the loads. Internal compensation is included and there are also independent run and soft-start pins.
Output voltage tracking and sequencing are also included in the part and there is a power-good pin to indicate when all four channels are within regulation. Drop-out voltage is typically 400 mV at 1 A loads.
The maximum quiescent current of the part is about 4 mA with a shutdown current of typically 4 µA. Efficiencies depend on the difference between input and output voltages as well as the switching frequency but they are typically going to be above 70% and, in some cases, approaching 90% for loads higher than about 300 mA. The load regulation is better then 2% across all loads.
The LT3504 is ideally suited for automobile battery regulation applications but will also find uses in avionics, wall wort supplies, and industrial supplies. It does a great job in a small package and the design allows for a really small amount of total real estate for a four-channel solution.
The LT3504 is in production in a thermally-enhanced QFN-28 priced at $3.50 and $3.85 (for an extended -40ºC to +125ºC operating version) both in 1000-piece lots.