highpowerZONE Products for the week of December 12, 2011

Linear Technology Says…

LTM8047/8048: 1.5 W Isolated Low Noise Dc-Dc µModule Converters Include Transformer

Linear Technology Corporation introduced the LTM8047 and LTM8048 1.5W output DC/DC µModule converters with 725VDC galvanic isolation in a 9mm x 11.25mm x 4.92mm BGA (ball grid array) package. All components, including the transformer, control circuitry and power switches are housed in a small enclosed BGA package for superior interconnect reliability in high vibration applications. These compact and reliable products break ground loops in applications such as industrial, avionics and instrumentation equipment. Both devices operate from an input voltage of 3.1V to 32V, delivering a regulated output voltage on the secondary side, adjustable from 2.5V to 12V (LTM8047) and 1.2V to 12V (LTM8048). The LTM8048 includes a low noise linear post regulator that reduces the output ripple noise to 20µVrms at 300mA.

The LTM8048’s isolated post regulator delivers an output voltage accuracy of 2.5%. The low noise performance of the LDO delivers a clean supply rail, improving the performance of high accuracy mixed signal converters. Additional features available on both products include overcurrent protection and adjustable soft start.

EN-Genius Says…

The LTM8047 is a galvanically-isolated dc-dc converter with a 3.1 V to 32 V input range and a regulated output that can be set between 2.5 V and 12 V with a single external resistor. It can deliver up to 440 mA of output current. The isolation is provided by a transformer which is driven by a current-mode controller.

Frequency of the drive is variable and increases as the load increases (no details are given in the data sheet). At low loads the current through the transformer may be discontinuous. Isolation is rated at 725 Vdc (not 725 Vac rms) which is verified by the application of +725 V and then -725 V, both for one second. The µModule that houses the LTM8047 contains the controller, power switching element, transformer, power Schottky diodes for secondary rectification and 0.1 µF capacitance on the input line and 1 µF across the output. (External capacitance is still required with an input value of 2.2 µF and output values from 10 µF to 100 µF – increasing with lower output voltages.) Because the output is isolated the two connections can be used in either a +ve or –ve rail configuration. It must be ensured, because this is a flyback converter, that the input voltage is sufficiently high to support the load current at the required output load current. The data sheet graphs are excellent in this respect.

The control circuits are powered by an internal voltage regulator and will be driven by the input voltage unless the bias pin is taken higher than 3.1 V (improving efficiency). A RUN pin is also provided which can shut down the part and reduce the input current to less than 1 µA. A programmable self-start is provided but care must be taken when using a ceramic input capacitor to avoid voltage ringing (which might damage the converter) because of stray inductances forming a tank circuit with the input capacitance. Undervoltage lockout is also configurable.

The LTM8048 is the same part (with an output voltage of up to 13 V) but adds an internal low noise LDO to provide a second output rail from 1.2 V to 12 V (with <1 mV pk-pk ripple). This rail is separately adjusted by another single external resistor and can provide up to 300 mA of load.

Efficiencies range, with both products, from about 70% to 85% depending on the load and input/output voltage configurations. These are relatively high numbers for a galvanic isolation product.

The µModule range from Linear has been a huge engineering and commercial success mainly because they are known-sized packages with extremely high potential current ratings: with the range of current and power ratings available in the dc-dc converter families it is the ideal drop-in solution, in many cases, when power supply needs are finalized shortly before a product goes to manufacturing. Here, however, the use of the module is being ‘stretched’ and one has to have the feeling that it is being employed because it is available and not because it is the best physical or real estate solution for the majority of users. The price also reflects the additional complication of the module compared to $5 products such as the ADUM5000 from ADI, and the DCH…, DCV…, DCP… products from TI. Time will obviously tell in this respect and we will watch with interest.

The LTM8047 and LTM8048 are both in production in Linear’s 11.25mm x 9mm x 4.92mm µModule BGA packaging with prices starting at $8.25 and $8.75, respectively, in 1000-piece lots for the lowest temperature grade. Extended MP grade -55ºC to +125ºC products are also available.

Data Sheet LTM8047  
Data Sheet LTM8048