Point-of-load dc/dc micromodule

Paul Boughton

Existing point-of -load dc/dc module regulators such as point-of-load (POL) power supplies are assembled on printed circuit boards (PCBs) with leads for through-hole or surface mount placement on a system board.

For many advanced system architectures with densely populated boards such as AdvancedTCA or CompactPCI, these power supplies are too tall (profile) and occupy too much space, which for a US$15000 24cmx35cm system board is a significant cost to pay for a power supply. These smaller form-factor systems demand higher power requirements in a limited amount of board space.

This means that the power regulator must deliver more power at very high efficiencies to limit the amount of power dissipated in the smaller footprint. Also the point-of -load regulator must be a high frequency switching regulator with a very fast transient response to limit the overall solution size of the design.

As a result, system developers have been seeking a smaller, lower profile, and higher performance point-of-load regulator in the form-factor of an IC for easy assembly.
Such a regulator could take advantage of board space on the bottom of the system board where room is available for a low profile and highly integrated dc/dc solution. .

Limited board space

The LTM4600 µModule provides a solution for several point-of-load regulator designs that are very limited in board space.

The µModule is contained in a 15mmx15mmx2.8mm LGA surface mount package. The package is designed to provide heat-sinking from the top and bottom of the device.
The constant on time current mode architecture provides very fast transient response and accurate current limit control for distributed power.

The µModule switches at 800kHz in a synchronous typology to offer very high efficiency in a small form factor.

The LTM4600EV µModule operates from an input supply range of 4.5V to 20V, and the LTM4600HVEV µModule operates from 4.5V to 28V.

The output voltage can be programmed from 0.6 to 5 volts with some constraints on the input to output voltage difference as specified in the datasheet. The output current can be up to 14A peak, and 10A continuous.

Fault protection features include over-voltage protection, and over-current protection. Fig.1 shows the LTM4600 µModule in comparison with a quarter. This small solution can even be mounted on the backside of a system board.

Typical design

Fig.2. shows a typical LTM4600EV design for a 2.5V point-of-load regulator. The design requires some external input capacitors for the RMS ripple current rating discussed in the datasheet.

Two low ESR 10µF 25V ceramic capacitors are used for the input. Input power is typically connected through a low inductance path to the input of the module. This requirement eliminates any resonant LC ringing from occurring at the input and thus eliminates any over voltage at the higher input voltage range.

If a harness is used to bring power to the module input, then a small aluminum electrolytic can be used in parallel with the ceramics to critically damp any ringing resulting from the low ESR ceramics and the harness inductance.

The output voltage is set with an external resistor from the VOSET pin to ground. The internal reference is 0.6V, and the output voltage setting equation is VOUT=0.6V*(100k+R2)/R2). The output capacitors are selected for low esr to maintain an initial voltage drop of the output voltage to approximately ?VOUT=(Istep*Rth-esr) in a transient step, where Istep is the load step current and Rth-esr is the parallel equivalent esr of all of the output capacitors.

Control loop recovery

The output bulk capacitance requirement is necessary for holding the output voltage in a specific range during the control loop recovery.

This is approximately equal to ?vout=(Istep*Loop turn around time)/Cout. Most of the transient waveforms in the datasheet show that the control loop responds with no clock latency.

This is unique to the valley model control architecture of the LTM4600.
The output voltage is typically turning around in 4 to 6 microseconds, and fully recovering in the 20 to 25 microsecond timeframe.

An example of a 5A step with 6 microseconds of turn around with the 470µF pos cap and the three 22µF ceramics calculates to ~55mV. This is approximate and should be validated on the actual circuit. Fig. 5 shows the transient waveform of the 2.5V design.
The Run/SS pin has a dual function that can be used to control the soft start of the output voltage, and the turn on of the regulator.

Built-in soft start

The datasheet refers to the amount of built-in soft start in the LTM4600 device. The Comp pin can be used for tying modules in parallel for increased current.

The EXTVCC pin can be used to improve efficiency at higher input voltages by reducing the internal power loss due to the internal 5V LDO regulator.

If an external 5V supply with at least 50mA is available, then it can be applied to the EXTVCC pin to supply the gate drive currents to the module, thus lowering the internal power dissipation of the module. This is especially true for input voltages above 12V.
The FADJ pin is used to decouple the frequency adjust path with a capacitor if needed. The frequency is internally set.

The FCB pin can be tied to the VOUT pin to put the power module into discontinuous power savings mode at light loads for battery operated products. The output voltage must be greater than the FCB threshold voltage specified in the datasheet.
A graph in the datasheet displays the efficiency differences at light load as a function of the FCB pin.

Figs.3,4 and 5 show the efficiency, ripple noise, and load transient of the above design.
Two LTM4600 µModules can be placed in parallel to double the output current in higher power regulator applications.

The module architecture is current mode control, and therefore the two modules will current share in the application.

High voltage 24V input application

The design uses the LTM4600HVEV device which is rated to 28V input maximum, so a tightly regulated 24V supply would be required.

The 5V output is connected back to the EXTVCC pin to operate the internal gate drives from the output. This improves the efficiency by removing gate drive currents from the 24V input.

An external heat sink is recommended to achieve the 7A output current. The total output power is 35W at 7amps of load current, and 5W of loss.

Further de-rating of the output current will be necessary at higher ambient temperatures to maintain the total temperature rise to less than 100°C on the device. The total temperature rise is ambient plus the device temperature rise due to power loss.
Air flow can improve the output power with the external heat sink..

Space-cinstrained designs

The LTM4600 provides an ideal solution for space-constrained power designs. The µModule is a unique power device that integrates all the necessary components for a high performance power supply into a small form factor.

The µModule can be soldered like any other surface mount integrated circuit, and requires very few external components. The µModule can be placed in parallel to double the power output. Product design cycles are ever decreasing, and the ease of use of the LTM4600 will significantly reduce time-to-market.

Afshin Odabaee, Product Marketing Engineer, Power Management Products, and Eddie Beville, Development Manager, uModule Products, are with Linear Technology, San Jose, CA, USA. For more information, visit www.linear.com

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