Controller saves converter space
01 March 2007
Designers of POL DC/DC converters for embedded systems face challenges due to the multiple constraints of limited space and cooling, and the need for the correct power supply tracking for improved system reliability
Most embedded systems are powered via a 48V backplane. This voltage is normally stepped-down to a lower intermediate voltage of either 12V or 5V to allow power to the racks of boards within the system. However, most of the subcircuits or ICs on these boards have to be powered from 0.8V to 3.3V at currents ranging from tens of mA to tens of A. As a result, a point-of-load (POL) DC/DC converter is needed, to step down the 12V or 5V to the desired voltage and current level required by the sub-circuits or ICs.
Since space and cooling are at a premium in these systems, it is important for any POL converters to be compact and efficient. Furthermore, many microprocessors and DSPs need a core power supply and an I/O power supply which must be sequenced during start-up. Designers have to consider the relative voltage and timing of core and I/O voltage supplies during power-up and - down operations to comply with manufacturers' specifications. Without proper power supply sequencing, latch-up or excessive current draw may occur that could lead to damage to the microprocessor's I/O ports or the I/O ports of a supporting device such as memory, PLDs, FPGAs, or data converters. To ensure that the I/O loads are not driven until the core voltage is properly biased, tracking of the core supply voltage and the I/O supply voltage is necessary.
Although start-up and shutdown tracking can be implemented externally for any given DC/DC converter, the power supply sequencing requirements will vary from system-to-system. These solutions include ASSPs that can be configured via a programmable interface or by external components; programmable microcontrollerbased solutions and FPGA solutions.
This growing demand for increased current at ever-decreasing voltages continues to drive power supply development. Much of the progress in this area can be traced to gains made in power conversion technology, particularly improvements in power ICs and power semiconductors. In general, these components contribute to enhance power supply performance by permitting increased switching frequencies with minimal impact on power-conversion efficiency. This is made possible by reducing switching and on-state losses while allowing for the efficient removal of heat. However, the migration to lower output voltages places more pressure on these factors, which in turn, creates significant design challenges.
Multi-phase topology
Multi-phase is a general term for topologies where a single input is processed by two or more converters, where the converters are run synchronously with each other but in different, locked, phases. This approach reduces the input ripple current, the output ripple voltage and the overall RFI signature while allowing high current single outputs, or multiple lower current outputs with fully regulated output voltages. It also allows smaller external components to be used which, in the case of a monolithic device, increases output current capability, as multiple, smaller MOSFETs can be easily fabricated 'eon-chip'. This also improves thermal management.
Linear Technology designates multi-phase, single output circuits as PolyPhase circuits, whereas multiple output, single input, supplies are regarded as 'eplain vanilla' multiphase.
Multi-phase topologies can be configured as step-down (buck), step-up (boost), and even forward, although buck is the prevalent application. Linear Technology manufactures both monolithic solutions, where all the power semiconductors are integrated into the device, and controller solutions. Generally, controller solutions are for higher power, typically above 15W to 20W and require external discrete MOSFETs.
PolyPhase is used to generate one high current output, say as a replacement for a 'ebrick' style DC-DC converter, while multiphase operation would be used where multiple outputs of differing voltages are required, for example 2.xV and 1.xV for an FPGA or processor power in a small system.
Feature-rich IC
The penalty for poor tracking or sequencing is often irreparable damage to devices in an embedded system. FPGAs, PLDs, ASICs, DSPs and microprocessors typically have diodes between the core and I/O supplies as a component of ESD protection. If supplies violate the tracking requirements and forward bias the protection diodes, the device may be damaged.
Voltage sequencing, tracking, and multiple output rails have become popular features in DC-to-DC converter modules, although these functions are less commonly found in DC/DC controller ICs. However, a threephase, triple output synchronous step-down controller incorporating all three power management functions is available.
The LTC3773 offers three independent high current outputs in a single IC for a simple, compact, efficient and feature-rich solution. It is also distinguished by its fast transient response, a PLL that allows synchronisation to a system clock, and a highly accurate reference.
The three-phase DC/DC controller can handle inputs as high as 36V and support one, two or three output voltages from 0.6V to 5V with currents in excess of 15A per phase. Two of these phases may be tied together as a 30A output; in this case the two channels can be operated anti-phase to minimise stress on the input capacitors. All three channels can also regulate a single output providing over 45A of current. Each channel can be tracked individually, in the ratiometric or coincident configurations; they may also be enabled and disabled sequentially with few external components. When all three channels are disabled, the controller typically draws just 18mA in shutdown mode. At light load, the LTC3773 may operate in BurstMode for maximum efficiency, forced continuous mode (constant frequency operation for the lowest ripple), or pulse skipping as a compromise between the two.
Switching frequency can be phase-locked to an external source of 160kHz to 700kHz, or can be set with a DC voltage on the PLLFLTR pin. Typical pin-selectable frequencies of 220kHz, 400kHz and 560kHz are also available. In either case, the CLKOUT pin expresses the operating frequency at zero, 60 to 180 degrees with respect to channel 1fs switching frequency, a useful feature where multiple controller ICs operate form the same set of input capacitors.
Where three 15A outputs are required in the smallest possible footprint, the LTC3773 delivers three low voltage, high current outputs from a single, loosely regulated supply (see figure 1).
Each output reference is guaranteed to remain within }1% over temperature. During startup, ratiometric tracking holds the feedback references of Vout 2 and 3 to 0.6V (Vout 1 is 2.5V), so that the three outputs reach their command levels at the same time (see figure 2). TRACK1 ramps up by charging the 0.01mF capacitor with an internal 1mA source. Where tracking is not required, all TRACK pins can be connected to external capacitors, so that they softstart their respective channels without regard to external voltage sources. Pulling the positive node of a TRACK divider to zero does not always produce zero volts at the respective output; the minute pull-up current in the TRACK pins could create unwanted offsets in the voltage dividers to which they are connected, producing unwanted low output voltages or hiccupping on channels 2 and 3. A 30mV offset in its tracking circuits, disables each channel's diver until its TRACK pin sees at least 30mV. This offset disappears as the TRACK level rises to 100mV, so that channels 2 and 3 can track predictable when near their final values.
Monitoring the sense resistor that is in series with the inductor provides accurate current limiting. The controller protects against excessive inrush current during start-up and limits current through the inductor and main MOSFET during shortcircuits on the output. It pulls the output down by turning on the synchronous MOSFET whenever the feedback pin VFB is 3.75 per cent above the 0.6V reference voltage, protecting the output capacitors and the load; it shuts off whenever the bias supply VCC drops below 3.94V, ensuring that the external MOSFETs operate at safe gate drive levels. When the feedback voltage of any channel is not within }10 per cent of the 0.600V internal reference for 100msec, the open drain power good PGOOD pulls low.
Each channel can be stable with as little as two 47mF ceramic capacitors at each output, providing low ripple at moderate to heavy loads, and the fastest transient response possible. With current-mode operation, the converter responds quickly to input voltage transients, correcting the pulse width cycle-by-cycle as the input voltage swings widely. Channel-to-channel interaction is virtually non-existent even during a substantial load step on one channel.
Alternative for two outputs
Compared to single-phase switching regulators, two-phase converters impose lower ripple current on the input capacitors, reducing size and cost. This technique interleaves the current pulses coming from the switches, reducing the amount of time when they overlap together. Lower ripple current means less power dissipated and higher efficiency, as well as reduced EMI. Two-phase converters also double the effective switching frequency, lowering the output ripple voltage.
To fully realise these benefits, the two channels should be operated 180‡ out of phase. The LTC3773 allows channels 2 and 3 to be operated out of phase, useful when they are tied together as a single, high current output. As an example, channel 1's output could be 2.5V at 15A and channel 2 (channels 2 and 3 tied together) could be 1.8V at 30A. This second dual-phase channel will exhibit excellent current sharing, no channel-to-channel interaction and minimal output ripple (at twice the switch nodes' operating frequency).
TONY ARMSTRONG is product marketing manager, Power Products Group, Linear Technology Corporation
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