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Table 1-1 highlights the latest point-of-load DC/DC converters with integrated FETs that are applicable to Whitley and Cedar Island applications; however, they are designed to accommodate the requirements for a wide range of markets. These devices are designed to achieve fast transient response, high efficiency, good thermal performance, and high output voltage accuracy. Notice that different control architectures are suggested in the chart. Fixed-frequency control architectures provide a predictable switching frequency and can be synchronized to an external clock. Current mode and voltage mode control are desirable in noise-sensitive applications that use data converters and high-speed analog circuits. On the other hand, devices implementing constant on-time control deliver a faster transient response than voltage or current mode control to quickly changing loads, since there is no internal clock to control the switcing frequency. Several devices feature PMBus or I2C with Serial Voltage Identification (SVID), adaptive voltage scaling and margining. Devices integrating PMBus or I2C with telemetry report voltage, current, and temperature information to a host.
IOUT RANGE | DEVICE (CONSTANT ON-TIME CONTROL) | DEVICE (FIXED FREQUENCY CONTROL) | PMBus / I2C W/O TELEMETRY | PMBus / I2C WITH TELEMETRY |
---|---|---|---|---|
DC/DC CONVERTERS WITH INTEGRATED MOSFETS | ||||
≤ 2 A | TPS62912 | N/A | N/A | |
2 A – 3 A | TPS62913 | N/A | N/A | |
3 A – 6 A | TPS54J061 | TPS542A50 | TPS53820 (SVID) | |
6 A – 10 A | TPS54JA20 | TPS542A50 | TPS546A24A | |
10 A – 15 A | TPS548A28 | TPS542A50 | TPS546B24A | |
15 A – 20 A | TPS548B28 | TPS549B22 | TPS546B24A | |
20 A - 25 A | TPS549B22 | TPS546D24A | ||
25 A – 40 A | TPS548D22 | TPS549D22 | TPS546D24A | |
DDR Memory Active Bus Termination | ||||
≤ 2 A | Source / Sink Linear Regulator with Tracking | |||
2 A - 6 A | DC/DC Converter with Tracking | |||
Low Drop Out Regulator | ||||
1.5 A | TPS74801 | Linear Regulator |
As the semiconductor process technology advances, processors require tighter voltage accuracy and lower operating voltages. The processor data sheet specifies the voltage tolerance as either a percentage or as a value in mV, which includes DC, AC and ripple variations over the entire operating temperature range. Designers also consider the tolerance of the resistor divider used by the DC/DC converter, the routing and trace losses of the circuit board, and the variations of the application, like the input voltage variations, temperature swings, and fast changes in the load.
Check the feedback voltage accuracy of the DC/DC converter in the data sheet rather than the front page. Table 2-1 shows the regulated feedback voltage, or the internal voltage reference specification of the TPS548A28, which is a 2.7 V to 16 V, 15 A converter, and shows that the reference accuracy is ±6 mV or ±1% over the full temperature range. The total output voltage accuracy is improved by choosing tighter tolerance resistors. If more headroom is needed, designers can choose 0.1% or 0.5% resistors, even though they may cost a little bit more. The additional headroom allows a total ±3% or ±5% output voltage variation to be met with less bulk and bypass capacitance. (1)
PARAMETER | TEST CONDITION | MINIMUM | TYPICAL | MAXIMUM | UNIT |
---|---|---|---|---|---|
Internal Voltage Reference | TJ = –40°C to 125°C, Vcc = 3 V | 594 | 600 | 606 | mV |
TJ = 0°C to 85°C, Vcc = 3 V | 597 | 600 | 603 | mV |
Layout constraints, connectors, and board density requirements often affect the total output voltage accuracy. A remote sense feature of a DC/DC converter compensates for voltage drops from long trace lines to accommodate processors needing high accuracy output voltage. This feature is especially useful when routing higher currents since the voltage drop can be a large portion of the overall DC error. Figure 2-1 shows the TPS543B20 using the remote sense feature with voltage feedback resistors used to program the output voltage. Figure 2-2 shows the TPS543B20 using the remote sense feature without voltage feedback resistors when the VSEL pin selects the reference voltage. The RSP and RSN pins are extremely high-impedance input terminals of a true differential remote sense amplifier.
Since the load profile can change dramatically in an enterprise rack server, it is important to consider AC transient performance. Choosing a DC/DC converter with a fast transient response using non-linear control techniques, such as constant on-time or D-CAP3™, allows a fast transient response with minimal output capacitance. A typical D-CAP3 converter design has three primary considerations for deciding the value of the output capacitance: transient (which includes load step and slew rate of the load step), output ripple, and stability. In applications where the load transient is stringent, the output capacitance is predominantly driven by the transient requirement. Today, a tight ripple requirement is becoming important in some high-end, high-performance application-specific integrated circuit (ASIC) and field-programmable gate array (FPGA) designs. The LC output power stage must be designed to meet the ripple criteria. For a D-CAP3-based design, there is a minimum capacitance requirement in terms of small signal stability. This requirement prevents subharmonic, multiple-pulsing behavior in all D-CAP modulators. Figure 3-1 shows how D-CAP3 control mode works in a load transient case. The Calculating Output Capacitance to Meet Transient and Ripple Requirements of an Integrated POL Converter Design Based on D-CAPx™ Modulators application report shows how to design the output filter for a 1-V output to accommodate a ±10 mV ripple requirement and a ±30 mV transient voltage deviation using the D-CAP3 control mode.
The TPS543C20A converter features a new control mode called Advanced Current Mode (ACM), which is an internally compensated emulated peak-current-mode control with a clock synchronizable, fixed-frequency modulator. The internal integrator and directly amplifying ramp tracking loop eliminates the need for external compensation over a wide range of frequencies. The TPS543C20A also features Asynchronous Pulse Injection (API) and body braking to improve transient performance by significantly reducing undershoot and overshoot, respectively, reducing the external capacitance requirement. When the API and body diode braking are activated, ACM delivers a similar transient performance as D-CAP3. Figure 3-2 shows examples of undershoot and overshoot reduction with API and body braking. (2)