Step-Down DC/DC Controllers Employ PMBus Functions to Enhance System Performance

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THE POWER Management Bus (PMBus) will finally do what its developers always intended it to do: manage and monitor the operation of a dc-dc converter. These capabilities are integrated into Linear Technology Corporation's LTC3880 and LTC3880-1 that are dual output, high efficiency synchronous step-down DC/DC controller ICs with an I²C-based PMBus interface for digital power management. The ICs combine an analog switching regulator with precision mixed signal data conversion to provide power system design, management, and monitoring.

By using the PMBus functions, multiple designs can be easily calibrated and configured in firmware to optimize a single hardware design for a range of applications. The converter loop gain does not change if the power supply parameters are modified, so compensation remains optimized for multiple configurations. Products best served by these PMBus functions include high current power supplies employed in telecom, datacom, computing and storage systems. For more information on PMBus, see the box titled “Power Management Bus (PMBus)”.

PMBUS IN ACTION

The LTC3880/-1 allows digital programming and read back for real-time control and monitoring of critical point-of-load converter functions. Configurations are saved to internal EEPROM over the device's I²C serial interface supported by Linear Technology's LTpowerPlay GUI-based development software. With configurations stored on-chip, the controller powers up autonomously without burdening the host processor.

PMBus functions include the ability to program specific power supply management parameters, including:

• Output Voltage
• Input Voltage
• Current Limit
• Switching Frequency
• Overvoltage and undervoltage
• On and Off Delay Times
• Output Rise/Fall Times

In addition, PMBus functions include the ability to monitor power supply operation, including:

• Internal Controller Temperature
• External System Temperature
• PWM Duty Cycle
• Output Current
• Output Voltage
• Input Voltage
• Input Current

Plus, there is also the ability to shutdown power supply operation by allowing all faults to be individually masked with operation in either unlatched (hiccup) or latched modes. Individual status commands enable fault reporting over the I²C serial bus to notify the host processor of the specific fault event, including:

• Internal/external overtemperature
• Output undervoltage/overvoltage
• Input undervoltage/overvoltage
• Input and output overcurrent
• Communication, memory or logic Fault

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PMBUS AND HOT-SWAP

Many system users insist that they experience virtually no downtime. This means that equipment must be very reliable, so if something begins to fail it must be replaced very quickly. To replace a defective power supply without interfering with system operation, requires removal of the defective unit and insertion of a new one without turning off system power, a process called “hot-swap.” PMBus functions provided by the LTC3880 and LTC3880-1 can combine with a hot-swap IC in the same power supply. When the PMBus fault data identifies an impending failure, it can notify the host processor to signal a technician to replace the defective power supply (without turning off system power).

LTC3880/1 OPERATION

Both the LTC3880 and LTC3880-1 controllers use a constant frequency, current mode architecture. You can set switching frequency, output voltage, and device address using external configuration resistors. Or, you can set parameters via the digital interface or stored in EEPROM. Voltage, current, internal/external temperature and fault status can be read back through the I2C bus interface. Fig. 1 shows a simplified version of the LTC3880 employed as a multiphase regulator.

There is a slight difference between the two LTC3880 versions that will be referred to as LTC3880/-1. The LTC3880 has an onboard LDO for controller and gate drive power whereas the LTC3880-1 allows an external 5V bias voltage for highest efficiency. Both ICs are available in a thermally enhanced 6mm × 6mm QFN-40 package. The extended temperature range grade is specified over a -40°C to 85°C operating junction temperature range. The industrial grade part is specified over a -40°C to 125°C operating junction temperature range.

A single LTC3880/-1 can either regulate two independent outputs or be configured for a two-phase single output. Up to six phases can be interleaved and paralleled for accurate sharing among multiple loads, minimizing input and output filtering requirements for high current and/or multiple output applications. An integrated amplifier provides true differential remote output voltage sensing, which enables high accuracy regulation, independent of board copper trace voltage drops.

The LTC3880/-1 features high current integrated gate drivers to drive all N-channel power MOSFETs from input voltages ranging from 4.5V to 24V, and it can produce ± 0.50% accurate output voltages from 0.5V to 5.5V with output currents up to 30A per phase over the full operating temperature range.

Among the features of these controllers are constant frequency current mode control with cycle-by-cycle current limit, adjustable soft start, optional synchronized switching frequency, and programmable GPIO pins to indicate IC status and provide autonomous recovery from faults.

You can configure the LTC3880/-1 to use either DCR (inductor dc resistance) sensing or low value resistors to sense output current. The LTC3880/-1 can nominally account for the temperature dependency of the DCR sensing element. The accuracy of the current reading and current limit are typically limited by the accuracy of the DCR resistor. Thus, current sensing resistors provide the most accurate current sense and limiting for the application. Highest efficiency is achieved by sensing the voltage drop across the output inductor to determine output current. Programmable DCR temperature compensation cancels the inductorís temperature coefficient, providing an accurate and constant current limit over a broad temperature range.

The LTC3880/-1 has two ranges of current limit programming and a total of eight levels within each range. Within each range the error amplifier gain is fixed, resulting in constant loop gain. The 75mV setting is best current limit accuracy.

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Once the inductor value is determined, you can select the type of inductor. Core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance. As the inductance increases, core losses go down. Unfortunately, increased inductance requires more turns of wire and therefore copper losses increase. Ferrite designs have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core materials saturate hard, which means that inductance collapses abruptly when exceeding the peak design current. This abruptly increases inductor ripple current and consequent output voltage ripple.

External component selection is driven by the load requirement, and begins with the selection of RSENSE (if RSENSE is used) and inductor value. Next, select the power MOSFETs. Then, select the input and output capacitors. Finally, the current limit is next. All of these components and ranges need to be determined prior to calculating the external compensation components.

Two external power MOSFETs must be selected for each controller in the LTC3880/-1: one N-channel MOSFET for the top (main) switch, and another N-channel MOSFET for the bottom (synchronous) switch. The peak-to-peak drive levels are set by the INTVCC voltage that is typically 5V. In most applications logic-level threshold MOSFETs are the best choice. The only exception is if low input voltage is expected (VIN< 5V); then, use sub-logic level threshold MOSFETs (VGS(TH < 3V). Pay close attention to the BVDSS specification for the MOSFETs as well; most of the logic level MOSFETs are limited to 30V or less.

Selection criteria for the power MOSFETs include the on-resistance, RDS(ON), Miller capacitance, CMILLER, input voltage and maximum output current. Miller capacitance can be approximated from the gate charge curve usually provided on the MOSFET manufacturersí data sheet.

Power Blocks similar to those in Fig. 2 can be employed with theLTC3880/-1. In addition, the gate drive outputs can also be used with DRMOS devices.

OPERATING MODES

The LTC3880 has three operating modes that include high efficiency Burst Mode® operation, discontinuous conduction mode or forced continuous conduction mode. Mode selection is done by command (discontinuous conduction is always the start-up mode, forced continuous is the default running mode).

In Burst Mode operation, the peak current in the inductor is set to approximately one-third of the maximum sense voltage even though the voltage on the ITH pin indicates a lower value. If the average inductor current is higher than the load current, the error amplifier, EA, will decrease the voltage on the ITH pin. When the ITH voltage drops below approximately 0.5V, the internal sleep mode asserts and both external MOSFETS are turned off. In sleep mode, the load current is supplied by the output capacitor. As the output voltage decreases, the EA output begins to rise. When the output voltage drops sufficiently, sleep mode is deasserted, and the controller resumes normal operation by turning on the top external MOSFET on the next PWM cycle.

You can array multiple LTC3880/-1s in order to provide a balanced load-share solution by bussing the necessary pins. Fig. 3 illustrates the shared connections required for load sharing. If current sharing uses the same IC it is recommended to assert the PMBus command (MFR_PWM_CONFIG) that causes channel 1 to use the feedback of channel 0.

The LTC3880/-1's switching frequency can be established with internal clock references or with an external time-base. You can configure it for an external clock input through the programmed value in the EEPROM, a PMBus command or setting a resistor. If the LTC3880/-1 is configured as the oscillator output on SYNC, you can select the switching frequency source with either external configuration resistors or I2C bus programming.

The internal EEPROM (nonvolatile memory) stores configuration settings and fault log information. EEPROM endurance retention and mass write operation time are specified over the operating temperature range. Read operations performed at temperatures between 85°C and 125°C will not degrade the EEPROM. Writing to the EEPROM above 85°C can cause a degradation of retention characteristics.

LTPOWERPLAY

LTpowerPlay™ is a Windows-based development environment that supports digital power ICs like the LTC3880/-1. The software supports various tasks; for example, it can be used to evaluate ICs by connecting to a demo board or the user application. It can also be used in an offline mode (with no hardware present) to build multiple IC configuration files that can be saved and reloaded at a later time. Also, it provides diagnostic and debug features during board bring-up to program or tweak the power system or to diagnose power issues when bring-up rails. LTpowerPlay utilizes Linear Technology's USB-to-I2C/SMBus/PMBus controller to communication with one of the many potential targets including the DC1590B-A/B demo board, the DC1709A socketed programming board, or a user target system. Fig. 4 shows a portion of the LTPowerPlay screen used to establish the operating parameters of the PMBus-enabled LTC3880/-1.

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