DC/DC controller design integrating digital power management and analog control loop

 The LTC3883/LTC3883-1 is a general-purpose, single-channel, multi-phase (PolyPhase), step-down current limits , margining voltage, overvoltage and undervoltage supervision limits, startup characteristics and timing, and fault response can be programmed directly through the serial bus without the need for external components such as resistors, sequencers, and supervisory ICs.

Thanks to digital power system management, complex multi-rail systems can be developed quickly and efficiently. LTpowerPlay™ software further simplifies the design work and enables monitoring of the circuit board and adjustment of parameters through a PC. This enables designers to perform debugging and online testing (ICT) without rewiring the circuit board or replacing components.

Figure 1: Digital power system management using the LTC3883

Figure 2: High efficiency 500kHz 1.8V step-down converter with DCR sensing

INTERFACE: Interface

Figure 3: Complete development platform using LTpowerPlay software

USB to PMBus Controller: USB to PMBus Controller

Demo Board: Demonstration circuit board

Socketed Programming Board: Socketed Programming Board

Customer Board with LTC3883/LTC3883-1: Customer Board with LTC3883/LTC3883-1

Demonstration Kit: Demonstration Kit

Or:

Socketed Programming: Socketed Programming

In-Circuit Serial Programming: Online settings via serial bus

Features Overview

The LTC3883/LTC3883-1 is a single output synchronous step-down DC/DC controller with integrated power FET gate drivers and an analog current mode control loop that can operate in PolyPhase mode with 6 phases. The frequency can be set from 250kHz to 1MHz, and if an external oscillator is available, the internal phase-locked loop can synchronize the LTC3883/LTC3883-1 to any frequency in the same range.

The LTC3883/LTC3883-1 features optimized gate driver dead time to minimize switching losses and body diode conduction, maintaining high efficiency under all operating conditions. The device supports a wide VIN range of 4.5V to 24V and a VOUT range of 0.5V to 5.5V. A precise reference, 12-bit DAC, and temperature-compensated analog current mode control loop produce a ±0.5% DC output voltage accuracy, and an integrated high-side input current sense amplifier allows accurate input current sensing and automatic inductor DCR calibration.

The 16-bit data acquisition system provides digital readback of input and output voltage and current, duty cycle and temperature. Users can read back the peak values ​​of important parameters. Key controller parameters can be set via PMBus. Fault logging includes interrupt flags and a black box recorder in non-volatile memory that stores the operating status immediately before the fault.

The LTC3883 features an internal LDO regulator for increased integration, while the LTC3883-1 is powered from an external 5V bias voltage for improved efficiency. Both devices are available in a thermally enhanced 32-lead 5mm x 5mm QFN package with an operating junction temperature range of either -40°C to 105°C (E grade) or -40°C to 125°C (I grade).

PMBus Control

The LTC3883/LTC3883-1 PMBus interface allows key power supply parameters to be set digitally as well as important real-time status data to be read back. Using Linear Technology’s LTpowerPlay development software, configuration parameters can be downloaded to the internal EEPROM. Figure 5 shows the PC-based LTpowerPlay development platform with a USB to I2C/SMBus/PMBus adapter. Once the device is configured as desired, it can operate autonomously without host control, so no additional firmware or microcontroller is required.

The following power supply parameters can be set via PMBus:

Output voltage and margin voltage

Temperature compensated current limit threshold based on inductor temperature

Switching frequency

Overvoltage and Undervoltage High-Speed ​​Supervisor Thresholds

Output voltage on/off time delay

Output voltage rise/fall time

Input voltage on/off threshold

Output rail on/off

Output rail margin high/margin low

Response to internal/external faults

Fault Propagation

PMBus enables the user to monitor the following power supply states:

Output/Input Voltage

Output/input current

Internal chip temperature

External Inductor Temperature

Device Status

Fault Status

System Status

Peak output current

Peak output voltage

Peak internal/external temperature

Fault Log Status

Analog Control Loop

Many functions of the LTC3883/LTC3883-1 are digitally programmable, including output voltage, current limit set point and sequencing. However, the control loop remains purely analog, eliminating the quantization effects of a digital control loop, providing the best loop stability and transient response.

Figure 4 compares the ramp curves of a controller IC with an analog feedback control loop and a controller IC with a digital feedback control loop. The analog loop ramps up smoothly, while the digital loop ramps up in steps, which can cause stability issues, slower transient response, the need for larger output capacitors in some applications, and greater output ripple and jitter on the PWM control signal due to quantization effects.

The current mode control loop produces the best loop stability, cycle-by-cycle current limit, and fast and accurate line and load transient response. Simple loop compensation is not affected by operating conditions and converter configuration. The device also supports continuous, discontinuous and burst mode inductor current control.

Figure 4: Comparison of the analog control loop and the digital control loop of the LTC3883. The analog loop has a smooth rising curve, while the digital loop has a step-by-step approach, which can cause stability issues due to quantization effects, slower transient response, the need for larger output capacitors in some applications, and greater output ripple and jitter on the PWM control signal.

ANALOG CONTROL LOOP: Analog control loop

ANALOG CURRENT WAVEFORM: Analog current waveform

FIXED: Fixed

DIGITAL CONTROL LOOP: Digital control loop

DIGITAL RAMP: Digital ramp

Automatic calibration of inductor DCR

Using the DC resistance of the inductor rather than a sense resistor to sense the output current of a DC/DC converter has several advantages, including lower power loss, circuit complexity, and cost. However, any difference between the specified nominal inductor DCR and the actual inductor DCR will result in a proportional error in the measured output current and peak current limit.

Using Linear Technology’s patent-pending algorithm, the LTC3883/LTC3883-1 can measure and compensate for the tolerance of the inductor DCR from its nominal value. A simple 180ms calibration procedure is accomplished via PMBus commands while the converter is in steady state and there is sufficient load current to accurately measure the input and output currents.

The LTC3883/LTC3883-1 accurately measures inductor temperature to maintain accurate current readback over the entire operating temperature range. The LTC3883 dynamically models the temperature rise from the external temperature sensor to the inductor core to understand the effects of inductor self-heating. This patent-pending algorithm simplifies external temperature sensor placement requirements, achieves extremely stable conditions, and compensates for transient temperature errors from the inductor core to the primary heat sink.

Multi-IC System

Large multi-rail power boards typically consist of an isolated intermediate bus converter that converts the -48V voltage from the backplane to a lower intermediate bus voltage (IBV), typically 12V, which is distributed throughout the PC board. Individual point-of-load (POL) DC-DC converters step down the IBV to the required rail voltage, typically 0.5V to 5V, with output currents ranging from 0.5A to 120A. These boards are densely populated, and the digital power system management circuitry cannot take up much PC board real estate.

Linear Technology's high performance PMBus controllers (e.g., LTC3883/LTC3883-1) and companion ICs (e.g., LTC2978) work together to efficiently and seamlessly meet the stringent digital power management requirements of today's complex circuit boards. These requirements include sequencing, voltage accuracy, overcurrent and overvoltage limiting, margining, monitoring, and fault control. Any combination of these devices makes sequencing designs very easy for any number of power supplies. Using a time-based algorithm, users can sequence the rails on and off in any order with simple programmable delays. Sequencing can be done across multiple chips using the single-wire SHARE_CLK bus and one or more bidirectional general purpose IO (/GPIO) pins.

LTpowerPlay Software

LTpowerPlay software makes it easy to control and monitor multiple PMBus-enabled Linear Technology devices simultaneously. The configuration of the DC/DC controller can be modified in real time by downloading system parameters to the internal EEPROM of the LTC3883/LTC3883-1. This allows the system configuration to be adjusted with software without having to complete demanding tasks such as replacing components and manually rewiring the circuit board, thus saving design development time. Figure 5 shows how to control the output voltage, OV/UV protection limits, and turn-on/off ramps. The waveform shows the soft start and soft stop of the output voltage. Alarm and fault conditions are also shown in the figure.

Figure 5: Power systems are simplified. LTpowerPlay puts comprehensive power control at your fingertips

in conclusion

The LTC3883/LTC3883-1 combines high performance analog switch regulation with precise data conversion and flexible digital interface. Multiple LTC3883s can be used in parallel with other devices to easily build an optimized multi-rail digital power system.

The LTpowerPlay software development system supports all Linear Technology PMBus products, which helps board designers quickly debug the system. LTpowerPlay can be used to monitor, control and adjust power supply voltages, limits and sequencing. Production margin testing can be easily performed using only a few standard PMBus commands. Integrating the LTC3883/LTC3883-1 and other Linear Technology PMBus products is the best way to quickly bring digitally controlled power supplies to market.