July 11, 2016
Power Systems Architect and MCM Design Expert, Exar Corporation
The DC/DC power converter subsystems in today's applications must provide flexibility and also support external control of many aspects of their operation as programmed by the user. Further, in designs with multiple supply rails – as most now are – there's an increasing need for sophisticated management of the power conversion by a sophisticated controller IC and design tool, which will effectively support various converter combinations.
For this reason, programmable controllers for power conversion have been gaining in popularity, as they can provide the system design engineer with the required features and functions, along with ease of implementation. [Note that this "programmable controller" is very different than the programmable logic controller (PLC), often also called a programmable controller, used for control of industrial processes.]
This controller must implement three to four complete, independent pulse-width-modulated controllers including a light load mode for low power dissipation and high efficiency at low output currents. It also must provide a number of critical safety features, such as overcurrent protection (OCP), overvoltage Protection (OVP) and over temperature protection (OTP) plus input undervoltage lockout (UVLO). In addition, a number of key health-monitoring features are needed, such as warning-level flags for the safety functions and a power good (PGOOD) indication along with full monitoring of system voltages and currents. These functions are all programmable and/or readable from an SMBus and many are steerable to the GPIO ports for hardware monitoring by the system controller. Of course, all this must be housed in a small package for space efficiency.
Required Feature Set
The list of desired controller features is extensive and illustrates how much the design engineer now expects the selected controller to do. The Exar XRP7724 quad-channel digital PWM/PFM programmable power management system (Figure 1) meets these many requirements including:
Fig 1: The simplified schematic of XRP7724 a quad-channel programmable controller shows the level of complexity and features incorporated in the IC. (click to zoom)
The functional block diagram of the regulation loops for an output channel, Figure 2, shows the four separate, parallel control loops: pulse width modulation (PWM), pulse frequency modulation (PFM), ultrasonic and oversampling (OVS). Each of these loops is fed by the analog front end (AFE) at the left of the diagram. The AFE consist of an input voltage-scaling function, programmable voltage reference DAC, error amplifier and window comparator.
Fig 2: The XRP7724 regulation loops shows the four separate, parallel control loops within the device. (click to zoom)
To provide current level information, the output inductor's current is measured by a differential amplifier that reads the voltage drop across the on-resistance (RDSON) of the lower MOSFET during its "on" time. This voltage is converted to a digital value by the current ADC block and the resulting current value is used to determine when PWM-to-PFM mode transitions should occur, see sidebar "PWM- and PFM-mode control loops."
When powering CPU and digital-power modules, transient response is of the utmost importance, Figure 3 and Figure 4. That is where oversampling mode is most effective, so this feature was added to the XRP7724 to improve transient responses. In OVS mode, the output voltage is sampled four times per each switching cycle and is monitored by the AFE window comparator. If the voltage goes outside the set high or low limits, the OVS control electronics can immediately modify the pulse width of the high-side or low-side gate drivers to respond accordingly without having to wait for the next cycle to start. OVS has two types of responses, depending on whether the high limit is exceeded during an unloading transient (overvoltage) or the low limit is exceeded during a loading transient (undervoltage).
Fig. 3: The XRP7724 has excellent response for a 0A to 6A load transient, when operating at 300 kHz in PWM mode only.
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