Sungkeun Lim, Intersil
Today’s digital lifestyles are driving demands for ultra-mobile computers: thin, lightweight, always-connected PCs that allow their owners to work wherever they need to be, unhindered by any need for frequent recharging.
Efficient power management significantly influences the computer’s ability to meet users’ expectations in relation to battery lifetime as well as overall size, thickness and weight. Power specifications such as Intel’s VR12.x or Mobile Voltage Positioning (IMVP) seek to simplify and standardize power-supply functionality for PC processors, and support operating modes designed to maximize energy efficiency in both active and standby states.
The IMVP8 specification for processors such as the Intel Atom family, for example, allows the CPU to reduce its core supply voltage in active modes when the current demand is highest. This not only reduces power consumption according to the square of the voltage (P = V2/R), thereby minimizing drain on the battery, but also reduces the heat dissipated by the processor. Reducing heat dissipation holds the key to simplifying thermal design, which in turn enables the weight and thickness of the enclosure to be reduced.
Other advances in power management help the PC extend battery life while at the same time delivering the instant readiness expected by today’s smartphone-savvy users. The Connected Standby (CS) specification developed by Intel and Microsoft introduces a new primary OFF mode that keeps network connections active in a low-power state. This enables the PC to resume much more quickly than is possible from traditional ACPI Sleep or Hibernate modes, and so delivers a modern user experience free from any delays associated with reconnecting.
Integration saves space and power
To build a power supply that meets specifications for the chosen platform, designers can either implement the specified switched-mode regulators, LDOs and control circuitry using separate ICs, MOSFET drivers, power MOSFETs, and the necessary passive components, or use a Power Management IC (PMIC) designed specifically for the target processor platform.
Using an integrated PMIC solution can save approximately 80% of the board area occupied by an equivalent discrete implementation. The space savings give designers more flexibility, enabling them to add extra features, reduce motherboard size or add another battery to extend the operating time of the mobile device.
For example, Intersil’s latest PMICs, the ISL95906 and ISL95908, are presented as the first single-chip devices to support the Intel/Windows CS mode. These devices enable credit card-size motherboards for high-end ultrabook and tablet computers powered by two-cell Li-ion batteries.
The ISL95906, for VR12.6 platforms, and ISL95908, for IMVP platforms, both integrate the controller with power MOSFETs and drivers, the VTT LDO regulator, independent enable and power-good indicators, I2C interfaces, and fault protection/monitoring for eight synchronous buck regulators.
Figure 1 shows how the ISL95908 is used in an IMVP8 platform, in combination with an NVDC charger, VCORE regulator, and 2-cell battery. The high level of feature integration holds the key to significantly reducing the PCB real estate occupied by the PC’s internal power supply. Switching frequencies up to 1MHz allow the use of smaller capacitor and inductor sizes, which also contribute to miniaturization.
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Figure 1. PMICs with support for CS mode deliver valuable space and power savings.
Both PMICs employ Intersil’s proprietary R4 modulation technology, a synthetic current-mode hysteretic controller that overcomes the challenge of ensuring high noise immunity and rapid transient response within the tight space constraints of mobile applications. System designers often rely on an external R-C compensation network to get the desired performance, whereas the R4 modulator eliminates any need for these components thereby saving board area.
Under steady-state conditions, the desired switching frequency is maintained within the entire specified range of input voltages, output voltages, and load currents. During load transients, the controller will increase or decrease the PWM pulses and switching frequency to maintain output voltage regulation.
Figure 2 illustrates this effect during a load insertion, and Figure 3 shows the output voltage of the VDDQ switcher during load insertion. As the window voltage starts to climb from a load step, the time between PWM pluses decreases as the switching frequency increases to keep the output voltage within regulation. This enhanced load-transient response also allows the total output capacitance to be reduced.
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Figure 2. Modulator waveform during load current insertion
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Figure 3: VDDQ Output Voltage waveform during load current insertion.
Boosting light-load efficiency
As well as saving space through feature integration, Intersil’s new PMICs also implement a number of techniques such as Diode Emulation (DE) that help to increase light-load efficiency. Improving light-load efficiency is known to be the most effective way of prolonging battery life in mobile devices.
Operating in diode emulation (DE) mode allows the low-side MOSFET to conduct only when the current is flowing from source to drain, and does not allow reverse current. As the name suggests, this emulates the behavior of a diode in a standard buck regulator configuration.
As Figure 4 shows, when LGATE is on, the low-side MOSFET conducts, creating negative voltage on the phase node due to the voltage drop across the ON-resistance. The controller monitors the current through monitoring the phase node voltage. It turns off LGATE when the phase node voltage reaches zero to prevent the inductor current from reversing.
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Figure 4: Operation in Diode Emulation Mode
If the load current reaches the critical conduction point (50% of inductor current ripple) the inductor current will reach and stay at zero before the next phase node pulse. Under these conditions the regulator is in discontinuous conduction mode (DCM).
When the load current rises above the critical conduction point, the inductor current will not cross zero Amps in a switching cycle and the regulator will operate in continuous conduction mode (CCM) although the controller is in DE mode. Switching losses are therefore reduced in DE mode and light-load efficiency is improved by reducing the switching frequency.
When the PC is in Connected Standby mode, the PMIC receives the appropriate signal via its #CS pin (see Figure 1). The PMIC then takes all switching converters out of R4 modulation mode and instead uses a low-IQ constant on-time modulator to reduce controller power and switching losses. The constant on-time modulator slows the DCM switching frequency of all the switched-mode regulators on-chip, thereby reducing the switching losses in each regulator. Figure 5 compares the DCM pulse of the VDDQ switcher in normal operating mode and in CS mode.
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Figure 5. VDDQ DCM pulse in normal operation mode and CS mode.
Over-current, over-voltage and over-temperature fault protections, as well as under-voltage and over temperature warnings, further improve the efficiency of the system. A dedicated temperature-warning signal indicates if the PMIC is operating at elevated temperature, and there is also a general ALERT that indicates if any other fault or warning has occurred.
While expectations for mobile systems continue to increase, the power management capability is increasing as well, enabling next generation mobile products that are more functional and need to be charged less often. The latest advances in PMIC technology support the efficient new PC operating modes, and can replace discrete solutions to enable a smaller footprint and even more energy-conscious battery management.