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Posted on 02 July 2019

Integrated All-in-One Buck Converters Make for Great Macros

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High current gate drives, coupled with optimum dead times yield higher efficiency

Low power Point of Load (POL) Buck Regulators have evolved dramatically, particularly in netcom and storage markets. While consuming less power than CPU’s, function specific ASICs, impose ever tighter accuracy and space constraints as well as mandating special features.

By Mehrzad Koohian, Staff Field Engineer, International Rectifier

 

The number of these rails on a typical motherboard or line card far exceeds the number of high current (CPU, Memory, Graphics) regulators. Hence efficiency is important when assessing the power losses of all these rails combined.

A key consideration for board designers is the ability to modularize the regulator. The designer creates a “macro” which is easily ported to any location on any board without the need to redesign and re-layout the regulator. The macro approach is gaining adoption in lower power applications as it significantly reduces the time to market by reducing the layout and validation time. The integrated controller + MOSFETs IR3837 family, offered in 5 x 6 mm, is well suited for such macro designs. Integration of the power MOSFETs reduces parasitic inductance and capacitance, often a nuisance for design engineers trying to avoid EMI and ringing issues. Since a common footprint is used for a wide current range regulators (IR3839, 6A; IR3838, 10A and IR3837,14A), one macro layout can accomplish all rails under 14A. The IR3837/38/39 is a single supply regulator. The internal LDO of the IR3837 family supplies a 5.2V Vcc for the internal MOSFET gate drivers. The Vcc can also be externally back driven to voltages up to 6.5V, bypassing the internal LDO and achieve higher enhancement of MOSFETs for lower RDS(on).

Among the features recently demanded by netcom ASICs is the ability to track between rails, where the regulator output closely tracks a higher voltage rail at power up and shut-down. Hence the voltage difference between any two rails never exceeds a diode drop during start up and shutdown until each rail reaches its final value. This is in order to prevent cross conduction between rails through internal parasitic diodes in ASICs that have several supply voltages (as most ASICs do). Simultaneous tracking simplifies the ASIC designer’s task, who would otherwise have to add schottky diodes between rails or carefully time regulator ramp up and ramp down under all load and turn on/off conditions to avoid such parasitic currents. The IR3837 family has a dedicated track input which can be configured to track a higher voltage on the board. When then higher voltages ramps up, a voltage divider from that rail sets the reference for the tracking rail. Once the track voltage exceeds the reference input, tracking is complete and the high accuracy reference voltage takes over the regulation. Figures 1 and 2 shows tracking during turn-on and turn-off at zero and full load current. Note the two rails are rising and falling simultaneously.

Repetitive startup and shutdown, Vout=Vp (track input) Green Vp, Purple Vout, Brown ILoad=0A

Repetitive startup and shutdown, Vout=Vp (track input) Green Vp, Purple Vout, Brown ILoad=14A

Another desirable feature in regulators used in netcom and storage applications is the ability for a digital to analog converter (DAC) to tweak and margin the output voltage. While margining is used in testing the robustness of the ASIC during final production tests, (by changing the supply voltage +/-5%, for example and assuring the ASIC is functional at various functional loads), the tweakfunction is used for ASIC binning. Historically, ASICs do not function optimally at the nominally set regulator voltage are discarded as rejects. With tweaking the regulator output voltage such ASIC’s may operate at their optimum performance by adjusting the voltage using a DAC. The IR3837 reference has been designed to allow an external analog signal, such as a DAC, to over drive it with a sink/source current greater than 20uA. Hence low cost, current output DACs, can be used to tweak the regulator reference voltage. Additionally, the power good circuit uses this externally driven voltage (rather than a fixed internal reference voltage) for its window comparator, so the Pgood circuit tracks the external DAC. This simplifies both margining and tweaking, as the Pgood remains valid even at large DAC voltage swings. Floating this pin relinquishes regulator control to the internal, 0.6V reference for normal operation.

The IR3837 family has been designed to switch at high frequencies (<=1.5Mhz) at low duty cycles. Special care has been taken to minimize or eliminate jitter at such low pulse widths. Figure 3 shows the operation of the switcher at 1Mhz, and 0.6V output, with a 12Vinput bus. The pulse width is less than 50ns, exhibiting no jitter. (Scope is set to infinite persistence display mode).

Switching at 1 Mhz with negligible pulse width jitter

The higher frequency allows for smaller inductors, which in turn improve transient response and improve efficiency. Increasing the switching frequency also reduces output droop during transients by reducing latency errors, thus eliminating the need for additional capacitors.

High current gate drives, coupled with optimum dead times yield higher efficiency, even at high frequencies. It is important to note that high speed, high performance MOSFETs need to be utilized with optimized drive circuits that take full advantage of their improved parameters, such as gate charge, gate resistance and RDS(on). IR is focused on this exact challenge, and the IR3837 family has been optimized for best and most cost efficient MOSFET and gate driver combinations.

Figure 4 shows the efficiency of the IR383X family at the specified operating conditions. The efficiency curves include driver and inductor power loss.

Efficiency of the IR383X family with Vin=12V, including driver and inductor power loss

Figure 5 shows the infrared thermal image of the IR3837 at above test conditions, (14A load current). Note that the thermal rise is about 60 degrees C, for a power loss of approximately 3Watts. This yields a thermal resistance of 20C/Watt with a PCB size of 3x3 and 2oz copper on top and bottom layers.

Thermal image at Vin=12V, Vout=1.8V, Iout=14A

Netcom applications feature several POL rails, often as high as 10 to 15 rails on the same board. With so many switching frequencies at slightly different clocks, there is possibility of aliasing and/or cross talk among adjacent switchers. IR3837 family has a sync input to allow external synchronization to avoid cross talk and aliasing between rails. The synchronization also enables the user to use phase shifted clock inputs to reduce input current ripple, the same way input ripple current cancellation is achieved in multi-phase converters.

The IR3837 family has a wide bandwidth, high slew rate error amplifiers to allow high loop cross over frequencies (above 100Khz). Higher BW coupled with higher slew rate optimizes converters’ transient response and reduces output capacitor size and cost.

Finally, the IR3837 includes an accurate threshold EN function which can be configured as a UVLO for another rail. Additionally, the device features Hiccup mode over current, pre-bias and over temperature protection functions, as well as internal digital soft start.

A user friendly, interactive, web-based tool is available at http://mypower.irf.com/SupIRBuck. Once basic requirements are entered, the tool allows the user to capture schematics, create a reference design along with associated Bill of Materials (BOM), view waveforms, estimate efficiency and loop compensation, and perform complex thermal and application analysis quickly and easily to dramatically accelerate development time.

Also on the website, www.irf.com, is a complete guide for PCB layout and examples of regulator design and PCB implementations (evaluation boards PNs are IRDC3837/38/39).

Visit www.IRF.com to download datasheets and user guides.

 

 

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