COVER STORY FEATURE


applications in power distribution, datacom and telecom as well as emerging 48V automotive systems. The LTC7821 operates over a 10V to 72V (80V abs max) input voltage range and can produce output currents in multiple 10s of amps, depending on the choice of external components. External MOSFETs switch at a fixed frequency, programmable from 200kHz to 1.5MHz. In a typical 48V to 12V/20A application, an efficiency of 97% is attainable with the LTC7821 switching at 500kHz. The same efficiency can only be


achieved in a traditional synchronous step-down converter by switching at one-third the operating frequency, forcing the use of much larger magnetics and output filter components. The LTC7821’s powerful 1Ω N-channel MOSFET gate drivers maximise efficiency and can drive multiple MOSFETs in parallel for higher power applications. Due to its current mode control


architecture, multiple LTC7821’s can be operated in a parallel, multiphase configuration with excellent current sharing and low output voltage ripple to enable much higher power applications without hot spots. The LTC7821 implements many protection features for robust performance in a wide range of applications. A design based on this device also eliminates the inrush current typically associated with switched capacitor circuits by pre-balancing the capacitors on startup. This device also monitors the system voltage, current and temperature for faults and uses a sense resistor for overcurrent protection. It stops switching and pulls the /FAULT pin low when a fault condition occurs. An onboard timer can be set for appropriate restart/retry times. It’s EXTVCC


pin permits the LTC7821 to be


powered from the lower voltage output of the converter or other available sources up to 40V, reducing power dissipation and improving efficiency. Additional features include +/- 1% output voltage accuracy over temperature, a clock output for multiphase operation, a power good output signal, short circuit protection, monotonic output voltage start-up, optional external reference, undervoltage lockout and internal charge balance circuitry. Figure 3 shows the schematic of the LTC7821 when converting a 36V to 72V input to a 12V/20A output. The efficiency curves shown in Figure 4 represent a comparison of three different converter types for the application that converts a 48VIN


to a 12VOUT at 20A as / ELECTRONICS


follows: 1. A single stage buck running at 125kHz with a 6V gate drive (Blue Curve)


2. A single stage buck running at 200kHz with a 9V gate drive (Red Curve)


3. The LTC7821 Hybrid running at 500kHz with a 6V gate drive (Green Curve)


A LTC7821-based circuit running at up to three times the operating frequency of the other converters has the same efficiency as the other solutions. This higher operating frequency results in a 56% reduction of the inductor size and up to a 50% reduction to the total solution size.


CAPACITOR PRE-BALANCING A switched capacitor converter usually has a very high inrush current when the input voltage is applied or when the converter is enabled, resulting in the possibility of supply damage. The LTC7821 has a proprietary scheme to pre-charge all switching capacitors before the converter PWM signal is enabled. Therefore, the inrush current during power up is minimised. In addition, this device also has a programmable fault protect window to


Figure 3:


LTC7821 schematic, 36VIN


output to 72VIn /12V/20A


further ensure reliable operation of the power converter. These features result in the output voltage having a smooth soft-start just like any other conventional current mode buck converter. See the LTC7821 datasheet for additional details.


MAIN CONTROL LOOP Once the capacitor balancing phase is completed, normal operation begins. MOSFETs M1 and M3 are turned ON when the clock sets the RS latch, and turned off when the main current comparator, ICMP, resets the RS latch. MOSFETs M2 and M4 are then turned on. The peak inductor current at which ICMP resets the RS latch is controlled by the voltage on the ITH


pin, which is the output of the


error amplifier EA. The VFB


pin receives the voltage


feedback signal, which is compared to the internal reference voltage by the EA. When the load current increases, it causes a slight decrease in VFB relative to the 0.8V reference, which in turn causes the ITH


voltage to increase until the


average inductor current matches the new load current. After MOSFETs M1 and M3 have turned


off, MOSFETs M2 and M4 are turned on until the beginning of the next cycle. During the switching of M1/M3 and M2/M4, capacitor CFLY


is alternately


connected in series with or parallel to CMID


. The voltage at MID will be approximately at VIN /2. So, this converter Figure 4:


Efficiency comparison and transformer size reduction


just operates like a conventional current mode buck converter with fast and accurate cycle-by-cycle current limit and option for current sharing. The combination of a switched capacitor circuit to halve the input voltage followed by a synchronous step- down converter (hybrid converter) provides up to a 50% reduction in DC/DC converter solution size compared to traditional buck converter alternatives. This improvement is enabled by a three times higher switching frequency without compromising efficiency. Alternatively, the converter can operate


with 3% higher efficiency in a footprint comparable to existing solutions. This new hybrid converter architecture also provides other benefits that include soft switching for reduced EMI and MOSFET stress. Multiple converters can be easily paralleled with active accurate current sharing when high power is needed.


Analog Devices Ltd. www.analog.com 01628 477 066


e: uksales@linear.com


ELECTRONICS | APRIL 2018 13


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