POWER
“
Figure 3. Circuit design for a hybrid step-down converter.
This can be quite surprising, since all the power on the 3.3V output needed to run through two individual switching regulator circuits. The efficiency of the circuit in Figure 1 is lower due to the short duty cycle and the resulting high inductor peak currents. When comparing single step-down architectures with intermediate bus architectures, there are many more aspects to consider besides power efficiency. One other solution to this basic problem is the LTC7821, hybrid step-down controller from Analog Devices. It combines charge pump action with a step-down buck regulation. This enables the duty cycle to be 2× VIN/VOUT and, thus, high step-down ratios can be achieved at high-power conversion efficiencies. Figure 3 shows the circuit setup of the LTC7821. It is a hybrid step-down synchronous controller. It combines a charge pump to halve the input voltage with a synchronous step-down converter utilising the buck topology. With it, conversion efficiencies of more than 97 percent for converting 48V to 12V at a 500kHz switching frequency are possible. With other architectures, this high efficiency would only be feasible with much lower switching frequencies. They would require larger inductors.
Four external switching transistors are activated. During operation, the capacitors C1 and C2 generate the charge pump function. The voltage generated in this way is converted into a precisely regulated output voltage with the synchronous buck function. To optimise the EMC characteristics, the charge pump is used with soft switching operations.
The combination of a charge pump and a buck topology offers the following advantages. Due to the optimal
The duty cycle is
the relationship between the on-time, when the main switch is turned on, and the off-time, when the main switch is turned off.
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Figure 4. Typical conversion efficiency for converting 48V to 5V at a switching frequency of 500kHz.
combination of charge pump and synchronous switching regulator, the conversion efficiency is extremely high. The external MOSFETs M2, M3 and M4 only have to withstand low voltages. The circuit is also compact. The coil is smaller and cheaper than in a single-stage converter approach. For this hybrid controller, the duty cycle for switches M1 and M3 is D = 2 × VOUT/VIN. For M2 and M4, the duty cycle is calculated as D = (VIN – 2 × VOUT)/VIN.
For charge pumps, many developers assume a power output limitation of approximately 100mW. The hybrid converter switch with the LTC7821 is designed for output currents of up to 25A. For even higher performance, multiple LTC7821 controllers can be connected in a parallel multiphase configuration with synchronised frequency
to share the overall load.
Figure 4 shows the typical conversion efficiency for a 48V input voltage and a 5V output voltage at different load currents. At approximately 6A, a conversion efficiency exceeding 90 percent is reached. Between 13A and 24A, the efficiency is even higher than 94 percent.
A hybrid step-down controller supplies extremely high conversion efficiency in a compact form. It offers an interesting alternative to a discrete two-stage switching regulator design with intermediate bus voltage and to a single-stage converter that is forced to operate at a very low duty cycle. Some designers will prefer a cascaded architecture, others a hybrid architecture. With these two available options, every design should be successful.
OCTOBER 2024 | ELECTRONICS TODAY 39
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