Monolithic boost converters can convert low input voltages to a higher output voltage, all in a compact solution size. However, when the output power requirements increase, so do current levels and heat dissipation. Limitations from the internal switch make it difficult to achieve those requirements. Thus, a two-phase boost converter can be better suited for these types of low-voltage applications.
By interleaving the switching action of the converter into two phases, the switch current ripple is effectively cut in half. This allows for smaller capacitors and inductors, as well as improved thermal performance.
One example is the LT8349, a monolithic synchronous boost converter equipped with two internal N-channel MOSFET stages switching out of phase. These power switches are rated at 8 V, 6 A and operate at a fixed switching frequency that can be programmed between 300 kHz and 4 MHz or synchronized to an external clock. Synchronous rectification increases efficiency and reduces power loss and heat dissipation over a wide range of loads, while Stage Shedding and optional Burst Mode help to improve efficiency at lighter loads. This boost converter can be used in applications such as handheld and industrial power supplies.
If the application requires low electromagnetic interference (EMI) emissions, spread-spectrum frequency modulation (SSFM) is an optional feature available to minimize noise.
This IC has a 2.5-V to 5.5-V input range, making it suitable for battery-powered applications, and the output voltage is programmable up to 8 V. This two-phase converter comes in a small, 1.9 × 2.6-mm WLCSP package, which helps minimize the overall footprint of any design.
Multiphase operation
The LT8349 uses a fixed-frequency, current-mode control scheme, providing excellent line and load regulation. The dual-phase architecture requires two inductors, but the IC equally divides current among the two phases and spaces out the switching action of each phase by 180°. This allows for substantially lower peak inductor currents and reduced output ripple. The peak inductor current is given by Equation 1:
where IOUT is the average load current, D is the PWM duty cycle, and ∆IL is the inductor ripple current.
High performance 6-V, 5-A power supply
Figure 1 shows a 6-V step-up application from a 2.5-V to 4.5-V input source. It can supply a maximum load current of 5 A when the input voltage is at 4.5 V and the switching frequency is programmed to 2 MHz using a 54.9-kΩ resistor at the RT pin.
Figure 1: 2.5-V to 4.5-V input, 6-V output boost converter (Source: Analog Devices Inc.)
Stage shedding
During heavy loads, the LT8349 operates as a two-phase boost converter. As load current decreases, so does the peak inductor current of each phase. When the peak current reduces to the shedding threshold (ISHED, DUAL) of approximately 1.7 A, Stage Shedding activates where the device operates as a single-phase boost converter instead of a two-phase. In this mode of operation, the second phase is turned off and the peak inductor current limit of Phase 1 is increased to ISHED, SINGLE, which is given by Equation 2:
As load is further reduced, this IC can be programmed to operate in a low-quiescent-current, low-output-ripple Burst Mode or a fixed-frequency forced continuous mode (FCM) by setting the SYNC/MODE pin. The Stage Shedding feature is further detailed in Figure 2, which shows the behavior of the inductor currents of each phase in both Burst Mode and FCM mode.
Figure 2: Simplified drawings of LT8349 operations as the load current decreases from high current to very low current (Source: Analog Devices Inc.)
When the SYNC/MODE pin is connected to signal ground, the IC operates in Burst Mode, where the output regulation voltage is maintained by reducing the switching frequency. The IC will deliver single pulses of current with peak IBURST programmed using the ISET pin. A sleep period will immediately follow the pulse, allowing for only 15 µA of quiescent current when there is no load at the output.
When the SYNC/MODE pin is floating, the IC operates in FCM at light loads. In this mode, the inductor current is allowed to go negative so the IC can switch at the programmed frequency over all ranges of load. This allows for consistent and predictable switching harmonics and EMI but at the cost of light-load efficiency. Figure 3 shows a comparison of the efficiency between Burst Mode and FCM.
Figure 3: Efficiency and power loss vs. output current (Source: Analog Devices Inc.)
SSFM
For applications in which EMI emissions are an important requirement, the LT8349 offers additional resources to further reduce noise. SSFM can be selected by configuring the SYNC/MODE pin and is compatible with Burst Mode and FCM operations. When SSFM is selected, the internal oscillator frequency is varied between the value programmed by the external RT resistor and approximately 25% higher than that value. Figures 4 and 5 show the conducted and radiated EMI results for CISPR 32 Class B standards, respectively.
Figure 4: CISPR 32 conducted EMI results (Source: Analog Devices Inc.)
Figure 5: CISPR 32 radiated EMI results (Source: Analog Devices Inc.)
This IC provides several benefits over the traditional single-phase boost converter. Higher output power can be achieved using the two-phase architecture and synchronous rectification. These two features help to increase efficiency, reduce power losses, and enhance thermal performance, all while keeping the overall footprint small despite requiring two inductors.
Stage Shedding and Burst Mode operation further increase efficiencies at light loads, while the optional SSFM helps to reduce EMI emissions. Additional features of the LT8349 include output soft start and output overvoltage lockout, providing protection for downstream components from excessively high voltage.
About the author
Michael Wu is a product applications engineer at Analog Devices Inc. He works in the high-performance power group, focusing on monolithic buck, boost, and buck-boost topologies. He studied electrical engineering at California Polytechnic State University San Luis Obispo (B.S. and M.S.).
The post Two-phase boost converter reduces power losses appeared first on Electronic Products.
