Feature: Power
Figure 6: The transient waveform for a 22-50A load step using a 28A/µs slew rate
Aſter determining the minimum output capacitance value, a 35mV (4.4%) VPP
was achieved in a 22-50A to 22A load transient with
28A/µs slew rate. Te transient waveform is shown in Figure 6. To verify the stability of the control loop, a Bode plot measure-
ment was made using a 50A load; see Figure 7. At 50A, the control loop has a bandwidth of 322kHz, with a 50° phase margin. For additional performance tests, the efficiency and full load
thermal performance are measured. Te efficiency was tested up to a 60A load for 12VIN
at 0.8VOUT ; see Figure 8. Including auxiliary
losses, the converter’s peak efficiency at 25A load was 89%, and at a 60A load it was 84%. The thermal performance of this four-phase design is shown in Figure 9 as a thermal image. With 60A load, the hottest IC is 66°C and the coolest is 61.6°C, giving a maximum temperature deviation of around 5°C between the four ICs – which is excellent current sharing between phases.
Design considerations As a peak current mode control IC, the LT8627SP can easily be configured for multiphase operation. Design considerations that need special care include: • Te VC pins of each IC should be tied together, as shown in Figure 1.
• To evenly interleave for a four-phase LT8627SPs, the CLKOUT of
each IC is configured to be phase-shiſted by 90° and fed into the SYNC pin of the next IC; see Figure 10. Interleaving is one of the biggest benefits of the multiphase buck.
Te evenly interleaved phases multiply the output voltage ripple frequency, significantly reducing the output capacitance. Te higher interleaved ripple frequency can also immunise the control loop against ripple noise at a higher bandwidth. Te LT8627SP can operate up to a 4MHz switching frequency with three different phase-shiſt clock configurations: 180°, 120° and 90°. Tis means
Figure 7: The Bode plot of the four-phase min VPP
solution
Figure 8: The efficiency of a four-phase interleaving the LT8627SP at 1MHz FSW, at 12VIN
and 0.8VOUT
24 November 2023
www.electronicsworld.co.uk
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