Ways to improve VRM output transient response

In addition to the frequent and high-speed changes in current caused by the ESR and ESL of the output filter capacitor, which cause changes in the VRM output voltage, other parasitic parameters in the circuit, such as the parasitic resistance and parasitic inductance of the leads, will also have a great impact on the transient voltage adjustment changes of the VRM. Therefore, the parasitic parameters of the leads should be minimized as much as possible.

There are several ways to improve VRM output transient response:

(1) Connect a decoupling capacitor in parallel to the output. However, the decoupling capacitor also has parasitic parameters such as ESR and ESL, which will also cause a transient voltage drop at the decoupling capacitor end. Simply increasing the decoupling capacitor to improve the transient characteristics of the VRM is not necessarily very beneficial. This is because even a parasitic inductance of a few nanohenries requires a 100 μF decoupling capacitor to keep the VRM transient voltage within the specified range. The larger the decoupling capacitor, the larger the space it occupies, which is not commensurate with the size of the microprocessor. Sometimes it is even impossible to achieve due to space constraints. Moreover, increasing the decoupling capacitor is only beneficial for improving the initial stage of the output transient response, and has little effect on the later stage of the transient response. The improvement in transient response time is also not significant.

(2) Use a multi-channel converter to interleave the output current wave to ensure the output voltage ripple and improve the output transient response.

Table 1 shows the comparison of the results of simply increasing the decoupling capacitor and using interleaved control of multi-channel converters to improve the output transient response.

Table 1 Comparison of the two methods to improve the output transient response

(3) Use voltage regulation technology to improve transient output response and reduce output capacitance. Intel has worked with other companies to jointly develop a two-phase VRM suitable for servers, workstations and desktop computers. Output 1.1~1.85 V, 60A, external capacitor 1200 μf×6, inductor 1 μH, bus voltage 12 V. Adopt ￣ADP3160 control chip, according to the principle of active voltage regulation to improve transient response; VRM uses two ADP3412 voltage regulator module controller chips and two drive MOSFETs, small size and low cost.

(4) Shorten the connection line between VRM and microprocessor to reduce parasitic parameters. Place VRM and microprocessor on the motherboard, make VRM and microprocessor as close as possible, and even make inductor into planar magnetic components. VRM can be integrated with microprocessor system into a whole (System Integration), eliminate the connection line, and greatly reduce L and C of output filter.