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Questions about MAX20751 and VT1697SB

Category: Hardware
Product Number: MAX20751

Hi, I have some questions about these two devices MAX20751 and VT1697SB. These two devices have been used in power supplies MGTAVCC (1.0 V @ 5 A) MGTAVTT (1.2 V @ 5 A) in the Kintex Ultrascale development board KCU 105. I did calculation for MGTAVCC (1.0 V @ 5 A), and MGTAVTT (1.2 V @ 5 A) but this did not match at 5A. 

Calculation for MGTAVCC (1.0 V @ 5 A):
Number of populated phase = 1, because only ISENSE2 along with PMW2 are connected to slave device which is one-phase according to Table 2. Phase Population Position of MAX20751 datasheet.
RSEL3 = 80.6 Ohm (Fsw = 600 KHz and slew rate = 1.25 V /ms) found from Table 6. Using R_SEL3 to Set the VOUT Slew Rate and Switching Frequency of MAX20751 datasheet.
RSEL2 = 402 Ohm (Vout = 0.970 V) found from Table 7 of MAX20751 datasheet.
RSEL1 = 576 Ohm (Vout = 0.030 V) found from Table 7 of MAX20751 datasheet.
RSEL0 = 0 Ohm (RDES = 604 Ohm) found from Table 7 of MAX20751 datasheet.

Vout = 0.030 + 0.970 = 1 V
Ioutmax = 0.85 x Iocp found from Table 5 of MAX20751 datasheet.

Iocp from Table 5 with RDES = 604 Ohm and one-phase is 25 A that gives Ioutmax = 0.85 x 25 = 21.25 A but in the schematic it says 5 A. How 5 A is calculated ?
PMAD [2:0] : 010: Please check and confirm the PMBus Address Bits ?
Do we need to mount RDES physically in the circuit, if yes then where. I don't found it in the KCU 105.

Calculation for MGTAVTT (1.2 V @ 5 A):
Number of populated phase = 1, because only ISENSE2 along with PMW2 are connected to slave device which is one-phase according to Table 2. Phase Population Position of MAX20751 datasheet.
RSEL3 = 1020 Ohm (Fsw = 800 KHz and slew rate = 0.5 V /ms) found from Table 6. Using R_SEL3 to Set the VOUT Slew Rate and Switching Frequency of MAX20751 datasheet.
RSEL2 = 576 Ohm (Vout = 1.170 V)
RSEL1 = 1020 Ohm (Vout = 0.030 V)
RSEL0 = 0 Ohm (RDES = 604 Ohm)

Vout = 0.030 + 1.170 = 1.2 V
Ioutmax = 0.85 x Iocp

Iocp from Table 5 with RDES = 604 Ohm and one-phase is 25 A that gives Ioutmax = 0.85 x 25 = 21.25 A but in the schematic it says 5 A. How 5 A is calculated ?
PMAD [2:0] : xx0 ? Please check and confirm the PMBus Address Bits ?
Do we need to mount RDES physically in the circuit, if yes then where. I don't found it in the KCU 105.

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  • Hello,

    25A is the lowest OCP can be selected for MAX20751, while i think 5A is designed output current for those two rails.

    PMBus has 7 bits, lowest 3 bits are selectable by R_SEL0 and R_SEL1, the upper 4 bits are constant at the value 1110b. 

    RDES value is set by R_SEL0

Reply
  • Hello,

    25A is the lowest OCP can be selected for MAX20751, while i think 5A is designed output current for those two rails.

    PMBus has 7 bits, lowest 3 bits are selectable by R_SEL0 and R_SEL1, the upper 4 bits are constant at the value 1110b. 

    RDES value is set by R_SEL0

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