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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 ?
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 ?
Do we need to mount RDES physically in the circuit, if yes then where. I don't found it in the KCU 105.

## Top Replies

• 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

• How it is 5 A that the main question. I have attached the two pages of schematic where it says 5A but my calculations mentioned in the post above does not match. How these power supplies are 5 A ?  • 25A is the lowest OCP can be selected for MAX20751 with 1-phase, which gives Ioutmax = 0.85x25=21.25A. Any selected designed Iout value below 21.25A, can only choose OCP value as 25A in the design. In another word, in Xilinx design, Iout = 5A, minimum OCP in theory will be 5/0.85 = 5.88A, however, MAX20751 does not have 5.88A OCP option for 1-phase, in the design, it can only be selected as 25A with RDES = 604ohm

• Thank you very much for reply. Yes, that is true that the lowest value of the OCP is 25 A that gives Ioutmax = 0.85x25=21.25A.

Question on the RDES is still there. I only see the resistors R_SEL0, R_SEL1, and R_SEL2 physically present in the schematic and placed in the layout as physical components but RDES having a value 604 Ohm from Table 5, where do we place it in the circuit ? It look like that RDES is only a value in theory and will be used in Table 7 to find the resistors R_SEL0, R_SEL1, and R_SEL2 for a given voltage. In other words, that RDES is not a physical resistor in the schematic, right ?

• Given the following values of the following resistors in Table 7 to get Vout = 0.030 + 0.970 = 1 V, how do we found three bits in the address PMAD [3:1] ?

RSEL2 = 402 Ohm (Vout = 0.970 V)
RSEL1 = 576 Ohm (Vout = 0.030 V)
RSEL0 = 0 Ohm (RDES = 604 Ohm)

My finding PMAD [3:1] = 010, is that correct ?

• Given the following values of the following resistors in Table 7 to get Vout = 0.030 + 1.170 = 1.2 V, how do we found three bits in the address PMAD [3:1] ?

RSEL2 = 576 Ohm (Vout = 1.170 V)
RSEL1 = 1020 Ohm (Vout = 0.030 V)
RSEL0 = 0 Ohm (RDES = 604 Ohm)

• That is right, R_DES is not a physical resistor in the schematic.

• PMAD is determined by R_SEL0, if R_SEL0 = 0 ohm, then PMAD =0. PMAD[2:1] is determined by R_SEL1, if R_SEL1 = 1020ohm, then PMAD[2:1] will be 11. You can refer to Table 7 in the datasheet.

• yes, it is correct.

• Alright. Thanks for reply once again. According to datasheet "R_SEL1 and R_SEL0 are the configuration resistors used to set the lowest 3 bits (PMAD[3:1])"

This means that only R_SEL1 and R_SEL0 are used to set the lowest 3 bits (PMAD[3:1])

R_SEL2 is NOT selecting the any of the bit (PMAD[3:1]).

However, R_SEL2 and R_SEL1 are the configuration resistors used to set the output voltage.