LTM4663 datasheet(16/30 Pages) AD

16 / 30 page

LT8722

Rev. 0

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vs transfer function (Figure 6) is unaffected by SYS_DC.

However, the SYS_DC register setting effects the LDR

slope and the min/max duty cycle of the PWM driver as

shown in Figure 7 and Figure 8.

3V = 0.2•15V

VLDR

VSFB

SPIS_DAC CODE (MILLIONS)

–13.0 –8.7

–4.3

0.0

4.3

8.7

13.0

8722 F07

Figure 7. VLDR and VSFB vs SPIS_DAC Code

when VIN = 15V, SYS_DC[1:0] = 2b00,

fSW = 2MHz

1.5V = 0.1•15V

VLDR

VSFB

SPIS_DAC CODE (MILLIONS)

–13.0 –8.7

–4.3

0.0

4.3

8.7

13.0

8722 F08

Figure 8. VLDR and VSFB vs SPIS_DAC Code

when VIN =15V, SYS_DC[1:0] = 2b10,

fSW = 2MHz

The integrated high resolution DAC is used to set LDR,

SFB and the corresponding output voltage. In a regulation

feedback loop, a software controlled PID loop measures

a desired parameter, then adjusts the output voltage by

configuring the SPIS_DAC register through SPI. SPIS_

DAC is stored in 2’s complement format. The 7MSB bits,

SPIS_DAC[31:26] are sign-extended and are decided by

the SPIS_DAC[25] bit. Table 5 shows how to set VDAC

through the SPIS_DAC register. Note that VDAC is equal to

V1P25 when SPIS_DAC is 0x00000000. The output voltage

can be calculated by Equation 6 and Equation 7.

VOUT = VLDR − VSFB = − 16 • VDAC − V1P25

(

) (6)

VDAC = V1P25 − SPIS_DAC • V2P5 • 2−25

(7)

The integrated 25-bit DAC is used to set the output dif-

ferential voltage, VOUT, as per Equation 8. VOUT can be

changed by setting the SPIS_DAC register through the

SPI. SPIS_DAC is stored in 2’s complement format. The

7MSB bits, SPIS_DAC[31:26] are sign-extended and are

decided by the SPIS_DAC[25] bit. Table 5 shows how to

set VDAC through the SPIS_DAC register. Note that VOUT is

equal to zero when SPIS_DAC is 0x00000000. The output

voltage VOUT be calculated by Equation 8.

VOUT = 16 •SPIS_DAC • V2P5 • 2−25

(8)

Where 2–25 is approximately 29.802nV.

Table 5. VDAC vs SPIS_DAC

SW_DAC_VTEC BITS

DESCRIPTION, VDAC

0xFF000000

V1P25 + 16777216 • V2P5 • 2–25V = 2.5V

0xFF000001

V1P25 + 16777215 • V2P5 • 2–25V = 2.49999997V

• • •

0xFF999998

V1P25 + 6710888 • V2P5 • 2–25V = 2.00000003V

0xFF999999

V1P25 + 6710887 • V2P5 • 2–25V = 2.0V

0xFF99999A

V1P25 + 6710886 • V2P5 • 2–25V = 1.99999997V

• • •

0xFFFFFFFF

V1P25 + 1 • V2P5 • 2–25V = 1.25000003V

0x00000000

V1P25 + 0 • V2P5 • 2–25V = 1.25V

0x00000001

V1P25 – 1 • V2P5 • 2–25V = 1.24999997V

• • •

0x00666666

V1P25 – 6710886 • V2P5 • 2–25V = 0.75000003V

0x00666667

V1P25 – 6710887 • V2P5 • 2–25V = 0.75V

0x00666668

V1P25 – 6710888 • V2P5 • 2–25V = 0.74999997V

• • •

0x00FFFFFE

V1P25 – 16777214 • V2P5 • 2–25V = 0.00000006V

0x00FFFFFF

V1P25 – 16777215 • V2P5 • 2–25V = 0.00000003V

OPERATION

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