
Data Sheet
AD5757
Rev. D | Page 31 of 44
Software Register
The software register has three functions. It allows the user to
perform a software reset to the part. It can be used to set the
user toggle bit, D11, in the status register. It is also used as part
of the watchdog feature when it is enabled. This feature is useful
to ensure that communication has not been lost between the
MCU and the AD5757 and that the datapath lines are working
properly (that is, SDIN, SCLK, and SYNC).
When the watchdog feature is enabled, the user must write
0x195 to the software register within the timeout period. If this
command is not received within the timeout period, the ALERT
pin signals a fault condition. This is only required when the
watchdog timer function is enabled.
DC-to-DC Control Register
The dc-to-dc control register allows the user control over
the dc-to-dc switching frequency and phase, as well as the
maximum allowable dc-to-dc output voltage. The dc-to-dc
Table 21. Programming the Software Register
MSB
LSB
D15
D14
D13
D12
D11 to D0
1
0
User program
Reset code/SPI code
Table 22. Software Register Functions
Bit
Description
User Program
This bit is mapped to Bit D11 of the status register. When this bit is set to 1, Bit D11 of the status register is set
to 1. Likewise, when D12 is set to 0, Bit D11 of the status register is also set to zero. This feature can be used to
ensure that the SPI pins are working correctly by writing a known bit value to this register and reading back
the corresponding bit from the status register.
Reset Code/SPI Code
Option
Description
Reset code
Writing 0x555 to D[11:0] performs a reset of the AD5757.
SPI code
If the watchdog timer feature is enabled, 0x195 must be written to the software
register (D11 to D0) within the programmed timeout period.
Table 23. Programming the DC-to-DC Control Register
MSB
LSB
D15
D14
D13
D12 to D7
D6
D5 to D4
D3 to D2
D1 to D0
0
1
DC-DC Comp
DC-DC phase
DC-DC Freq
DC-DC MaxV
1 X = don’t care.
Table 24. DC-to-DC Control Register Options
Bit
Description
DC-DC Comp
Selects between an internal and external compensation resistor for the dc-to-dc converter. See t
he DC-to-DC Converterinformation.
0 = selects the internal 150 k compensation resistor (default).
1 = bypasses the internal compensation resistor for the dc-to-dc converter. In this mode, an external dc-to-dc
compensation resistor must be used; this is placed at the COMP
DCDC_x pin in series with the 10 nF dc-to-dc compensation
capacitor to ground. Typically, a ~50 k resistor is recommended.
DC-DC Phase
User programmable dc-to-dc converter phase (between channels).
00 = all dc-to-dc converters clock on the same edge (default).
01 = Channel A and Channel B clock on the same edge, Channel C and Channel D clock on opposite edges.
10 = Channel A and Channel C clock on the same edge, Channel B and Channel D clock on opposite edges.
11 = Channel A, Channel B, Channel C, and Channel D clock 90° out of phase from each other.
DC-DC Freq
00 = 250 ± 10% kHz.
01 = 410 ± 10% kHz (default).
10 = 650 ± 10% kHz.
DC-DC MaxV
Maximum allowed V
BOOST_x voltage supplied by the dc-to-dc converter.
00 = 23 V + 1 V/1.5 V (default).
01 = 24.5 V ± 1 V.
10 = 27 V ± 1 V.
11 = 29.5 V ± 1V.