WM8945
Production Data
The resistance RREF is a function of the circuit gain and a function of the parallel combination of
RSOURCE and RLOAD. When VB_GAIN = 0 (0dB gain), the current gain of the video buffer is 5, as
described by the equation IVBOUT = 5 x IVBREF
.
The resistor RREF should be set equal to 5 x (RSOURCE // RLOAD), where (RSOURCE // RLOAD) is the effective
resistance of the parallel combination of RSOURCE and RLOAD. (Note that the required resistance RREF is
the same for both settings of VB_GAIN.)
In a typical application, RLOAD = 75, RSOURCE = 75, RREF = 187.
RECOMMENDED VIDEO BUFFER INITIALISATION SEQUENCE
Power Up (Video signal AC coupled to Video Buffer input):
ACTION
LABEL
REGISTER[BITS]
Turn on external supplies and wait for the
supply voltages to settle.
Reset registers to default state (software
reset)
SW_RESET
R0 (00h) [15:0]
VMID_FAST_START = 1
STARTUP_BIAS_ENA = 1
BIAS_SRC = 1
R7 (07h) [11]
R7 (07h) [8]
R7 (07h) [7]
R7 (07h) [6:5]
Enable VMID Fast Start and Start up
Bias
Select Start-Up Bias and set VMID soft
start for start-up ramp
VMID_RAMP[1:0] = 01
If using VMID as the reference voltage
for the LDO then select VMID fast start or
set to 0 if using the Bandgap as the
reference voltage for LDO.
LDO_REF_SEL_FAST = 1
LDO_BIAS_SRC = 1
R53 (35h) [14]
R53 (35h) [5]
Select LDO Start-Up Bias and enable
LDO
LDO_ENA = 1
R53 (35h) [15]
Delay 300ms for LDO to settle
BIAS_ENA = 1
VMID_BUF_ENA = 1
R2 (02h) [3]
R2 (02h) [2]
Enable VMID Buffer and Master Bias
Set VMID_SEL[1:0] for fast start-up
VMID_SEL[1:0] = 11
R2 (02h) [1:0]
Enable VMID
VMID_ENA = 1
R7 (07h) [4]
Delay 150ms to allow VMID to settle
LDO_REF_SEL_FAST = 0
LDO_BIAS_SRC = 0
R53 (35h) [14]
R53 (35h) [5]
Set LDO for normal operation
VMID_FAST_START = 0
STARTUP_BIAS_ENA = 0
R7 (07h) [11]
R7 (07h) [8]
Set VMID for normal operation
Set VMID divider for normal operation
VMID_SEL = 01
VMID_ENA = 1
R2 (02h) [1:0]
R7 (07h) [4]
Enable VMID
Delay 150ms to allow VMID to settle
LDO_REF_SEL_FAST = 0
LDO_BIAS_SRC = 0
R53 (35h) [14]
R53 (35h) [5]
Set LDO for normal operation
Set Video Buffer Gain as required
VB_GAIN
R38 (26h) [5]
R38 (26h) [6]
Set Video Buffer Filter Q Boost as
required
VB_QBOOST
Enable Video Buffer Clamp
Enable Video Buffer Pulldown
Enable video buffer
VB_CLAMP = 1
VB_PD = 1
R38 (26h) [0]
R38 (26h) [1]
R38 (26h) [7]
VB_ENA = 1
Delay 20ms for buffer to capture input
level
Disable Video Buffer Pulldown
VB_PD = 0
R38 (26h) [1]
PD, May 2011, Rev 4.1
82
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WOLFSON [ WOLFSON MICROELECTRONICS PLC ]
