wm8400 Wolfson Microelectronics plc, wm8400 Datasheet - Page 143

• Manufacturer Part Number: wm8400
• Description: Wolfson Audioplustm Hi-fi Audio Codec And Power Management Unit For Mobile Multimedia
• Manufacturer: Wolfson Microelectronics plc

WM8400
Pre-Production
For best accuracy, the fixed default voltages should be used if possible. These default settings
(selected by connecting VSEL pins to 0V or to 2.4V) are intended to meet the requirements of the
most likely usage scenarios. In this configuration, the resistance of the VSEL connection to 0V or
2.4V should be as low as possible, and must be less than 50kΩ.
If the programmable start-up voltage method is used (ie. connecting VSEL to a resistive divider), the
resistors should be selected as described in Table 89. It is also an important requirement that the
parallel combination of resistors R1 and R2 must equate to at least 100kΩ.
The parallel combination of resistors R1 and R2 is calculated using the equation below. If the parallel
resistance is too small, then R1 and R2 should be scaled in order to achieve a suitable value.
Some limitations of the programmable start-up voltage method should be noted. In start-up, the
WM8400 initially selects the lowest valid output voltage for each applicable LDO or DC-DC
Converter. The control circuit then steps incrementally through the available voltage selections (listed
in Table 87 and Table 88) until the desired output voltage is reached. The control circuit detects the
desired output voltage using a comparator connected to the VSEL pin. The comparator reference is
0.625V.
If the LDO or DC-DC Converter is loaded during start-up, this may result in the selection of a slightly
higher output voltage setting than the value set by R1 and R2. The voltage drop caused by the load
causes the VSEL voltage to be reduced also. In this case, the control circuit continues to increment
the LDO or DC-DC Converter output voltage until the correct voltage is sensed at VSEL. When this
has been achieved, the output voltage of the converter may be slightly higher than under ideal
conditions. The magnitude of the error may be larger in the case of the LDOs, as the voltage control
steps are larger. The voltage error is more likely to arise under high load conditions.
An important consequence of this behaviour is that the maximum LDO or DC-DC Converter output
voltages cannot be selected in start-up. This is because, in attempting to achieve the required
voltage on VSEL, the control circuit may attempt to select an output voltage higher than the
maximum, which can lead to an unstable condition.
The maximum recommended start-up output voltage selected by R1 and R2 is 3.1V (for LDO1 and
LDO2) and 3.975V (for DC1 and DC2). (As noted earlier, the performance figures in the Electrical
Characteristics are valid for DC-DC output voltages of up to 3.4V only.)
The limitations described above are only applicable during the start-up phase of the WM8400. It is
recommended that the voltage select registers (see Table 87 and Table 88) should be set in software
at the earliest opportunity after start-up in order to ensure accurate output voltage regulation for
normal running conditions. Note that, after start-up, the VSEL pins are no longer used.
PP, April 2009, Rev 3.0
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