Information

33.3.2.3 SC[MODE_LV] = 10
The internal VREF bandgap is on. The low power buffer is enabled to generate a buffered
1.2 V voltage to VREF_OUT. It can also be used as a reference to internal analog
peripherals such as an ADC channel or analog comparator input.
If this mode is entered from the standby mode (SC[MODE_LV] = 00, SC[VREFEN] = 1)
there will be a delay before the buffer output is settled at the final value. This is the buffer
start up delay (Tstup) and the value is specified in the appropriate device data sheet. If
this mode is entered when the VREF module is enabled then you must wait the longer of
Tstup or until SC[VREFST] = 1.
In this mode, a 100 nF capacitor is required to connect between the VREF_OUT pin and
VSSA.
33.3.2.4 SC[MODE_LV] = 11
Reserved
33.4 Initialization/Application Information
The Voltage Reference requires some time for startup and stabilization. After
SC[VREFEN] = 1, SC[VREFST] can be monitored to determine if the stabilization and
startup is completed.
When the Voltage Reference is already enabled and stabilized, changing SC[MODE_LV]
will not clear SC[VREFST] but there will be some startup time before the output voltage
at the VREF_OUT pin has settled. This is the buffer start up delay (Tstup) and the value
is specified in the appropriate device data sheet. Also, there will be some settling time
when a step change of the load current is applied to the VREF_OUT pin. When the 1.75V
VREF regulator is disabled, the VREF_OUT voltage will be more sensitive to supply
voltage variation. It is recommended to use this regulator to achieve optimum
VREF_OUT performance.
The TRM[CHOPEN], SC[REGEN] and SC[ICOMPEN] bits are written to 1 during
factory trimming of the VREF voltage. These bits should be written to 1 to achieve the
perfromance stated in the device data sheet.
Initialization/Application Information
K20 Sub-Family Reference Manual, Rev. 2, Feb 2012
666 Freescale Semiconductor, Inc.