AN239
AN239 is Expanding ADC1 Dyanmic Range manufactured by Silicon.
..
E X PA N D I N G ADC1 D Y N A M I C R A N G E F O R T H E Si8250
1. Introduction
The Si8250 data sheet specifies a mon mode input range of 0.6 to 1.2 V for ADC1. This range effectively limits the dynamic voltage output range in voltage-controlled converters. For example, the output range for a 3.3 V power supply designed to generate a 1.00 V sense would have an absolute maximum dynamic range of 1.98 to 3.96 V. In reality, the active regulation point would need to be something greater than 1.98 V and less than 3.96 V since these are on the very fringes of the mon mode specification. If a design requires wider dynamic output voltage range (e.g., 2.00- 5.70 V) then a simple resistor divider will not work. This application note discusses a simple and inexpensive solution to expand the dynamic range for applications such as power factor correction and wide variable output power supplies.
2. ADC1 mon Mode Limits
The specified mon mode range for ADC1 (Figure 1) is from 0.6 V to the voltage reference which is typically 1.2 V. This simply means that the input voltages to ADC1 from both VSENSE and the REFDAC should stay within the range of 0.6 to 1.2 V referenced to the mon ground to achieve good linear response. For input voltages much lower than the mon mode specification, ADC1 exhibits non-linear characteristics. Figure 2 shows an example of the ADC1 dc response when the inputs are significantly out of specification. For input voltages above the mon mode range the input is effectively “cut off” yielding no change in the ADC output.
200 k Hz 12-Bit ADC
Bits [11:6]
- +
+
Bits [8:3]
REFDAC0H REFDAC0L
VSENSE
+
- 6
10 MHz 6-Bit ADC
1 0 2 3
6 To PID Filter
-1
REFDAC
PID Input MUX
ADC1DAT
SFR Bus READ/WRITE
REFDAC0H REFDAC0L
Figure 1. ADC1 Block Diagram
ADC1 Output 6 m V Step, VSENSE = 301 m V, VREF = 1.226 V
-32 -22 -12
ADC1DATA
-2 8 18 28 0 50 100 150...
