AFS090 datasheet(105/334 Pages) MICROSEMI

105 / 334 page

Fusion Family of Mixed Signal FPGAs

R e visio n 2

2 - 89

To initiate a current measurement, the appropriate Current Monitor Strobe (CMSTB) signal on the AB

macro must be asserted low for at least tCMSLO in order to discharge the previous measurement. Then

CMSTB must be asserted high for at least tCMSET prior to asserting the ADCSTART signal. The CMSTB

must remain high until after the SAMPLE signal is de-asserted by the AB macro. Note that the minimum

sample time cannot be less than tCMSHI. Figure 2-69 shows the timing diagram of CMSTB in relationship

with the ADC control signals.

Figure 2-70 illustrates positive current monitor operation. The differential voltage between AV and AC

goes into the 10× amplifier and is then converted by the ADC. For example, a current of 1.5 A is drawn

from a 10 V supply and is measured by the voltage drop across a 0.050

Ω sense resistor, The voltage

drop is amplified by ten times by the amplifier and then measured by the ADC. The 1.5 A current creates

a differential voltage across the sense resistor of 75 mV. This becomes 750 mV after amplification. Thus,

the ADC measures a current of 1.5 A as 750 mV. Using an ADC with 8-bit resolution and VAREF of 2.56

V, the ADC result is decimal 75. EQ 3 shows how to compute the current from the ADC result.

EQ 3

where

I is the current flowing through the sense resistor

ADC is the result from the ADC

VAREF is the Reference voltage

N is the number of bits

Rsense is the resistance of the sense resistor

Figure 2-69 • Timing Diagram for Current Monitor Strobe

VADC

tCMSET

CMSTBx

ADCSTART can be asserted

after this point to start ADC

sampling.

tCMSHI

ADCSTART

tCMSLO

ADC

AREF

() 10 2

sense

()

⁄

AFS090 Datasheet(PDF) 105 Page - Microsemi Corporation