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ISL45042AIRZ-T Datasheet(PDF) 6 Page - Intersil Corporation |
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ISL45042AIRZ-T Datasheet(HTML) 6 Page - Intersil Corporation |
6 / 8 page 6 FN6158.3 August 29, 2007 Table 1 gives the calculated value of VOUT for resistors values of: RSET = 24.9kΩ, R1 = 200kΩ, R2 = 243kΩ, and AVDD = 10V. RSET Resistor The external RSET resistor sets the full-scale sink current that determines the lowest voltage of the external voltage divider R1 and R2 (Figure 1). The voltage difference between the VOUT pin and ISET pin (Figure 5) has to be greater than 1.75V. This will keep the output MOS transistor in the saturation region. Expected current settings and 7-Bit accuracy occurs when the output MOS transistor is operating in the saturation region. Figure 5 shows the internal connection for the output MOS transistor. The value of the AVDD supply sets the voltage at the source of the output transistor. This voltage is equal to (Setting/128) x (AVDD/20). The ISET current is therefore equal to (Setting/128) x (AVDD/20 x RSET). The value of the Drain voltage is found using Equation 2. The values of R1 and R2 (Equation 2) should be determined (setting equal to 128) so the minimum value of VOUT is greater than 1.75V + AVDD/20. Power Supply Sequence The recommendation for power supply sequence would be to power down the part first (VDD, AVDD), after 100ms if programming has occurred, and then power down the control power supplies (CTL, CE). Verifying the Programmed Value The following sequence can be used to verify the programmed value without having to sequence the VDD supply. To verify the programmed value, follow the following steps. The ISL45042A will read memory contents and be set to that value when the CE pin is grounded. 1. Power up the ISL45042A. 2. CE pin = VDD. 3. Change counter value with CTL pin to desired value. 4. CTL = more than 4.9V and 200ms. Counter value programmed. 5. Change the counter value with CTL pin to a different value. 6. CE pin = Ground. 7. Check that the output value is the one programmed in Step 4. Generating VDD and CE supply from a Larger Voltage Source The CE pin has an internal pull-down resistor (RINTERNAL Figure 6). The impedance of this resistor is 400k Ω to 500kΩ. If your design is using a resistor divider network to generate the 3.3V supply (for both VDD and CE to enable programming) from a larger voltage source, the 400k Ω (worst case) resistor needs to be taken into account as a parallel resistance when the CE pin is connected to this source. Another design concern is to be able to provide enough supply current during programming. The ISL45042A draws about 2mA during this process. Recommended resistor values are shown in Figure 6. This design will result in an additional 0.83mA quiescent current flowing through resistors RA and RB. TABLE 1. CALCULATED VCOM OUTPUT VOLTAGES SETTING VALUE VOUT 1 5.468 10 5.313 20 5.141 30 4.969 40 4.797 50 4.625 60 4.453 70 4.281 80 4.109 90 3.936 100 3.764 110 3.592 128 3.282 FIGURE 5. OUTPUT CONNECTION CIRCUIT EXAMPLE AVDD = 15V RSET VOUT PIN R1 R2 AVDD VSAT 0.5V ISET PIN SETTING 128 ---------------------------- x AV DD 20 ------------------ FIGURE 6. APPLICATION GENERATING VDD AND VCE VOLTAGES VCC = 5V SCHMITT TRIGGER CE LOGIC RINTERNAL = 400kΩ to 500kΩ RA RB 2k Ω 4k Ω ISL45042A CE VCE ISL45042A |
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