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MAX749 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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MAX749 Datasheet(HTML) 7 Page - Maxim Integrated Products |
7 / 12 page Figure 4. Count-Up Operation Figure 5. Reset Sequence without Shutdown. The device is not shut down during reset. Figure 6. Reset Sequence with Shutdown In Figure 3, the MAX749 is reset when it is taken out of shutdown, which sets the output at mid-scale. Figure 4 shows how to increment the counter. Figure 5 illustrates a reset without shutting the device down. Figure 7 provides an example of a sequence of opera- tions: Starting from shutdown, the device is turned on, incremented, reset to mid-scale without being shut down, incremented again, and finally shut down. Shutdown Mode When CTRL and ADJ are both low, the MAX749 is shut down (Table 1): The internal reference and biasing cir- cuitry turn off, the output voltage drops to zero, and the supply current drops to 15µA. The MAX749 retains its DAC setting, simplifying software control. Reset Mode If ADJ is high when CTRL is low, the DAC set point is reset to mid-scale and the MAX749 is not shut down. Mid-scale is 32 steps from the minimum, 31 steps from the maximum. Design Procedure _________and Component Selection Setting the Output Voltage The MAX749’s output voltage is set using an external resistor and the internal current-output DAC. The full- scale output voltage is set by selecting the feedback resistor RFB according to the formula: -VOUT(MAX) = RFB x 20µA (Figure 1). The device is in regulation when VFB = 0V. DAC Adjustment On power-up or after a reset, the counter sets the DAC output to mid-range, and -VOUT = RFB x 13.33µA. Each rising edge of ADJ increments the counter (and there- fore the DAC output) in the direction of -VOUT(MAX) by one count. When incremented beyond -VOUT(MAX), the Digitally Adjustable LCD Bias Supply _______________________________________________________________________________________ 7 Figure 3. Shutdown-Reset-On-Shutdown Sequence of Operation. The device is not shut down during reset. Figure 7. Control Sequence Example (see Output Voltage Control section) ADJ CTRL SHUTDOWN RESET ON SHUTDOWN tR tSD ADJ CTRL HIGH tSH tSL ADJ CTRL ON RESET ON tRS tRH tR ADJ CTRL SHUTDOWN RESET ON SHUTDOWN INCREMENT INCREMENT ADJ CTRL SHUTDOWN RESET ON tRH tR |
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