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MAX1747EUP Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX1747EUP Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 13 page Triple Charge-Pump TFT LCD DC-DC Converter ______________________________________________________________________________________ 11 in the Typical Operating Characteristics). The main out- put voltage is controlled to be in regulation within 4096 clock cycles (1/fOSC). The negative and positive low- power charge pumps are controlled to be in regulation within 2048 clock cycles. Power-Up Sequencing Upon power-up or exiting shutdown, the MAX1747 starts a power-up sequence. First, the reference pow- ers up. Then the primary charge pump powers up with soft-start enabled. Once the main charge pump reach- es 90% of its nominal value (VFB > 1.125V), the nega- tive charge pump turns on. When the negative output voltage reaches approximately 90% of its nominal value (VFBN < 125mV), the positive charge pump starts up. Finally, when the positive output voltage reaches 90% of its nominal value (VFBP > 1.125V), the active-low ready signal (RDY) goes low (see Power Ready). Fault Detection Once RDY is low, and if any output falls below its fault detection threshold, RDY goes high impedance. For the reference, the fault threshold is 1.05V. For the main charge pump, the fault threshold is 88% of its nominal value (VFB < 1.1V). For the negative charge pump, the fault threshold is approximately 88% of its nominal value (VFBN > 140mV). For the positive charge pump, the fault threshold is 88% of its nominal value (VFBP < 1.1V). Once an output faults, all outputs later in the power sequence shut down until the faulted output rises above its power-up threshold. For example, if the nega- tive charge-pump output voltage falls below the fault- detection threshold, the main charge pump remains active while the positive charge pump stops switching and its output voltage decays, depending on output capacitance and load. The positive charge-pump out- put will not power up until the negative charge-pump output voltage rises above its power-up threshold (see Power-Up Sequencing). Power Ready Power ready is an open-drain output. When the power- up sequence is properly completed, the MOSFET turns on and pulls RDY low with a typical 125 Ω RON. If a fault is detected, the internal open-drain MOSFET appears as a high impedance. Connect a 100k Ω pullup resistor between RDY and IN for a logic level output. Voltage Reference The voltage at REF is nominally 1.25V. The reference can source up to 50mA with excellent load regulation (see Typical Operating Characteristics). Connect a 0.22µF bypass capacitor between REF and GND. During shutdown, the reference is disabled. Design Procedure Efficiency Considerations The efficiency characteristics of the MAX1747 regulat- ed charge pumps are similar to a linear regulator. They are dominated by quiescent current at low output cur- rents and by the input voltage at higher output currents (see Typical Operating Characteristics). Therefore, the maximum efficiency may be approximated by: Efficiency ≅ VOUT / (2 ✕ VSUPM) for the main charge pump Efficiency ≅ - VNEG / (VSUPN ✕ N) for the negative low-power charge pump OSC SUPP DRVP FBP PGND VREF 1.25V MAX1747 GND C3 D1 D2 R3 R4 C4 VPOS VPOS = [1 + (R3/R4)] ✕ VREF VREF = 1.25V VSUPP = 2.7V TO 13V Figure 4. Positive Charge-Pump Block Diagram MAX1747 IN SHDN GND RFREQ VON = VIN CIN RFREQ = kFREQ ✕ (VON - 0.7V)/fOSC RFREQ IS IN k Ω, kFREQ IS 45.5MHz/mA, AND fOSC IS IN MHz. OSC Figure 5. Frequency Adjustment |
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