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MAX1763 Datasheet(PDF) 11 Page - Maxim Integrated Products |
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MAX1763 Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 16 page using an external P-channel MOSFET pass device. The gain-block output is a single-stage transconductance amplifier that drives an open-drain N-channel MOSFET. The transconductance (GM) of the entire gain-block stage is 10mS. The internal gain block amplifies the dif- ference between AIN and the internal 0.938V reference. To provide a power-OK signal, connect the gain-block input, AIN, to an external resistor-divider (Figure 3). The input bias current into AIN is less than 30nA, allowing large-value divider resistors without sacrificing accura- cy. Connect the resistor voltage-divider as close to the IC as possible, within 0.2in (5mm) of AIN. Choose an R4 value of 270k Ω or less, then calculate R3 using: R3 = R4((VTRIP / VAIN ) - 1) where VAIN is 0.938V. Figures 4 and 5 show the gain block used in a linear- regulator application. The output of an external P-chan- nel pass element is compared to an internal 0.938V reference. The difference is amplified and drives the gate of the pass element. Use a logic-level PFET, such as Fairchild’s NDS336P (RDS(ON) = 270m Ω). When the linear-regulator output voltage is in regulation, the MOSFET will not be full on; thus, the on-resistance will not be important. However, if the linear regulator is used in dropout, the MOSFET on-resistance will determine the dropout voltage (VDROPOUT = IOUT ✕ RDS(ON)). If a lower RDS(ON) PFET is used, increase the linear-regula- tor output filter capacitance to maintain stability. 1.5A, Low-Noise, 1MHz, Step-Up DC-DC Converter ______________________________________________________________________________________ 11 Figure 3. Using the Gain Block as a Power-OK Comparator Table 3. On/Off Logic Control ONA ONB MAX1763 00 On 0 1 Off 10 On 11 On MAX1763 POUT AO AIN R6 150k R3 R4 POWER-OK OUTPUT TO VIN OR VOUT GND PGND AO POUT OUT LX C1 47 μF BOOST OUTPUT COUT 47 μF LINEAR- REGULATED OUTPUT RG 20k R5 4.7 Ω C2 1.0 μF 0.22 μF VIN 1.8V TO 5.5V L1 1.5 μH FB C4 220 μF REF AIN R3 R4 R2 30k SIGNAL GROUND POWER GROUND R1 MAX1763 ISET CLK/SEL ONA ONB P GND PGND POUT OUT LX C4 220 μF R5 4.7 Ω C2 1 μF C3 0.22 μF VIN 1.5 μH FB COUT 47 μF C1 47 μF REF AIN R3 165k R4 100k MAX1763 ISET CLK/SEL AO ONB ONA MBRO520L 3.3V 2.5V RG 20k Figure 4. Using the Gain Block as a Linear Regulator from the Boosted Output Voltage Figure 5. Powering a Gain-Block Linear Regulator from the Input Voltage |
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