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MAX877 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX877 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page Active Rectifier The internal active rectifier of the MAX877/MAX878/ MAX879 replaces the external Schottky catch diode in normal boost operation. The rectifier consists of a PNP pass transistor and a unique control circuit which, in shutdown mode, entirely disconnects the load from the source. This is a distinct advantage over standard boost topologies, since it prevents battery drain in shutdown. The MAX877/MAX878/MAX879 can withstand a momen- tary short at the output in normal operation. The active rectifier also acts as a zero-dropout regulator if the input exceeds the regulated output.The device still switches to deliver power to the output, and the differ- ence between the input and output voltage appears across the rectifier. Efficiency is similar to that of a linear regulator if the MAX877/MAX878/MAX879 are used as step-down converters. The maximum output current (IOUT (MAX)) with larger input/output differentials is deter- mined by package power dissipation. IOUT (MAX) can be approximated by: PDISS IOUT (MAX) ≈ (—————— )x 0.9 (VIN - VOUT) Shutdown Shutdown (S — H — D — N – ) is a high-impedance, active-low input. Connect it to IN for normal operation. Keeping S — H — D — N – at ground holds the converters in shutdown mode. Since the active rectifier is turned off in this mode, the path from input to load is cut, and the output effectively drops to 0V. The supply current in shutdown mode ranges from 4µA at VIN = 1V to 50µA at VIN = 5V. The shutdown-circuit threshold is set nominally to VIN/2 + 250mV. When S — H — D — N – is below this threshold, the device is shut down; it is enabled with S — H — D — N – above the threshold. When driven from external logic, S — H — D — N – can be driven to a higher voltage than VIN, (6.2V max). Current Limit Connecting ILIM to IN sets an LX current limit of 1A. For smaller output power levels that do not require the maxi- mum peak current, reduce the peak inductor current by connecting a resistor between ILIM and IN. This opti- mizes overall efficiency and allows very small, low-cost coils with lower current ratings. See Figure 2 to select the resistor (see also Inductor Selection section). Output Voltage Selection The MAX877’s output voltage is fixed at 5V. The MAX878’s output voltage can be set to 3V by leaving the SEL pin open, or to 3.3V by connecting SEL to AGND. The MAX879’s output voltage is set by two resistors, R1 and R2 (Figure 3), which form a voltage divider between the output and the FB pin. The output voltage can be set from 2.5V to 6.0V by the equation: VOUT = VREF (R1 + R2) R2 where VREF = 0.2025V. To simplify the resistor selection: R1 = R2 ( VOUT VREF - 1 ) Since the input current at FB has a maximum of 40nA, large values (10k Ω to 50kΩ for R2) can be used without significant accuracy loss. For 1% error, the current through R2 should be at least 100 times FB’s bias current. When large values are used for the feedback resistors (R1 > 50k Ω), stray output impedance at FB can add a “lag” to the feedback response, destabilizing the regula- tor and creating a larger ripple at the output. Lead lengths and circuit board traces at the FB node should be kept short. Reduce ripple by adding a “lead” compensa- tion capacitor (C3, 100pF to 50nF) in parallel with R1. 5V/3.3V/3V/Adjustable-Output, Step-Up/Step-Down DC-DC Converters 8 _______________________________________________________________________________________ 1200 02468 10 12 14 CURRENT-LIMIT RESISTOR vs. PEAK INDUCTOR CURRENT RESISTOR VALUE (k Ω) 1000 600 800 400 200 0 VIN = 2.5V Figure 2. Current-Limit Resistor vs. Peak Inductor Current |
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Descripción similar - MAX877 |
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