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MAX750ACPA Datasheet(PDF) 9 Page - Maxim Integrated Products |
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MAX750ACPA Datasheet(HTML) 9 Page - Maxim Integrated Products |
9 / 16 page tors’ leakage currents rises sharply. The actual design limit for standby current is much less than the 100µA specified in the Electrical Characteristics (see the Standby Current vs. Temperature graph in the Typical Operating Characteristics). However, testing to tighter limits is prohibitive because the current takes several seconds to settle to a final value. For normal operation, connect SHDN to V+. Coming out of shutdown mode initiates a soft-start cycle. Continuous-/Discontinuous- Conduction Modes The input voltage, output voltage, load current, and inductor value determine whether the IC operates in continuous or discontinuous mode. As the inductor value or load current decreases, or the input voltage increases, the MAX750A/MAX758A tend to operate in discontinuous-conduction mode (DCM). In DCM, the inductor-current slope is steep enough so it decays to zero before the end of the transistor off-time. In contin- uous-conduction mode (CCM), the inductor current never decays to zero, which is typically more efficient than DCM. CCM allows the MAX750A/MAX758A to deliver maximum load current, and is also slightly less noisy than DCM because the ripple current in the out- put capacitor is smaller. Internal Reference The +1.23V bandgap reference supplies up to 100µA at REF. Connect a 0.01µF bypass capacitor from REF to GND. Oscillator The internal oscillator of the MAX750A typically oper- ates at 170kHz (160kHz for the MAX758A). The Typical Operating Characteristics indicate stability of the oscil- lator frequency over temperature and supply voltage. __________Applications Information Figure 3 shows the MAX750A/MAX758A configured for a standard 5V step-down application. Table 2 lists the com- ponents for the desired operating temperature range. These circuits are useful in systems that require high cur- rent at high efficiency and are powered by an unregulated supply, such as a battery or wall-plug AC-DC transformer. They will operate over the entire line, load, and tempera- ture ranges using the single set of component values shown in Figure 3 and listed in Table 2. Inductor Selection The MAX750A/MAX758A require no inductor design because they are tested in-circuit, and are guaranteed to deliver the power specified in the Electrical Characteristics with high efficiency using a single 100µH (MAX75_AC) or 33µH (MAX75_AE/AM) inductor The inductor’s incremental saturation-current rating should be greater than 1A, and its DC resistance should be less than 0.8 Ω. Table 2 lists inductor types and suppliers for various applications. The surface- mount inductors and the larger-size through-hole inductors have nearly equivalent efficiencies. Adjusting the Output Voltage The MAX750A/MAX758A have outputs adjustable from 1.25V to the input voltage. To set the output voltage, connect a voltage divider to the feedback input pin (CC) as shown in Figure 3. The output voltage is set by R2 and R3 as follows: Let R3 be any resistance in the 10k Ω to 20kΩ range (typically 10k Ω), then R2 = R3 [((VOUT/1.22V) - 1)] Output tolerance over temperature is ±4.5% plus exter- nal resistor tolerances. Output Filter Capacitor Selection The primary criterion for selecting the output filter capacitor is low equivalent series resistance (ESR). The product of the inductor-current variation and the ESR of the output capacitor determines the amplitude of the sawtooth ripple seen on the output voltage. In addition, the ESR of the output filter capacitor should be minimized to maintain AC stability. The ESR of the capacitor should be less than 0.25 Ω to keep the output ripple less than 50mVp-p over the entire current range (using a 100µH inductor). Capacitor ESR ususally rises Adjustable, Step-Down, Current-Mode PWM Regulators _______________________________________________________________________________________ 9 SS MAX750A MAX758A SS CLAMP 1M ±35% 1.23V R1 510k C1 FROM SHDN Figure 2. Block Diagram of Soft-Start Circuitry |
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