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MAX747 Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX747 Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 12 page With decreasing loads, as the inductor current becomes discontinuous, traditional PWM converters continue to switch at a fixed frequency, decreasing light-load efficiency. However, the MAX747’s idle-mode comparator increases the peak inductor current, allowing more energy to be transferred per cycle. Since fewer cycles are required, the switching frequency is reduced. This keeps the external P- FET off for longer periods, minimizing switching losses and increasing efficiency. The light-load output noise spectrum widens due to variable switching frequency in idle-mode, but output ripple remains low. Using the Typical Operating Circuit, with a 9V input and a 125mA load current, output ripple is less than 40mV. Soft-Start and Current Limiting The MAX747 draws its highest current at power-up. If The power source to the MAX747 cannot provide this initial elevated current, the circuit may not function correctly. For example, after prolonged use, a battery’s increased series resistance may prevent it from providing adequate initial surge currents when the MAX747 is brought out of shutdown. Using Soft-Start (SS) minimizes the possibility of overloading the incoming supply at power-up by gradually increasing the peak current limit. Connect an external capacitor from SS to ground to reduce the initial peak currents drawn from the supply. The steady-state SS pin voltage is typically 3.8V. On power-up, SS sources 1µA until the SS voltage reaches 3.8V. The current-limit comparator inhibits EXT switching until the SS voltage reaches 1.8V. The maximum current limit is set by: Figure 3 shows how the peak current limit increases as the voltage on SS rises for two RSENSE values. Shutdown Mode When SHDN is high, the MAX747 enters shutdown mode. In this mode, the internal biasing circuitry (including EXT) is turned off, VOUT drops to 0V, and the supply current drops to 0.6µA (20µA max). This excludes external component leakage, which may add several microamps to the shutdown supply current for the entire circuit. SHDN is a TTL/CMOS logic-level input. Connect SHDN to GND for normal operation. Low-Battery Detector The MAX747 provides a low-battery comparator that compares the voltage on LBI to the reference voltage. LBO, an open-drain output, goes low when the LBI voltage is below VREF. Use a resistor-divider network as shown in Figure 4 to set the trip voltage (VTRIP) to the desired level. In this circuit, LBO goes low when V+ ≤ V TRIP. LBO is high impedance in shutdown mode. __________________Design Procedure Setting the Output Voltage The MAX747’s output voltage can be set to 5V by grounding FB, or adjusted from 2V to 14V using external resistors R4 and R5, configured as shown in Figure 5. Select feedback resistor R4 from the 10k Ω to 1M Ω range. R5 is given by: Selecting RSENSE First, approximate the peak current assuming IPK is (1.1)(ILOAD), where ILOAD is the maximum load current. Once all component values have been determined, the actual peak current is given by: Next, determine the value of RSENSE such that: For example, to obtain 5V at 3A, IPK = 3.3A and RSENSE = 125mV/3.3A = 38m Ω. The sense resistor should have a power rating greater than (IPK2)(RSENSE) (with an adequate safety margin). With a 3A load current, IPK = 3.3A and RSENSE = 38mΩ. The power dissipated by the resistor (assuming an 80% R V I 125mV I SENSE LIMIT (MIN) PK PK == I I V (2L) (f ) 1 V V PK LOAD OUT OSC OUT IN =+ − R5 (R4) V 2V 1 OUT =− I V R 150mV (typ) R PK LIMIT SENSE SENSE == High-Efficiency PWM, Step-Down P-Channel DC-DC Controller 8 _______________________________________________________________________________________ 0 SOFT-START VOLTAGE (V) 4 2 1 13 3 2 0 RSENSE = 50mΩ RSENSE = 100mΩ V+ –VCS = 150mV Figure 3. Peak Current Limit vs. Soft-Start Voltage |
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