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MAX774CPA View Datasheet(PDF) - Maxim Integrated

 Part Name Description Manufacturer MAX774CPA -15V or adjustable, high-efficiency, low IQ inverting DC-DC controller Maxim Integrated MAX774CPA Datasheet PDF : 16 Pages
 First Prev 11 12 13 14 15 16 -5V/-12V/-15V or Adjustable, High-Efficiency, Low IQ Inverting DC-DC Controllers 700 600 RSENSE = 0.05Ω RSENSE = 0.06Ω RSENSE = 0.07Ω 500 VOUT = -15V 400 300 200 100 3 RSENSE = 0.08Ω RSENSE = 0.09Ω 4 5 6 7 INPUT VOLTAGE (V) Figure 8. MAX776 Maximum Output Current vs. Input Voltage (VOUT = -15V) 800 VOUT = -24V RSENSE = 0.05Ω 600 RSENSE = 0.06Ω RSENSE = 0.07Ω 400 200 0 34 RSENSE = 0.08Ω RSENSE = 0.09Ω 5 6 7 8 9 10 11 12 13 14 15 INPUT VOLTAGE (V) Figure 9. MAX774/MAX775/MAX776 Maximum Output Current vs. Input Voltage (VOUT = -24V) To choose the proper current-sense resistor, simply fol- low the two-step procedure outlined below. 1) Determine: • Input voltage range, V+ • Maximum (absolute) output voltage, VOUT • Maximum output current, ILOAD For example, let V+ range from 4V to 6V, and choose VOUT = -24V and IOUT = 150mA. 2) Next, referring to Figure 9, find the curve with the lowest current limit whose output current (with the lowest input voltage) meets your requirements. In our example, we want a curve where IOUT is >150mA with a 4V input and a -24V output. The RSENSE = 80mΩ (shown in Figure 9) shows only approximately 125mA of output current with a 4V input, so we look next at the RSENSE = 70mΩ line. It shows IOUT >150mA for V+ = 4V and VOUT = -24V. The cur- rent limit will be 0.210V / 0.070Ω = 3A. These curves take into account worst-case inductor (±10%) and current-sense trip levels, but not sense-resistor toler- ance. The switch on resistance is 70mΩ. Standard wire-wound and metal-film resistors have an inductance high enough to degrade performance. Metal-film resistors are usually deposited on a ceramic rod in a spiral, making their inductances relatively high. Surface-mount (or chip) resistors have very little induc- tance and are well suited for use as current-sense resistors. If you want to use through-hole resistors, IRC has a wire resistor that is simply a band of metal shaped as a “U” so that inductance is less than 10nH (an order of magnitude less than metal-film resistors). These are available in resistance values between 5mΩ and 0.1Ω. External Switching Transistor The MAX774/MAX775/MAX776 are capable of driving P-channel enhancement-mode MOSFET transistors only. The choice of power transistor is dictated by input and output voltage, peak current rating, on resistance, gate- source threshold, and gate capacitance. The drain-to- source rating must be greater than the V+ - VOUT input-to-output voltage differential. The gate-to-source rating must be greater than V+ (the source voltage) plus the absolute value of the most negative swing of EXT. For bootstrapped operation, the most negative swing of EXT is VOUT. In non-bootstrapped operation, this may be ground or some other negative voltage. Gate capac- itance is not normally a limiting factor, but values should be less than 1nF for best efficiency. For maximum effi- ciency, the MOSFET should have a very low on resis- tance at the peak current and be capable of handling that current. The transistor chosen for the typical oper- ating circuit has a 30V drain-source voltage limit and a 0.07Ω drain-source on resistance at VGS = -10V. Table 1 lists suppliers of switching transistors suitable for use with the MAX774/MAX775/MAX776. ______________________________________________________________________________________ 13