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MAX8734A Folha de dados(PDF) 29 Page - Maxim Integrated Products |
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MAX8734A Folha de dados(HTML) 29 Page - Maxim Integrated Products |
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29 / 33 page  High-Efficiency, Quad-Output, Main Power- Supply Controllers for Notebook Computers ______________________________________________________________________________________ 29 the minimum V+ is: Calculating with h = 1 yields: Therefore, V+ must be greater than 6.65V. A practical input voltage with reasonable output capacitance would be 7.5V. Use of Coupled Inductors to Create Auxiliary Outputs A coupled inductor or transformer can be substituted for the inductor in the 5V or 3.3V SMPS to create an auxiliary output (Figure 12). The MAX8732A/MAX8733A/ MAX8734A are particularly well suited for such applica- tions because they can be configured in ultrasonic or forced-PWM mode to ensure good load regulation when the main supplies are lightly loaded. An additional postregulation circuit can be used to improve load regula- tion and limit output current. The power requirements of the auxiliary supply must be considered in the design of the main output. The trans- former must be designed to deliver the required current in both the primary and the secondary outputs with the proper turns ratio and inductance. The power ratings of the synchronous-rectifier MOSFETs and the current limit in the MAX8732A/MAX8733A/MAX8734A must also be adjusted accordingly. Extremes of low input-output dif- ferentials, widely different output loading levels, and high turns ratios can further complicate the design due to par- asitic transformer parameters such as interwinding capacitance, secondary resistance, and leakage induc- tance. Power from the main and secondary outputs is combined to get an equivalent current referred to the main output. Use this total current to determine the cur- rent limit (see the Determining the Current Limit section): where ITOTAL is the equivalent output current referred to the main output and PTOTAL is the sum of the output power from both the main output and the secondary output: where LPRIMARY is the primary inductance, N is the transformer turns ratio, VSEC is the minimum-required rectified secondary voltage, VFWD is the forward drop across the secondary rectifier, VOUT(MIN) is the minimum value of the main output voltage, and VRECT is the on- state voltage drop across the synchronous rectifier MOSFET. The transformer secondary return is often con- nected to the main output voltage instead of ground to reduce the necessary turns ratio. In this case, subtract VOUT from the secondary voltage (VSEC - VOUT) in the transformer turns-ratio equation above. The secondary diode in coupled-inductor applications must withstand flyback voltages greater than 60V, which usually rules out most Schottky rectifiers. Common sili- con rectifiers, such as the 1N4001, are also prohibited because they are too slow. This often makes fast silicon rectifiers such as the MURS120 the only choice. The fly- back voltage across the rectifier is related to the VIN - VOUT difference, according to the transformer turns ratio: VFLYBACK = VSEC + (VIN - VOUT) ✕ N where N is the transformer turns ratio (secondary wind- ings/primary windings), VSEC is the maximum secondary DC output voltage, and VOUT is the primary (main) out- put voltage. If the secondary winding is returned to VOUT instead of ground, subtract VOUT from VFLYBACK in the equation above. The diode’s reverse breakdown voltage rating must also accommodate any ringing due to leak- age inductance. The diode’s current rating should be at least twice the DC load current on the secondary output. The optional linear postregulator must be selected to deliver the required load current from the transformer’s rectified DC output. The linear regulator should be con- figured to run close to dropout to minimize power dissi- pation and should have good output accuracy under those conditions. Input and output capacitors are cho- sen to meet line regulation, stability, and transient requirements. There is a wide variety of linear regulators appropriate for this application; consult the specific lin- ear-regulator data sheet for details. Widely different output loads affect load regulation. In particular, when the secondary output is left unloaded while the main output is fully loaded, the secondary out- put capacitor may become overcharged by the leakage inductance, reaching voltages much higher than intend- ed. In this case, a minimum load or overvoltage protec- L VV V VI LIR N VV VV PRIMARY OUT IN MAX OUT IN MAX TOTAL SEC FWD OUT MIN RECT = − ׃ × × = + + ( () () () IP V TOTAL TOTAL OUT = / V VV s s VV V MIN += + () − µ× µ +−= () . . . .. . 50 1 1 035 1 225 01 01 604 V VV s s VV V MIN += + () − µ× µ +−= () . .. . .. . 50 1 1 035 1 5 225 01 01 665 |
Nº de peça semelhante - MAX8734A |
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Descrição semelhante - MAX8734A |
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