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MAX8734A Datasheet(PDF) 21 Page - Maxim Integrated Products |
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MAX8734A Datasheet(HTML) 21 Page - Maxim Integrated Products |
21 / 33 page High-Efficiency, Quad-Output, Main Power- Supply Controllers for Notebook Computers ______________________________________________________________________________________ 21 undervoltage-protection circuit, this current-limit method is effective in almost every circumstance. A negative current limit prevents excessive reverse inductor currents when VOUT sinks current. The nega- tive current-limit threshold is set to approximately 120% of the positive current limit and therefore tracks the positive current limit when ILIM_ is adjusted. The current-limit threshold is adjusted with an external voltage-divider at ILIM_. The current-limit threshold adjustment range is from 50mV to 300mV. In the adjustable mode, the current-limit threshold voltage is precisely 1/10th the voltage at ILIM_. The threshold defaults to 100mV when ILIM_ is connected to VCC. The logic threshold for switchover to the 100mV default value is approximately VCC - 1V. Carefully observe the PC board layout guidelines to ensure that noise and DC errors do not corrupt the cur- rent-sense signals at CS_. Mount or place the device close to the synchronous rectifier or sense resistor (whichever is used) with short, direct traces, making a Kelvin-sense connection to the sense resistor. The cur- rent-sense accuracy of Figure 8 is degraded if the Schottky diode conducts during the synchronous recti- fier on-time. To ensure that all current passes through the sense resistor, connect the Schottky diode in paral- lel with only the synchronous rectifier (Figure 9) if the voltage drop across the synchronous rectifier and sense resistor exceeds the Schottky diode’s forward voltage. Note that at high temperatures, the on-resis- tance of the synchronous rectifier increases and the forward voltage of the Schottky diode decreases. MOSFET Gate Drivers (DH_, DL_) The DH_ and DL_ gate drivers sink 2.0A and 3.3A, respectively, of gate drive, ensuring robust gate drive for high-current applications. The DH_ floating high-side MOSFET drivers are powered by diode-capacitor charge pumps at BST_. The DL_ synchronous-rectifier drivers are powered by LDO5. The internal pulldown transistors that drive DL_ low have a 0.6 Ω typical on-resistance. These low on-resis- tance pulldown transistors prevent DL_ from being pulled up during the fast rise time of the inductor nodes due to capacitive coupling from the drain to the gate of the low-side synchronous-rectifier MOSFETs. However, for high-current applications, some combinations of high- and low-side MOSFETS may cause excessive gate-drain coupling, which leads to poor efficiency and EMI-producing shoot-through currents. Adding a resis- tor in series with BST_ increases the turn-on time of the MAX8732A MAX8733A V+ DH_ DL_ CS_ LX_ OUT_ Figure 8. Current Sensing Using Sense Resistor (MAX8732A/MAX8733A) MAX8732A MAX8733A V+ DH_ DL_ CS_ LX_ OUT_ Figure 9. More Accurate Current Sensing with Adjusted Schottky Connection MAX8732A MAX8733A MAX8734A 5V VIN 10 Ω BST DH LX Figure 10. Reducing the Switching-Node Rise Time |
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