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MAX8734A データシート(PDF) 28 Page - Maxim Integrated Products |
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MAX8734A データシート(HTML) 28 Page - Maxim Integrated Products |
28 / 33 page High-Efficiency, Quad-Output, Main Power- Supply Controllers for Notebook Computers 28 ______________________________________________________________________________________ Rectifier Selection Current circulates from ground to the junction of both MOSFETs and the inductor when the high-side switch is off. As a consequence, the polarity of the switching node is negative with respect to ground. This voltage is approximately -0.7V (a diode drop) at both transition edges while both switches are off (dead time). The drop is IL x RDS(ON) when the low-side switch conducts. The rectifier is a clamp across the synchronous rectifier that catches the negative inductor swing during the dead time between turning the high-side MOSFET off and the synchronous rectifier on. The MOSFETs incorporate a high-speed silicon body diode as an adequate clamp diode if efficiency is not of primary importance. Place a Schottky diode in parallel with the body diode to reduce the forward-voltage drop and prevent the N2/N4 MOSFET body diodes from turning on during the dead time. Typically, the external diode improves the efficiency by 1% to 2%. Use a Schottky diode with a DC current rating equal to 1/3 of the load current. For example, use an MBR0530 (500mA-rated) type for loads up to 1.5A, a 1N5819 type for loads up to 3A, or a 1N5822 type for loads up to 10A. The rectifier’s rated reverse-breakdown voltage must be at least equal to the maximum input volt- age, preferably with a 20% derating factor. Boost Supply Diode A signal diode, such as a 1N4148, works well in most applications. Use a small (20mA) Schottky diode for slightly improved efficiency and dropout characteris- tics, if the input voltage can go below 6V. Do not use large power diodes, such as 1N5817 or 1N4001, since high-junction capacitance can force LDO5 to excessive voltages. Applications Information Dropout Performance The output voltage-adjust range for continuous-conduc- tion operation is restricted by the nonadjustable 350ns (max) minimum off-time, one-shot. Use the slower 5V SMPS for the higher of the two output voltages for best dropout performance in adjustable feedback mode. The duty-factor limit must be calculated using worst-case val- ues for on- and off-times, when working with low input voltages. Manufacturing tolerances and internal propaga- tion delays introduce an error to the tON K-factor. Also, keep in mind that transient-response performance of buck regulators operated close to dropout is poor, and bulk output capacitance must often be added (see the VSAG equation in the Output-Capacitor Selection section). The absolute point of dropout occurs when the inductor current ramps down during the minimum off-time ( ∆IDOWN) as much as it ramps up during the on-time ( ∆IUP). The ratio h = ∆IUP/∆IDOWN indicates the ability to slew the inductor current higher in response to increased load, and must always be greater than 1. As h approaches 1, the absolute minimum dropout point, the inductor current is less able to increase during each switching cycle and VSAG greatly increases unless additional output capacitance is used. A reasonable minimum value for h is 1.5, but this can be adjusted up or down to allow tradeoffs between VSAG, output capacitance, and minimum operating voltage. For a given value of h, the minimum operating voltage can be calculated as: where VDROP1 and VDROP2 are the parasitic voltage drops in the discharge and charge paths (see the On- Time, One-Shot section), tOFF(MIN) is from the EC table, and K is taken from Table 2. The absolute minimum input voltage is calculated with h = 1. Operating frequency must be reduced or h must be increased and output capacitance added to obtain an acceptable VSAG if calculated V+(MIN) is greater than the required minimum input voltage. Calculate VSAG to be sure of adequate transient response if operation near dropout is anticipated. Dropout Design Example MAX8733A: With VOUT5 = 5V, fsw = 400kHz, K = 2.25µs, tOFF(MIN) = 350ns, VDROP1 = VDROP2 = 100mV, and h = 1.5, V VV th K VV MIN OUT DROP OFF MIN DROP DROP += + () − × +− () _ () 1 21 1 MAX8732A MAX8733A MAX8734A V+ 12V POSITIVE SECONDARY OUTPUT 5V MAIN OUTPUT DL_ DH_ T1 10 µH 1:2.2 T1 = TRANSPOWER TECHNOLOGIES TTI-5870 MAX1658/ MAX1659 LDO Figure 12. Transformer-Coupled Secondary Output |
同様の部品番号 - MAX8734A |
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同様の説明 - MAX8734A |
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