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MAX15004 Datasheet(PDF) 17 Page - Maxim Integrated Products |
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MAX15004 Datasheet(HTML) 17 Page - Maxim Integrated Products |
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17 / 27 page 
IO is the load current, ΔVQ is the portion of the ripple due to the capacitor discharge, and ΔVESR is the contribution due to the ESR of the capacitor. DMAX is the maximum duty cycle at the minimum input voltage. Use a combina- tion of low-ESR ceramic and high-value, low-cost alu- minum capacitors for lower output ripple and noise. Calculating Power Loss in Boost Converter The MAX15004A/MAX15005A devices are available in a thermally enhanced package and can dissipate up to 1.7W at +70°C ambient temperature. The total power dissipation in the package must be limited so that the junction temperature does not exceed its absolute max- imum rating of +150°C at maximum ambient tempera- ture; however, Maxim recommends operating the junction at about +125°C for better reliability. The average supply current (IDRIVE-GATE) required by the switch driver is: where Qg is total gate charge at 7.4V, a number avail- able from MOSFET datasheet. The supply current in the MAX15004A/B/MAX15005A/B is dependent on the switching frequency. See the Typical Operating Characteristics to find the supply current ISUPPLY of the MAX15004A/B/MAX15005A/B at a given operating frequency. The total power dissipa- tion (PT) in the device due to supply current (ISUPPLY) and the current required to drive the switch (IDRIVE- GATE) is calculated using following equation. MOSFET Selection in Boost Converter The MAX15004A/B/MAX15005A/B drive a wide variety of n-channel power MOSFETs. Since VCC limits the OUT output peak gate-drive voltage to no more than 11V, a 12V (max) gate voltage-rated MOSFET can be used with- out an additional clamp. Best performance, especially at low-input voltages (5VIN), is achieved with low-threshold n-channel MOSFETs that specify on-resistance with a gate-source voltage (VGS) of 2.5V or less. When selecting the MOSFET, key parameters can include: 1) Total gate charge (Qg). 2) Reverse-transfer capacitance or charge (CRSS). 3) On-resistance (RDS(ON)). 4) Maximum drain-to-source voltage (VDS(MAX)). 5) Maximum gate frequencies threshold voltage (VTH(MAX)). At high switching, dynamic characteristics (parameters 1 and 2 of the above list) that predict switching losses have more impact on efficiency than RDS(ON), which pre- dicts DC losses. Qg includes all capacitances associat- ed with charging the gate. The VDS(MAX) of the selected MOSFET must be greater than the maximum output volt- age setting plus a diode drop. The 10V additional margin is recommended for spikes at the MOSFET drain due to the inductance in the rectifier diode and output capacitor path. In addition, Qg helps predict the current needed to drive the gate at the selected operating frequency when the internal LDO is driving the MOSFET. Slope Compensation in Boost Configuration The MAX15004A/B/MAX15005A/B use an internal ramp generator for slope compensation to stabilize the current loop when operating at duty cycles above 50%. It is advisable to add some slope compensation even at lower than 50% duty cycle to improve the noise immunity. The slope compensations should be optimized because too much slope compensation can turn the converter into the voltage-mode control. The amount of slope compensation required depends on the downslope of the inductor cur- rent when the main switch is off. The inductor downslope depends on the input to output voltage differential of the boost converter, inductor value, and the switching fre- quency. Theoretically, the compensation slope should be equal to 50% of the inductor downslope; however, a little higher than 50% slope is advised. Use the following equation to calculate the required compensating slope (mc) for the boost converter: The internal ramp signal resets at the beginning of each cycle and slews at the rate programmed by the external capacitor connected to SLOPE. Adjust the MAX15004A/B/MAX15005A/B slew rate up to 110mV/μs using the following equation: where CSLOPE is the external capacitor at SLOPE in farads. C mc mV s SLOPE = × − 25 10 9 . () μ mc VV R L mV s OUT IN S = ×× () − − () 10 2 3 μ PV I INMAX SUPPLY TDRIVE GATE I =× + − () IQ f DRIVE GATE g OUT − =× ESR V I C ID Vf ESR O OUT OMAX QOUT = = × × Δ Δ 4.5V to 40V Input Automotive Flyback/Boost/SEPIC Power-Supply Controllers ______________________________________________________________________________________ 17 |
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