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AMS2596S Datasheet(PDF) 8 Page - Advanced Monolithic Systems |
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AMS2596S Datasheet(HTML) 8 Page - Advanced Monolithic Systems |
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8 / 15 page  AMS2596 3A 30V Step-Down Converter 3/4/2010 www.advanced-monolithic.com Phone (925) 443-0722 8 Fax (925) 443-0723 Device Summary The AMS2596 is a high voltage fixed frequency step- down converter with a current capability of up to 5A. The peak current mode step-down converter has internal compensation and is stable with a wide range of ceramic, tantalum, and electrolytic output capacitors. The step-down converter output voltage is sensed through an external resistive divider that feeds the negative input to an internal transconductance error amplifier. The output of the error amplifier is connected to the input to a peak current mode comparator. The inductor current is sensed as it passes through the power switch, amplified and is also fed to the current mode comparator. The error amplifier regulates the output voltage by controlling the peak inductor current passing through the power switch so that, in steady state, the average inductor current equals the load current. The step-down converter has an input voltage range of 4.5V to 20V with an output voltage as low as 0.6V. Shutdown The enable input has two levels so that the step-down converter can be enabled independently of the LDO. The enable threshold for the step-down converter is 2.0V while the enable threshold for the linear regulator output is 2.5V typical. Fault Protection Short circuit and over-temperature shutdown disable the converter and LDO in the event of an overload condition. Application Inductor The step-down converter inductor is typically selected to limit the ripple current to 40% of the full load output current. Solve for this value at the maximum input voltage where the inductor ripple current is greatest. L= Vin-Vo · Vo Vin·Io·0.4·Fs L= 12V-5V · 5V 12V·3A·0.4·150kHz =16µH For most applications the duty cycle of the AMS2596 step down converter is less than 50% duty and does not require slope compensation for stability. This provides some flexibility in the selected inductor value. Given the above selected value, others values slightly greater or less may be examined to determine the effect on efficiency without a detrimental effect on stability. With and inductor value selected, the ripple current can be calculated: Ipp= (Vo+Vfwd)·(1-D) ·L·Fs Using the maximum input voltage values the ripple is: Ipp= (5V+0.2V)· 1-0.44 22μH·150kHz =0.88A Once the appropriate value is determined, the component is selected based on the DC current and the peak (saturation) current. Select an inductor that has a DC current rating greater than the full load current of the application. The DC current rating is also reflected in the DC resistance (DCR) specification of the inductor. The inductor DCR should limit the inductor loss to less than 2% of the step- down converter output power. The peak current at full load is equal to the full load DC current plus one half of the ripple current. As mentioned before, the ripple current varies with input voltage and is a maximum at the maximum input voltage. Ipkmax=Io+ (Vo+Vfwd)·(1-Dmin) 2·L·Fs Dmin= Vo Vinmax The duty cycle can be more accurately estimated by including the drops of the external Schottky diode and the internal power switch: Dmin= Vo+Vfwd Vinmax-V o+Vfwd Dmin= 5V+0.2V 12V-0.3V+0.2V =0.44 Vfwd is the diode freewheeling diode drop and Vsw is the collector to emitter drop of the internal power switch. With a good estimate of the duty cycle (D) the inductor peak current can be determined: Ipkmax=3A+ (5V+0.2V)·(1-0.44) 2·22µH·150kHz =3.44A |
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