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T491X107M016AS Datasheet(PDF) 11 Page - Advanced Monolithic Systems Ltd |
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T491X107M016AS Datasheet(HTML) 11 Page - Advanced Monolithic Systems Ltd |
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11 / 18 page  AMS4123 3A 20V Step-Down Converter + 1A LDO 3/5/2010 www.advanced-monolithic.com Phone (925) 443-0722 11 Fax (925) 443-0723 Device Summary The AMS4123 is combines a high voltage 3 Amp fixed frequency step-down converter combined with a 1 Amp low drop out (LDO) linear regulator on a single die. 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. The LDO operates from an input voltage ranging from 1V to 20V and a typical dropout voltage of 350mV at 1A. The input to the LDO can be supplied by the output of the Step-Down converter or some other available power source that must be 2V less than the input voltage (Vin). The LDO is also stable for a wide range of ceramic output capacitors ranging from as low as 1µF. Enable 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. Under Voltage Lockout The under-voltage lockout (UVLO) feature guarantees sufficient input voltage (Vin) bias for proper operation of all internal circuitry prior to activation. The input voltage (Vin) is internally monitored and the converter and LDO are enabled when the rising level of Vin reaches 4.2V. To prevent UVLO chatter 400mV of hysteresis is built in to the UVLO comparator so that the step-down converter and LDO are disabled when VIN drops to 3.8V. 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= 15V-2.5V · 2.5V 15V·2A·0.4·300kHz =9.4µH For most applications the duty cycle of the AMS4123 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= (2.5V+0.2V)· 1-0.23 10μH·300kHz =0.7A 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 |
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