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XRP6141ELTR-F Datasheet(PDF) 10 Page - Exar Corporation |
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XRP6141ELTR-F Datasheet(HTML) 10 Page - Exar Corporation |
10 / 14 page XRP6141 35A Synchronous Step Down COT Controller © 2015 Exar Corporation 10/14 Rev. 2.0.0 DETAILED OPERATION XRP6141 is a synchronous step-down proprietary emulated current-mode Constant On-Time (COT) controller. The on-time, which is programmed via RON, is inversely proportional to VIN and maintains a nearly constant frequency. The emulated current-mode control allows the use of ceramic output capacitors. Each switching cycle begins with GH signal turning the high-side (switching) FET for a preprogrammed time. At the end of the on-time the high-side FET is turned off and the low-side (synchronous) FET is turned on for a preset minimum time (250ns nominal). This parameter is termed Minimum Off-Time. After the minimum off-time the voltage at the feedback pin FB is compared to an internal voltage ramp at the feedback comparator. When VFB drops below the ramp voltage, the high-side FET is turned on and the cycle repeats. This voltage ramp constitutes an emulated current ramp and makes possible the use of ceramic capacitors, in addition to other capacitor types, for output filtering. ENABLE/MODE INPUT (EN/MODE) EN/MODE pin accepts a tri-level signal that is used to control turn on/off. It also selects between two modes of operation: ‘Forced CCM’ and ‘DCM/CCM’. If EN is pulled below 1.9V, the controller shuts down. A voltage between 1.9V and 3.0V selects the Forced CCM mode which will run the converter in continuous conduction at all times. A voltage higher than 3.0V selects the DCM/CCM mode which will run the converter in discontinuous conduction at light loads. Selecting the Forced CCM Mode In order to set the controller to operate in Forced CCM, a voltage between 1.9V and 3.0V must be applied to the EN/MODE pin. This can be achieved with an external control signal that meets the above voltage requirement. Where an external control is not available, the EN/MODE can be derived from VIN. If VIN is well regulated, use a resistor divider and set the voltage to 2.5V. If VIN varies over a wide range, the circuit shown in figure 28 can be used to generate the required voltage. Note that at VIN of 5.5V to 22V the nominal Zener voltage is 4.0V to 5.0V respectively. Therefore, for VIN in the range of 5.5V to 22V, the circuit shown in figure 28 will generate voltage at the EN/MODE pin required for Forced CCM. Selecting the DCM/CCM Mode In order to set the controller operation to DCM/CCM, a voltage between 3.1V and 5.5V must be applied to the EN/MODE pin. If an external control signal is available, it can be directly connected to the EN/MODE pin. In applications where an external control signal is not available, EN/MODE input can be derived from VIN. If VIN is well regulated, use a resistor divider and set the voltage to 4V. If VIN varies over a wide range, the circuit shown in figure 29 can be used to generate the required voltage. Figure 28. Selecting Forced CCM by deriving EN/MODE from VIN Figure 29. Selecting DCM/CCM by deriving EN/MODE from VIN PROGRAMMING THE ON-TIME The on-time TON is programmed via resistor RON according to following equation: ������������ = (3.4���� − 10) × ������������ ������������ The required TON for a given application is calculated from: ������������ = ���������������� ������������ × ���� |
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Similar Description - XRP6141ELTR-F |
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