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XR76203-Q Datasheet(PDF) 13 Page - Exar Corporation |
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XR76203-Q Datasheet(HTML) 13 Page - Exar Corporation |
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13 / 20 page  XR76203-Q/XR76205-Q/XR76208-Q 13 / 20 exar.com/XR76203-Q/XR705-Q/XR76208-Q Rev 1C ©2016-2017 Exar Corporation Functional Description XR76203-Q, XR76205-Q and XR76208-Q are synchro- nous step-down proprietary emulated current-mode Con- stant On-Time (COT) regulators. The on-time, which is programmed via RON, is inversely proportional to VIN and maintains a nearly constant frequency. The emulated cur- rent-mode control is stable with ceramic output capacitors. Each switching cycle begins with GH signal turning on the high-side (control) 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 mini- mum time (250ns nominal). This parameter is termed Mini- mum 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 con- trol turn on/off. It also selects between two modes of opera- tion: ‘Forced CCM’ and ‘DCM/CCM’. If EN is pulled below 1.8V, the Regulator shuts down. A voltage between 2.0V and 2.8V selects the Forced CCM mode which will run the Regulator in continuous conduction at all times. A voltage higher than 3.1V selects the DCM/CCM mode which will run the Regulator in discontinuous conduction at light loads. Selecting the Forced CCM Mode In order to set the Regulator to operate in Forced CCM, a voltage between 2.0V and 2.8V must be applied to EN/ MODE. This can be achieved with an external control sig- nal 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 31 can be used to gener- ate the required voltage. Note that at VIN of 5.5V and 40V the nominal Zever voltage is 4.0V and 5.0V respectively. Therefore for VIN in the range of 5.5V to 40V, the circuit shown in figure 31 will generate VEN required for Forced CCM. Selecting the DCM/CCM Mode In order to set the Regulator operation to DCM/CCM, a volt- age between 3.1V and 5.5V must be applied to EN/MODE pin. If an external control signal is available, it can be directly connected to EN/MODE. In applications where an external control 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 32 can be used to gener- ate the required voltage. Figure 31: Selecting Forced CCM by deriving EN/MODE from VIN Figure 32: Selecting DCM/CCM by deriving EN/MODE from VIN R1 30.1k, 1% RZ 10k EN/MODE R2 35.7k, 1% Zener MMSZ4685T1G or Equivalent IN V Zener MMSZ4685T1G or Equivalent RZ 10k EN/MODE V IN V EN |
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