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CS51414GD8G 데이터시트(PDF) 10 Page - ON Semiconductor |
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CS51414GD8G 데이터시트(HTML) 10 Page - ON Semiconductor |
10 / 20 page CS51411, CS51412, CS51413, CS51414 http://onsemi.com 10 independent of switching duty cycle. The maximum load current, given by the following formula under continuous conduction mode, is less than the Current Limit due to the ripple current. IO(MAX) + ILIM * VO(VIN * VO) 2(L)(VIN)(fs) where: fS = switching frequency, ILIM = current limit threshold, VO = output voltage, VIN = input voltage, L = inductor value. When the regulator runs undercurrent limit, the subharmonic oscillation may cause low frequency oscillation, as shown in Figure 13. Similar to current mode control, this oscillation occurs at the duty cycle greater than 50% and can be alleviated by using a larger inductor value. The current limit threshold is reduced to Foldback Current when the FB pin falls below Foldback Threshold. This feature protects the IC and external components under the power up or overload conditions. Figure 13. The Regulator in Current Limit BOOST Pin The BOOST pin provides base driving current for the power switch. A voltage higher than VIN provides required headroom to turn on the power switch. This in turn reduces IC power dissipation and improves overall system efficiency. The BOOST pin can be connected to an external boost−strapping circuit which typically uses a 0.1 mF capacitor and a 1N914 or 1N4148 diode, as shown in Figure 1. When the power switch is turned on, the voltage on the BOOST pin is equal to VBOOST + VIN ) VO * VF where: VF = diode forward voltage. The anode of the diode can be connected to any DC voltage other than the regulated output voltage. However, the maximum voltage on the BOOST pin shall not exceed 40 V. As shown in Figure 14, the BOOST pin current includes a constant 7.0 mA predriver current and base current proportional to switch conducting current. A detailed discussion of this current is conducted in Thermal Consideration section. A 0.1 mF capacitor is usually adequate for maintaining the Boost pin voltage during the on time. BIAS Pin (CS51412 and CS51414 Only) The BIAS pin allows a secondary power supply to bias the control circuitry of the IC. The BIAS pin voltage should be between 3.3 V and 6.0 V. If the BIAS pin voltage falls below that range, use a diode to prevent current drain from the BIAS pin. Powering the IC with a voltage lower than the regulator’s input voltage reduces the IC power dissipation and improves energy transfer efficiency. Figure 14. The Boost Pin Current Includes 7.0 mA Predriver Current and Base Current when the Switch is Turned On. The Beta Decline of the Power Switch Further Increases the Base Current at High Switching Current 0 0.5 1.0 1.5 SWITCHING CURRENT (A) 0 5 10 15 20 25 30 Shutdown The internal power switch will not turn on until the VIN pin rises above the Startup Voltage. This ensures no switching until adequate supply voltage is provided to the IC. The IC transitions to sleep mode when the SHDNB pin is pulled low. In sleep mode, the internal power switch transistor remains off and supply current is reduced to the Shutdown Quiescent Current value (20 mA typical). This pin has an internal pull-up current source, so defaults to high (enabled) state when not connected. |
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