Ordering number : ENA2021 LA5724MC Monolithic Linear IC Separately-Excited Step-Down Switching Regulator (Variable Type) http://onsemi.com Overview The LA5724MC is a separately-excited step-down switching regulator (variable type). Functions • Time-base generator (160kHz) incorporated. • Current limiter incorporated. • Thermal shutdown circuit incorporated. Specifications Absolute Maximum Ratings at Ta = 25°C Parameter Input voltage SW pin application reverse voltage Symbol Conditions Ratings Unit VIN 30 V VSW -1 V VOS pin application voltage VVOS Allowable power dissipation Pd max Mounted on a circuit board.* -0.2 to 7 V 0.8 W Operating temperature Topr -30 to +125 °C Storage temperature Tstg -40 to +150 °C Junction temperature Tj max 150 °C * Specified circuit board : 114.3×76.1×1.6mm3, glass epoxy board. Caution 1) Absolute maximum ratings represent the value which cannot be exceeded for any length of time. Caution 2) Even when the device is used within the range of absolute maximum ratings, as a result of continuous usage under high temperature, high current, high voltage, or drastic temperature change, the reliability of the IC may be degraded. Please contact us for the further details. Stresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above the Recommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affect device reliability. Recommended Operating Conditions at Ta = 25°C Parameter Input voltage range Symbol VIN Semiconductor Components Industries, LLC, 2013 August, 2013 Conditions Ratings Unit 4.5 to 28 V 32812 SY 20120207-S00001 No.A2021-1/6 LA5724MC Electrical Characteristics at Ta = 25°C, VIN = 15V Parameter Symbol Reference voltage VOS Reference pin bias current IFB Switching frequency fosc Short-circuit protection circuit min IO = 0.3A typ 1.20 128 η Efficiency Ratings Conditions Unit max 1.23 1.26 V 1 2 μA 160 192 kHz VOUT = 5V, IO = 0.3A fscp 82 % 30 kHz operating switching frequency Saturation voltage Vsat IOUT = 0.3A, VOS = 0V 1.2 V Maximum on duty D max VOS = 0V 100 % Minimum on duty D min VOS = 5V 0 % Output leakage current Ilk SWOUT = -1V Supply current Iin VOS = 2V Current limiter operating current IS Thermal shutdown operating 5 200 μA 10 mA 0.7 A TSD Designed target value. * 165 °C ΔTSD Designed target value. * 15 °C temperature Thermal shutdown Hysteresis width * Design target value : No measurement made. Package Dimensions unit : mm (typ) 3424 4.9 0.42 1.75 MAX 0.2 Allowable power dissipation, Pd max - W 0.835 0.375 6.0 3.9 2 0.175 1 1.27 Pd max -- Ta 1 8 Designated board : 114.3×76.1×1.6mm3 glass epoxy 0.8 0.75 Mounted on a board 0.6 0.4 0.2 0.15 0 --30 0 30 60 90 120 150 Ambient temperature, Ta - C SANYO : SOIC8 No.A2021-2/6 LA5724MC Pin Assignment NC NC GND NC LA5724MC VIN NC SWOUT VOS Block Diagram VIN 3 SWOUT 1 Reg. OCP Reset OSC Drive NC 2 Comp. NC 5 NC 7 NC 8 TSD 4 VOS Amp. VREF 6 GND Note : Since the NC pins are not connected within the IC package, they can be used as connection points. Application Circuit Example L1 VIN SWOUT LA5724MC C3 + + C2 D1 C1 VOS R2 GND R1 Note : Insomecases, the output may not turn on if power is applied when a load is connected. If this is a problem, increase the value of the inductor. No.A2021-3/6 LA5724MC Protection Circuit Functional Descriptions Overcurrent protection function The overcurrent protection function detects, on a pulse-by-pulse basis, the output transistor current and turns off that output transistor current if it exceeds 0.7A in a pulse-by-pulse manner. Limit current Inductor current SWOUT voltage Short circuit protection function This IC prevents the current from increasing when the outputs are shorted by setting the switching frequency to 30kHz if the VOS pin voltage falls below 0.8V. Note : At startup, since the switching frequency will be 30kHz while the VOS pin voltage is 0.8Vor lower, the current capacity is reduced. If the load is applied at startup and the applications has trouble starting, increase the value of the inductor to resolve this problem. Timing Chart VIN voltage 30kHz 160kHz SWOUT voltage 1.23V 0.8V VOS voltage 0V No.A2021-4/6 LA5724MC Part selection and set 1. Resistors R1 and R2 R1 and R2 are resistors to set the output voltage. When the large resistance value is set, the error of set voltage increases due to the VOS pin current. The output voltage may also increases due to the leak current of switching transistor at light load. In consequence, it is essential to see R1 and R2 currents to around 500μA. 1.23V R1 = ≈ 2.4kΩ 2.0kΩ to 2.4kΩ recommended 500μA VOUT R2 = ( 1.23V - 1) × R1 The calculation equation gives the output voltage set by R1 and R2. R2 VOUT = (1 + R1) × 1.23V(typ) 2. Capacitor C1, C2 and C3 The large ripple current flows through C1 and C2, so that the high-frequency low-impedance product for switching power supply must be used. Do not use, for C2, a capacitor eith extremely small equivalent series resistance (ESR), such as ceramic capacitor, tantalum capacitor. Otherwise, the output waveform may develop abnormal oscillation. The C2 capacitance and ESR value stabilization conditions are as follows: 1 ≤ 20kHz 2×π×C2×ESR C3 is a capacitor for phase compensation of the feedback loop. Abnomal oscillation may occur when the C2 capacitance value is small or the equivalent series resistance is small. In this case, addition of the capacitance of C3 enable phase compensation, contributing to stabilization of power supply. 3. Input capacitor: Effecitive-value current The AC ripple current flowing in the input capacitor is larger than that in the output capacitor. The equation expressing the effective-value current is as folloes. Use the capacitor wiyhin the reted current range. IC1 = 1 Vout Vout ) + × ΔIR 2 (Iout 2 (1 − 12 Vin Vin [Arms] 4. Output capacitor: Effective-value current The AC ripple current flowing in the capacitor is the triangular wave. Therefore, its effective value is obtained from the following equation. Select the output capacitor so that it does not exceed the allowable ripple current value. 1 VOUT(VIN-VOUT) IC2 = × [Arms] L×fSW×VIN 2 3 fSW = switching frequency ··· 160kHz 5. Choke coil Note that choke coil heating due to overload or load shorting may be a problem. The inductance valuecan be determined from the following equation once the input voltage, output voltage, and current ripple conditions are known. ΔIR indicates the ripple current value. Reference example: VIN = 12V, VOUT = 5V, ΔIR = 150mA VIN - VOUT - Vsat L= × Tom ΔIR = 12 - 5.0 - 0.4 × 2.8×10-6 0.15 ≈ 120µH T Ton = (V - V +1) IN OUT - Vsat) / (VOUT + VF) Toff = T - Ton T: Switching repetition period ··· 6.25µs is assumed for the calculation VF: Schottky diode forward voltage ··· 0.4V is assumed for the calculation No.A2021-5/6 LA5724MC 6. Inductance current: peak value The ripple current peak value must be held within the rated current values for the inductor used. Here, IRP id the ripple current. IRP can be determined from the folloeing equation. VIN - VOUT - Vsat × Tom IRP = IOUT + 2L = 0.5 + 12 - 5.0 - 0.4 × 2.8×10-6 2 × 120×10-6 ≈ 0.57A 7. Inducrance current: ripple current value Here ΔIR is the ripple current. ΔIR can be determined from the folloeing equation. If the load current becomes less than one half the ripple current, the icductor current will become discontinuous. VIN - VOUT - Vsat ΔIR = × Tom L = 12 - 5.0 - 0.4 × 2.8×10-6 120×10-6 ≈ 0.15A 8. Diode D1 A Schottky barrier diode must be used for the diode. If a fast recovery diode si used, it is possible that the IC could be destroyed be the applied reverse voltage due to the recovety and the on-state voltage. 9. Diode current: peak current Applecations must be designed so that the peak value of the diode current remains within the rated current of the diode. The peak value of the diode current will be the same current as the peak value of the inductor current. 10. Repetitive pwak reverse voltage Application must be designed so that the repetitive peak reverse voltage remains within the voltage rating og the diode. Here, VRRM is the repetitive peak reverse voltage. VRRM can be determined from the folloeing equation. VRRM ≥ VCC Since moise voltage and other terms will be added in actual in actual operation, the voltage headling caoacity of the device should be about 1.5 times that given by the above calculation. ON Semiconductor and the ON logo are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC owns the rights to a number of patents, trademarks, copyrights, trade secrets, and other intellectual property. A listing of SCILLC’s product/patent coverage may be accessed at www.onsemi.com/site/pdf/Patent-Marking.pdf. SCILLC reserves the right to make changes without further notice to any products herein. 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