Ordering number : ENA2022 LA5735MC Monolithic Linear IC Separately-Excited Step-Down Switching Regulator (Variable Type) http://onsemi.com Overview The LA5735MC is a separately-excited step-down switching regulator (variable type). Functions • Time-base generator (300kHz) 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 34 V VSW -1 V VOS pin application voltage VVOS Allowable power dissipation Pd max Mounted on a circuit board.* -0.2 to 7 V 0.75 W Operating temperature Topr -30 to +125 °C Storage temperature Tstg -40 to +150 °C Junction temperature Tjmax 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 32 V 32812 SY 20120207-S00002 No.A2022-1/6 LA5735MC Electrical Characteristics at Ta = 25°C, VIN = 15V Parameter Symbol Ratings Conditions min Reference voltage VOS Reference pin bias current IFB Switching frequency fosc Short-circuit protection circuit fscp IO = 0.3A typ 1.20 240 Unit max 1.23 1.26 V 1 2 μA 300 360 kHz 15 kHz operating switching frequency Saturation voltage Vsat IOUT = 0.3A, VOS = 0V Maximum on duty D max VOS = 0V 100 % Minimum on duty D min VOS = 5V 0 % Output leakage current Ilk SWOUT = -0.4V Supply current Iin VOS = 2V Current limiter operating current IS Thermal shutdown operating 1 1.15 5 V 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 : Design guarantee values are replaced with electrical measurements, and are not measured by temperature. Package Dimensions unit : mm (typ) 3424 4.9 0.2 Allowable power dissipation, Pd max - W 0.835 0.375 6.0 3.9 0.42 1.75 MAX 1.27 2 0.175 1 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.A2022-2/6 LA5735MC Pin Assignment NC NC GND NC VIN NC SWOUT VOS Block Diagram VIN 3 SWOUT 1 Reg. OCP Reset OSC Drive NC 2 NC 5 NC 7 NC 8 Comp. 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 LA5735MC C3 + C1 + C2 D1 VOS R2 GND R1 Note: Insome cases, 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.A2022-3/6 LA5735MC Protection Circuit Functional Descriptions 1. 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 2. Short circuit protection function This IC prevents the current from increasing when the outputs are shorted by setting the switching frequency to 15kHz if the VOS pin voltage falls below 0.8V. Note : At startup, since the switching frequency will be 15kHz while the VOS pin voltage is 0.8V or 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.A2022-4/6 LA5735MC 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 currnet to around 500μA. R1= 1.23V ≈ 2.4kΩ 500μA We recommend values in the range 2.0 to 2.4kΩ VOUT R2= 1.23V -1 × R1 The following equation gives the output voltage set by R1 and R2. R2 VO= (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. Abnormal oscillation may occur when the C2 capacitance value is small or the equivalent series resistance is small. In this case, addition od the capacitance of C3 enables phase compensation, contributing to stabilization of power supply. 3. Input capacitor: Effective-value current The AC ripple currents flowing in the input capacitor is large than that in the output capacitor. The equation expressing the effective-value current is as follows. Use the capacitor within the rated 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 output capacitor is the triabgular 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. VOUT (VIN - VOUT) 1 IC2 = 2 3 × L × fsw × VIN √ fsw = Switching frequency [Arms] 300kHz 5. Choke coil L1 Note that choke coil heating due to overload or load shorting may be a problem.The inductance value can 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 L= VIN - VOUT - Vsat × Ton ΔIR 12 - 5.0 - 1.0 × 1.58 × 10-6 0.15 ≈ 68μH T Ton = ((V - V Vsat)/(V IN OUT OUT + VF)) + 1 Toff = T - Ton t : Switching repetition period ··· 3.33μs is assumed for the calculation VF : Schottky diode forward voltage0.4V is assumed for the calculation = No.A2022-5/6 LA5735MC 6. Inductance current : peak value The ripple current peak value must be held within the rated current values for the inductor used. Here, IRP is the ripple current. IRP can be determined from the following equation. Reference example : VIN = 12V, VOUT = 5V, IOUT = 0.5A, L = 68μH VIN - VOUT - Vsat × Ton 2L 12 - 5.0 - 1.0 × 1.58 × 10-6 = 0.5 + 2 × 68 × 10-6 IRP = IOUT + ≈ 0.57A 7. Inductance current : ripple current value Here ΔIR is the ripple current. ΔIR can be determined from the following equation. If the load current becomes less than one half the ripple current, the inductor current will become discontinuous. VIN - VOUT - Vsat × Ton L 12 - 5.0 - 1.0 × 1.58 × 10-6 = 68 × 10-6 ≈ 0.15A ΔIR = 8. Diode D1 A Schottky barrier diode must be used for this diode. If a fast recovery diode is used, it is possible that the IC could be destroyed by the applied reverse voltage due to the recovery and the on-state voltage. 9. Diode current: peak current Applications 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 peak reverse voltage Applications must be designed so that the repetitive peak reverse voltage remains within the voltage rating of the diode. Here, VRRM is the repetitive peak reverse voltage. VRRM can be determined from the following equation. VRRM ≥ VCC Since noise voltage and other terms will be added in actual operation, the voltage handling capacity 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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