AND9095/D Operation of NCP5252 in Low Dropout Applications http://onsemi.com APPLICATION NOTE Introduction Low Dropout PWM Operation The NCP5252 is a single−phase buck regulator with integrated Power MOSFETs. The device is able to deliver up to 2 A current at an externally adjustable output voltage, which range is from 0.6 V to 5.0 V. The operation range of the input supply voltage is from 4.5 V to 13.2 V. To provide a design guide for low dropout applications, detailed operation behavior of the NCP5252 is described in this application note. Both measurement and calculation results of a 5 V−output application are provided. As a non−isolated buck converter, the input supply voltage VIN of the NCP5252 needs to be higher than the output voltage VOUT to provide a sustainable current to a load. In applications where VIN is much higher than VOUT, the NCP5252 operates in a selected fixed−frequency in CCM. When the input voltage VIN drops to be close to the output voltage VOUT, it is possible to see other two operation modes with the NCP5252. Figure 1 is a plot that illustrates key signals in these operation modes. VIN V IN_MIN1 Mode 1 V OUT V IN_MIN2 Mode 2 COMP RAMP Mode 3 V COMP _MAX 0 t ON PWM TOFF _MIN 0 T 1 D D MAX 0 Time Figure 1. PWM Regulation Modes with the NCP5252 © Semiconductor Components Industries, LLC, 2012 July, 2012 − Rev. 0 1 Publication Order Number: AND9095/D AND9095/D Mode 1 – Fixed−Frequency Operation switching frequency until the off time of the PWM signal reaches its minimum limit TOFF_MIN (225 ns typical in datasheet, but it would be better to use 260 ns in calculation to cover dead times of gate drivers.). The minimum required voltage VIN_MIN1 can be obtained by As shown in Figure 1, the NCP5252 operates in Mode 1 when the input supply voltage VIN is higher than a minimum required voltage VIN_MIN1. It is a fixed−frequency operation mode with an externally programmed frequency at FREQ_SET pin. The duty ratio of the PWM signal can be estimated by D+ V IN_MIN1 + V OUT ) I OUT @ ǒR DS_L ) DCRǓ V IN * I OUT @ ǒR DS_H * R DS_LǓ where IOUT is the load current, DCR is the DC resistance of the output filter inductor L, RDS_H is the conduction resistance of the integrated high−side MOSFET, and RDS_L is the conduction resistance of the integrated low−side MOSFET. To simplify analysis, the diode voltage drops of MOSFETs during dead times have not been taken into account in this calculation. To ensure good stability and line transient response, a VIN feedforward function has been implemented in the NCP5252’s ramp generator. The slew rate SRRAMP of the ramp signal is proportional to both the input voltage VIN and the nominal switching frequency FSW. If the input voltage drops below VIN_MIN1, the off time of the PWM signal cannot be reduced further due to TOFF_MIN. This forces the NCP5252 to reduce switching frequency to maintain the output regulation. The switching frequency in Mode 2 operation is V COMP_M2 + T ON_M2 @ SR RAMP ) V RAMP_OFFSET (eq. 6) T ON_M2 + T OFF_MIN Ǔ *1 V )I @ ǒR )DCRǓ OUT OUT DS_L V (eq. 3) )I IN ǒ @ R OUT *R DS_L DS_H 1* 0.25@F V @T SW *V COMP_MAX OFF_MIN RAMP_OFFSET ǒ @ V OUT ) I OUT @ ǒR DS_L ) DCRǓ Mode 3 – Maximum−Duty Operation SR RAMP 0.25@V 1)V Ǔ @F IN_MIN2 *V COMP_MAX @T SW OFF_MIN (eq. 11) RAMP_OFFSET In Mode 3, the output voltage drops as input voltage drops. The output voltage can be calculated by (eq. 9) The switching frequency FSW_M3 in Mode 3 does not decrease as much as that in Mode 2 when VIN drops. 1 T ON_M3 ) T OFF_MIN (eq. 8) 1 D MAX + If the input voltage drops below VIN_MIN2, the COMP signal is clamped to VCOMP_MAX, and thus the NCP5252 reaches its limitation in the on time TON_M3. V COMP_MAX * T RAMP_OFFSET (eq. 7) The maximum on time is limited by the maximum voltage level VCOMP_MAX of the COMP signal, which typical value is 3.5 V. A corresponding minimum input voltage VIN_MIN2 to maintain the target regulation voltage can be calculated by: V OUT ) I OUT @ ǒR DS_H ) DCRǓ V IN_MIN2 + (eq. 5) The control loop pushes the COMP voltage to a higher level VCOMP_M2 to get a wider on time TON_M2. where VRAMP_OFFSET is the offset/valley voltage (0.4 V typical) of the ramp signal. When the input voltage VIN drops, the duty ratio D increases to maintain the output regulation with the fixed F SW_M3 + 1*D . T OFF_MIN F SW_M2 + (eq. 2) V COMP_M1 + D @ T @ SR RAMP ) V RAMP_OFFSET, (eq. 4) Mode 2 – Frequency−Deduction Operation As a result of the input feedforward function, the voltage level VCOMP_M1 of the error signal COMP does not change much with the input voltage VIN, having a value of T ON_M3 + 1 * F SW @ T OFF_MIN ) I OUT @ ǒR DS_H * R DS_LǓ. (eq. 1) SR RAMP + 0.25 @ V IN @ F SW V OUT ) I OUT @ ǒR DS_L ) DCRǓ V OUT_M3 + V IN * I OUT @ ǒR DS_H * R DS_LǓ 1) (eq. 10) 0.25@V V @F IN COMP_MAX @T SW *V OFF_MIN (eq. 12) RAMP_OFFSET * I OUT @ ǒR DS_L ) DCRǓ The NCP5252 reaches its maximum limitation in the duty ratio, which has a value DMAX when the input voltage is equal to VIN_MIN2. The voltage drop between VIN and VOUT increases as IOUT increases. http://onsemi.com 2 AND9095/D 5 V−Output Application Figure 3 shows measurement results with a 1 A load current and three nominal switching frequency options. The higher the nominal frequency is selected, the higher VIN_MIN2 is required to maintain the 5 V output voltage. VIN Figure 4 shows measurement results of the switching frequency changing with the input voltage for three nominal frequency options. The NCP5252 is able to have about 60% off in switching frequency in this application. The 333 kHz frequency option is able to support the lowest minimum input voltage in the low dropout applications. To verify the operation of the NCP5252 in a low dropout application, a 5 V−output regulator has been built on a NCP5252 demo board. Three plots in Figure 2 show waveforms of three key signals (VOUT, COMP, and LX) under the three operation modes when VIN is close to VOUT. The load current is 1 A and the selected nominal frequency is 500 kHz in this test. The switching frequency drops smoothly as VIN drops. The minimum required input voltage for the 5 V/1 A output is about 5.5 V. The NCP5252 still be able to provide current to the output when VIN drops further, but the output voltage starts to drop below the 5 V target. http://onsemi.com 3 AND9095/D VOUT (1.0V/div) COMP (2.0V/div) LX (5.0V/div) Time (1us/div) (a) VIN = 7.0 V (Mode 1) VOUT (1.0V/div) COMP (2.0V/div) LX (5.0V/div) Time (1us/div) (b) VIN = 5.75 V (Mode 2) VOUT (1.0V/div) COMP (2.0V/div) LX (5.0V/div) Time (1us/div) (c) VIN = 5.0 V (Mode 3) Figure 2. PWM Operation Waveforms vs. Input Voltage (VOUT = 5 V, IOUT = 1 A, 500 kHz Option) http://onsemi.com 4 AND9095/D Figure 3. Output Regulation VOUT vs. Input Voltage VIN Figure 4. Switching Frequency FSW vs. Input Voltage VIN measurements and the calculations grows with an increasing in the load current, which may be caused by some omitted factors in the calculations such as body diode forward voltage during dead times, conduction resistance variations regarding to temperatures of inductor and MOSFETs, and other power lose factors. As shown in Figure 5, a result comparison between measurements and calculations has been done on the minimum required input voltage VIN_MIN2 to maintain 5 V output voltage under different load current conditions. Both results address that higher minimum input voltage is required for higher load current. Difference between the http://onsemi.com 5 AND9095/D Figure 5. Minimum Input Voltage VIN_MIN2 vs. Load Current IOUT (333 kHz Option) ON Semiconductor and 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. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. All operating parameters, including “Typicals” must be validated for each customer application by customer’s technical experts. SCILLC does not convey any license under its patent rights nor the rights of others. SCILLC products are not designed, intended, or authorized for use as components in systems intended for surgical implant into the body, or other applications intended to support or sustain life, or for any other application in which the failure of the SCILLC product could create a situation where personal injury or death may occur. Should Buyer purchase or use SCILLC products for any such unintended or unauthorized application, Buyer shall indemnify and hold SCILLC and its officers, employees, subsidiaries, affiliates, and distributors harmless against all claims, costs, damages, and expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized use, even if such claim alleges that SCILLC was negligent regarding the design or manufacture of the part. SCILLC is an Equal Opportunity/Affirmative Action Employer. This literature is subject to all applicable copyright laws and is not for resale in any manner. PUBLICATION ORDERING INFORMATION LITERATURE FULFILLMENT: Literature Distribution Center for ON Semiconductor P.O. Box 5163, Denver, Colorado 80217 USA Phone: 303−675−2175 or 800−344−3860 Toll Free USA/Canada Fax: 303−675−2176 or 800−344−3867 Toll Free USA/Canada Email: [email protected] N. American Technical Support: 800−282−9855 Toll Free USA/Canada Europe, Middle East and Africa Technical Support: Phone: 421 33 790 2910 Japan Customer Focus Center Phone: 81−3−5817−1050 http://onsemi.com 6 ON Semiconductor Website: www.onsemi.com Order Literature: http://www.onsemi.com/orderlit For additional information, please contact your local Sales Representative AND9095/D