AN2627 Application note ST8R00 syncronous boost converter with output current cut-off function Introduction The ST8R00 family of synchronous step-up DC-DC converters with current output cut-off function provide up to 1 A over an input voltage range of 4 V to 6 V and an output voltage range of 6 V to 12 V. The high switching frequency (1.2 MHz) allows the use of tiny surface-mount components. Along with the resistor divider to set the output voltage value, an inductor and two capacitors are required. A low output ripple is guaranteed by the current mode PWM topology and by the use of low ESR surface-mounted ceramic capacitors. The device is available in two versions: burst mode (ST8R00) and continuous mode (ST8R00W). The ST8R00 devices are thermal protected and available in the DFN8 4x4 package. Figure 1. Simplified schematic diagram LX OUT Thermal Ps Po Ns INH Inhibit PGND PWM control FB IN Vref PGND GND AM00001v1 December 2009 Doc ID 13913 Rev 2 1/19 www.st.com Contents AN2627 Contents 1 ST8R00 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.1 2 Inhibit function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Selecting components for applications . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1 Output voltage selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.2 Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.3 Inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.4 Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2.5 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3 Thermal considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4 Demonstration board usage recommendation . . . . . . . . . . . . . . . . . . . 13 4.1 External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.1.1 Capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 4.1.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5 BOM with most-used components . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 6 Footprint recommended data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 7 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 2/19 Doc ID 13913 Rev 2 AN2627 List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Simplified schematic diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 ST8R00 inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 ST8R00W inductor current at light load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ST8R00W inductor current at no load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 ST8R00 cut-off block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Current cut-off function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Inrush current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 ST8R00 application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Inhibit voltage vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Voltage feedback vs. temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 The ST8R00 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Demonstration board layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Efficiency vs. output current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Efficiency vs. output voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 ST8R00 efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 ST8R00W efficiency vs. inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 DFN8 4x4 recommended footprint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Doc ID 13913 Rev 2 3/19 ST8R00 description 1 AN2627 ST8R00 description The ST8R00 is a family of adjustable current mode PWM synchronous step-up DC-DC converters with internal 1 A power switch. It represents a complete 1 A switching regulator with internal compensation which eliminates the need for additional components. The two devices in the family, the ST8R00 and ST8R00W, operate at light load in two different ways. The ST8R00 works in power-save mode to achieve good efficiency, as shown in Figure 2. The ST8R00W, in order to guarantee the lowest switching ripple, operates in PWM (pulse width modulation) mode as show in Figure 3 and Figure 4. At medium and high load current, both versions operate in PWM mode. The thermal shutdown block turns off the regulator when the junction temperature exceeds 150 °C (typ), and the cycle-by-cycle current limiting provides protection against overcurrent sink. Figure 2. ST8R00 inductor current at light load IL LX VIN=5 V, VOUT=8 V, IOUT=60 mA, Ch1=LX, Ch4=IL 4/19 Doc ID 13913 Rev 2 AN2627 ST8R00 description Figure 3. ST8R00W inductor current at light load IL LX VIN=5 V, VOUT=8 V, IOUT=60 mA, Ch1=LX, Ch4=IL Figure 4. ST8R00W inductor current at no load IL LX VIN=5 V, VOUT=8 V, no load Ch1=LX, Ch4=IL For proper functioning of the device, only a few components are required: an inductor, two capacitors and the resistor divider. The inductor chosen must not saturate at the operating peak current. Its value should be selected taking into account that a large inductor value reduces output voltage ripple, while a smaller inductor can be selected when it is important to reduce package size and the total cost of the application. Finally, the ST8R00 family has been designed to work properly with X5R or X7R SMD ceramic capacitors both at the input and at the output. These types of capacitors, thanks to their very low series resistance (ESR), minimize the output voltage ripple. Other low ESR capacitors can be used in accordance with application requirements without compromising the correct functionality of the device. This device features an output current cut-off function. Two P-channel MOSFETs in a backto-back configuration, as shown in Figure 5, stop the output current when the inhibit is low (Figure 6). Doc ID 13913 Rev 2 5/19 ST8R00 description Figure 5. AN2627 ST8R00 cut-off block LX OUT Ps Po Ns PGND Figure 6. Current cut-off function Iout Vout Inh Vin Figure 7 shows the in-rush current at start-up. Initially, the COUT capacitor is completely discharged and the current limitation is due only to the equivalent series resistor of the inductor, the power MOSFET parasitic diode and the cut-off MOSFETs’ RDS(ON). As soon as the output voltage reaches the input voltage level, the device begins to switch and the current is limited cycle by cycle. 6/19 Doc ID 13913 Rev 2 AN2627 ST8R00 description Figure 7. Inrush current Vout Iin LX VIN=4.5 V, VOUT=7 V, VINH from 0 V to 3 V, RLOAD=13 Ω, L=10 µH, CIN=COUT=10 µF 1.1 Inhibit function The ST8R00 family of devices also include an inhibit function (pin 6). When the INH voltage is higher than 2 V, the device is ON and if it is lower than 0.8 V, the device is OFF. The INH pin does not have an internal pull-up, which means that the pin cannot be left floating. If the inhibit function is not used, the INH pin must be connected to VIN as in the schematic in Figure 8 below. Figure 8. ST8R00 application schematic L Vin 4 IN Rinh 6 Cin Cinh 1 7 Vout LX 8 OUT INH R1 ST8R00 HV GND PGND 3 Doc ID 13913 Rev 2 FB Cout 5 R2 2 7/19 ST8R00 description Vinh [V] Figure 9. AN2627 Inhibit voltage vs. temperature 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 Vin=4V, Vinh from 0 to 2V, Iout=50mA, L=4.7µH, Cin=10µF, Cout=10µF -75 -50 -25 0 25 50 T [°C] 8/19 Doc ID 13913 Rev 2 ON 75 OFF 100 125 150 175 AN2627 2 Selecting components for applications Selecting components for applications This section provides information to assist in the selection of the most appropriate components for applications. Figure 10 shows a typical application schematic diagram. Figure 10. Typical application schematic L Vin 4 1 IN OFF ON Cin 6 7 LX OUT INH ST8R00 FB HV GND Cout 5 R2 2 Output voltage selection The output voltage can be adjusted from 6 V up to 12 V by connecting a resistor divider between the output and the FB pin. The resistor divider should be chosen in accordance with the following equation: Equation 1 R V out = V FB 1 + ------1R2 with V FB = 1.22 V The feedback voltage versus temperature is shown Figure 11 below. It is recommended to use a resistor with a value in the range of 10 kΩ to 100 kΩ. Lower values can be suitable as well, but will increase current consumption. Figure 11. Voltage feedback vs. temperature Vfb [mV] 2.1 R1 PGND 3 Vout 8 1.3 1.28 1.26 1.24 1.22 1.2 1.18 1.16 1.14 1.12 1.1 Vin=Vinh=5V, Iout=50mA, L=4.7µH, Cin=10µF, Cout=10µF -75 -50 -25 0 25 50 75 100 125 150 175 T [°C] Doc ID 13913 Rev 2 9/19 Selecting components for applications 2.2 AN2627 Input capacitor The input capacitor must be able to provide AC ripple current to the inductor and to withstand the maximum input operating voltage. Another important function of the input capacitor is to limit noise and therefore the interference with the other blocks connected to the same network. The quality of these capacitors must to be quite high to minimize the power dissipation generated by the internal ESR, thereby improving system reliability and efficiency. Various capacitors can be considered: 2.3 ● Ceramic capacitors - These capacitors usually have a higher RMS current rating for a given physical dimension (due to the very low ESR). The drawback is the high cost of capacitors with very large values. ● Electrolytic capacitor - The availability of small size tantalum capacitors with very low ESR is increasing. However, they are subject to thermal damage if subjected to very high current during charge. Since they can, in fact, be subjected to high surge current when connected to the power supply, it is better to avoid using this type of capacitor for the input filter of the device. Aluminum capacitors are not the best choice due to their high ESR. Inductor The inductor value is very important because it establishes the ripple current. The approximate inductor value is obtained with the following formula: Equation 2 L= Vin ⋅ TON ΔIL where TON is the ON time of the internal switch, given by D · TSW. The ripple current, ΔIL, is usually fixed at 20-40% of IIN_MAX. Equation 3 IIN_MAX = IOUT_max ⋅ Vout Vin ⋅ η where η is the efficiency. 2.4 Output capacitor The output capacitor is very important to satisfy the output voltage ripple requirement. To reduce the output voltage ripple, a low ESR capacitor is required. The output voltage ripple (VRIPPLE), in continuous mode is: Equation 4 ( V out – V in ) ⎞ V RIPPLE = I out ⋅ ⎛ ESR + ------------------------------------------⎝ V out ⋅ C out ⋅ F SW⎠ where FSW is the switching frequency. 10/19 Doc ID 13913 Rev 2 AN2627 2.5 Selecting components for applications Layout considerations Due to the high switching frequency and peak current, the layout is an important design step for all switching power supplies. If the layout is not done carefully, important parameters such as efficiency and output voltage ripple could be out of specification. Short, wide traces must be implemented for main current and for power ground paths as shown in bold in Figure 12. The input capacitor must be placed as close as possible to the IC pins as well as the inductor and output capacitor. A common ground node minimizes ground noise, as shown in Figure 12. The HV pin must be floating or connected to GND and the exposed pad of the package must be connected to the common ground node. Figure 12. Layout considerations L Vin 4 1 IN OFF ON 6 LX OUT INH Cin 7 Vout 8 ST8R00 5 R1 Cout FB HV GND 3 PGND R2 2 Doc ID 13913 Rev 2 11/19 Thermal considerations 3 AN2627 Thermal considerations The dissipated power of the device is related to three different sources: ● Switching losses due to the (not negligible) RDS(ON). These are equal to: Equation 5 PON_N = RDSON_N⋅ [IOUT /(1 − D)]² ⋅ D and Equation 6 2 PON_P = RDSON_PEQ ⋅ IOUT ⋅ (1 − D) where D is the duty cycle of the application and RDS(ON)_PEQ=RDS(ON)_PS+RDS(ON)_PO. Note: the duty cycle is theoretically given by: V in1 – -----------V out but in practice it is quite higher than this value to compensate for the losses of the overall application. For this reason, the switching losses related to the RDS(ON) increase compared to an ideal case. ● Switching losses due to its turning on and off. These are calculated using the following equation: Equation 7 PSW = VIN ⋅ I OUT ⋅ (t ON + t OFF ) ⋅ FSW = VIN ⋅ I OUT ⋅ t R -F ⋅ FSW 2 where tON and tOFF are the overlap times of the voltage across the power switch and the current flowing into it during the turn-on and turn-off phases. tR-F is the equivalent switching time. ● Quiescent current losses: Equation 8 PQ = VIN ⋅ IQ where IQ is the quiescent current. The overall losses are: Equation 9 2 PTOT = RDSON_N ⋅ (IOUT / 1− D)² ⋅ D + RDSON_PEQ ⋅ IOUT ⋅ (1− D) + VIN ⋅ IOUT ⋅ tR-F ⋅ FSW + VIN ⋅ IQ The junction temperature of device will be: Equation 10 TJ = TA + R thJA ⋅ PTOT where TA is the ambient temperature and RthJA is the thermal resistance junction-toambient. 12/19 Doc ID 13913 Rev 2 AN2627 4 Demonstration board usage recommendation Demonstration board usage recommendation The demonstration board shown in Figure 13 is provided with a Kelvin connection, so for each pin there are two lines available: one used to supply or sink current, and the other used to perform the needed measurement. The ST8R00 inhibit pin does not have an internal pull-up, so the inhibit pin cannot be left floating. Figure 13. The ST8R00 demonstration board Figure 14. Demonstration board layers Top layer Bottom layer The board has one inhibit pin available, located on the top left of the board. The inhibit pin can be used to supply an external voltage higher than 2 V to turn on the device, or an external voltage lower than 0.8 V to turn off the device. Doc ID 13913 Rev 2 13/19 Demonstration board usage recommendation 4.1 AN2627 External component selection Figure 15 shows the schematic diagram of the demonstration board. Figure 15. Demonstration board schematic L Vin 4 1 LX 8 OUT IN OFF ON Cin 6 7 INH ST8R00 FB HV GND R1 5 Cout R2 PGND 3 Vout 2 In order to obtain the needed output voltage, the resistor divider must be selected based on the following formula: Equation 11 V out = V FB 1 + R1 -------R2 Table 1. with V FB = 1.22 V Recommended resistor divider Vout R1 R2 8V 56 kΩ 10 kΩ 9.5 V 68 kΩ 10 kΩ The resistors in Table 1 represent a good compromise in terms of current consumption and minimum output voltage. 4.1.1 Capacitor selection It is possible to use any X5R or X7R ceramic capacitor: 4.1.2 ● CIN=10 µF (ceramic) or higher ● COUT=10 µF (ceramic) or higher. It is possible to put several capacitors in parallel to reduce the equivalent series resistance and improve the ripple present in the output voltage. Inductor selection Due to the high (1.2 MHz) frequency, it is possible to use very small inductor values. In the demonstration board, the device was tested with inductors in the range of 1 µH to 10 µH, with very good efficiency performance (see Figure 18 and Figure 19). Because the device is able to provide an operating output current of 1 A, we strongly recommend the use of inductors capable of managing at least 3.5 A. 14/19 Doc ID 13913 Rev 2 AN2627 Demonstration board usage recommendation Figure 16. Efficiency vs. output current 100 Efficiency [%] 90 80 70 60 50 40 ST8R00 ST8R00W Vin=5V , Vinh=5V, L=4.7µH, Cin=10µF, Cout=10µF, Vout=8V 30 20 0 0.1 0.2 0.3 0.4 0.5 0.6 Iout [A] 0.7 0.8 0.9 1 Figure 17. Efficiency vs. output voltage 100 Efficiency [%] 95 90 85 80 75 70 65 ST8R00 ST8R00W Vin=5V , Vinh=5V, L=4.7µH, Cin=10µF, Cout=10µF, Iout=300mA 60 55 50 4.5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12.5 Vout [V] Figure 18. ST8R00 efficiency vs. inductor 100 Efficiency [%] 90 80 70 60 50 Iout=300mA Iout=0.5A Iout=1A Vin=5V , Vinh=5V, Vout=8V, Cin=10µF, Cout=10µF 40 30 20 0 2 4 6 L [µH] Doc ID 13913 Rev 2 8 10 15/19 BOM with most-used components AN2627 Figure 19. ST8R00W efficiency vs. inductor 100 Efficiency [%] 90 80 70 60 50 Iout=300mA Iout=0.5A Iout=1A Vin=5V , Vinh=5V, Vout=8V, Cin=10µF, Cout=10µF 40 30 20 0 5 4 6 L [µH] 8 10 BOM with most-used components Table 2. 16/19 2 Bill of materials Name Value Material Manufacturer Part numbers CIN 10 µF Ceramic Murata GRM31CR61E106KA12B COUT 10 µF Ceramic Murata GRM31CR61E106KA12B L 4.7 µH Coiltronics DR73-4R7 Doc ID 13913 Rev 2 AN2627 6 Footprint recommended data Footprint recommended data Figure 20. DFN8 4x4 recommended footprint Doc ID 13913 Rev 2 17/19 Revision history 7 AN2627 Revision history Table 3. 18/19 Document revision history Date Revision Changes 13-May-2008 1 Initial release 03-Dec-2009 2 Modified Equation 9: on page 12. Doc ID 13913 Rev 2 AN2627 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. No license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. If any part of this document refers to any third party products or services it shall not be deemed a license grant by ST for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoever of such third party products or services or any intellectual property contained therein. UNLESS OTHERWISE SET FORTH IN ST’S TERMS AND CONDITIONS OF SALE ST DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY WITH RESPECT TO THE USE AND/OR SALE OF ST PRODUCTS INCLUDING WITHOUT LIMITATION IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION), OR INFRINGEMENT OF ANY PATENT, COPYRIGHT OR OTHER INTELLECTUAL PROPERTY RIGHT. UNLESS EXPRESSLY APPROVED IN WRITING BY AN AUTHORIZED ST REPRESENTATIVE, ST PRODUCTS ARE NOT RECOMMENDED, AUTHORIZED OR WARRANTED FOR USE IN MILITARY, AIR CRAFT, SPACE, LIFE SAVING, OR LIFE SUSTAINING APPLICATIONS, NOR IN PRODUCTS OR SYSTEMS WHERE FAILURE OR MALFUNCTION MAY RESULT IN PERSONAL INJURY, DEATH, OR SEVERE PROPERTY OR ENVIRONMENTAL DAMAGE. ST PRODUCTS WHICH ARE NOT SPECIFIED AS "AUTOMOTIVE GRADE" MAY ONLY BE USED IN AUTOMOTIVE APPLICATIONS AT USER’S OWN RISK. Resale of ST products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by ST for the ST product or service described herein and shall not create or extend in any manner whatsoever, any liability of ST. ST and the ST logo are trademarks or registered trademarks of ST in various countries. Information in this document supersedes and replaces all information previously supplied. The ST logo is a registered trademark of STMicroelectronics. All other names are the property of their respective owners. © 2009 STMicroelectronics - All rights reserved STMicroelectronics group of companies Australia - Belgium - Brazil - Canada - China - Czech Republic - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan Malaysia - Malta - Morocco - Philippines - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States of America www.st.com Doc ID 13913 Rev 2 19/19