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| FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator December 2008 FAN8303 2A 23V Non-Synchronous Step-Down DC/DC Regulator Features 2A Output Current 0.22 Internal Power MOSFET Switch Wide 5V to 23V Operating Input Range Output Adjustable from 0.6 to 20V Stable with Low ESR Output Ceramic Capacitors Up to 90% Efficiency Less than 40A Shutdown Current Fixed 370kHz Frequency Thermal Shutdown with Hysteresis Cycle-by-Cycle Over-Current Protection Available in 8-Pin SOIC Package Description The FAN8303 is a monolithic, non-synchronous, stepdown (buck) regulator with internal power MOSFETs. It achieves 2A continuous output current over a wide input supply range with excellent load and line regulation. Current-mode operation provides fast transient response and eases loop stabilization. Fault condition protection includes cycle-by-cycle current limiting and thermal shutdown. The regulator draws less than 40A shutdown current. FAN8303 requires a minimum number of readily available standard external components. External compensation, enable, and programmable soft-start features allow design optimization and flexibility. Cycle-by-cycle current limit, frequency foldback, and thermal shutdown provide protection against shorted outputs. CBS 10 nF Applications Set-Top Box DSL and Cable Modems Distributed Power Systems Consumer Appliances (DVD) Auxiliary supplies INPUT 5~23V CIN 10 F VIN ENABLE SHUTDOWN EN BS SW L1 15 H OUTPUT 2.5V/2A D1 FAN8303 SS GND CSS 10 nF FB COMP R2 18 k COUT 22 F R3 RC 22 k 5.6 k CA OPEN CC 1nF Figure 1. Typical Application Ordering Information Part Number FAN8303MX Eco Status RoHS Operating Temperature Range -40C to +85C Package 8-SOIC Packing Method Reel For Fairchild's definition of "green" Eco Status, please visit: http://www.fairchildsemi.com/company/green/rohs_green.html. (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 www.fairchildsemi.com FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Internal Block Diagram Figure 2. Functional Block Diagram (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 www.fairchildsemi.com 2 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Pin Configuration BS VIN SW GND Figure 3. SS EN COMP FB Pin Configuration (Top View) Pin Definitions Name BS VIN SW Pin # 1 2 3 Type Bootstrap Supply Voltage Switch Description High-Side Drive BOOT Voltage. Connect through capacitor (CBS) to SW. The IC includes an internal synchronous bootstrap diode to recharge the capacitor on this pin to VCC when SW is LOW. Power Input. This pin needs to be closely decoupled to the GND pin with a 10F or greater ceramic capacitor. Power Switching Output. SW is the switching node that supplies power to the output. Power Return and Signal Ground for the IC. All internal control voltages are referred to this pin. Tie this pin to the ground island / plane through the lowest impedance connection. This pin is the ground reference for the regulated output voltage. Feedback Input. This pin is the center tap of the external feedback voltage resistive divider across the output. Compensation Node. Frequency compensation is accomplished at this node by connecting a series R-C to ground. Enable Input. EN is a digital input that turns the regulator on or off. Drive EN HIGH to turn on the regulator, drive it LOW to turn it off. For automatic startup, leave EN unconnected. External Soft-Start. A capacitor connected between this pin and GND can be used to set soft-start time. GND 4 Ground FB COMP EN SS 5 6 7 8 Feedback Compensation Enable Soft Start (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 www.fairchildsemi.com 3 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Absolute Maximum Ratings Stresses exceeding the absolute maximum ratings may damage the device. The device may not function or be operable above the recommended operating conditions and stressing the parts to these levels is not recommended. In addition, extended exposure to stresses above the recommended operating conditions may affect device reliability. The absolute maximum ratings are stress ratings only. All voltage values, except differential voltages, are given with respect to the network ground terminal. Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device Symbol VIN VSW VBS VFB VEN VCOMP VSS JA JC TJ TL TSTG ESD Supply Voltage, VIN to GND Switch Voltage, SW to GND Boost Voltage Feedback Voltage Enable Voltage Compensation Voltage Soft-Start Voltage Parameter Min. -0.3 -0.3 -0.3 -0.3 -0.3 Max. 25 VIN+0.3 VSW + 6 6.0 6.0 6.0 6.0 105 40 Unit V V V V V V V C/W C/W C C C kV Thermal Resistance, Junction-Air Thermal Resistance, Junction-Case Operating Junction Temperature Lead Temperature (Soldering, 5 Seconds) Storage Temperature Range Human Body Model, JEDEC JESD22-A114 Electrostatic Discharge Charged Device Model, JEDEC JESD22Protection Level C101 -65 3.0 2.5 -40 +125 +260 +150 Recommended Operating Conditions The Recommended Operating Conditions table defines the conditions for actual device operation. Recommended operating conditions are specified to ensure optimal performance to the datasheet specifications. Fairchild does not recommend exceeding them or designing to absolute maximum ratings. Symbol VIN TA Supply Voltage Parameter Operating Ambient Temperature Min 5 -40 Max. 23 +85 Unit V C (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 www.fairchildsemi.com 4 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Electrical Characteristics VIN=12V, TA= -40 to +85C, unless otherwise noted. Symbol VFB RON_H RON_L ILKG IPK fOSC VUVLO fSHORT DMAX TON_MIN VEN VEN_H IOFF IQ GCS GEA AVEA ISS TSD Parameter Feedback Voltage Upper Switch On Resistance Lower Switch On Resistance Upper Switch Leakage Current Peak Inductor Current Oscillator Frequency Under-Voltage Lockout Short Circuit Frequency Maximum Duty Cycle Minimum On Time Enable Threshold Enable Threshold Hysteresis Supply Current (Shutdown) Supply Current (Quiescent) Current Sense Gain Error Amplifier Transconductance Error Amplifier Voltage Gain Soft-Start Current Thermal Shutdown Temperature Condition 25C, 5V 0.58 Typ. 0.60 0.22 4 Max. 0.62 Unit V VEN=0V,VSW =0V 0 3.5 10 A A VFB>0.3V Rising VIN VFB<0.3V 315 4.2 25 370 4.6 45 90 210 435 5.0 55 kHz V kHz % ns 1.2 1.6 150 2.0 V mV VEN=0V VEN>1.6V; VFB=0.8V 10 1.0 2 380 400 6 155 40 2.0 A mA A/V A/V V/V A C (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 www.fairchildsemi.com 5 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Typical Performance Characteristics VIN = 12V, VOUT = 5V, L1 = 15H, CIN = 10F, COUT = 22F, TA = +25 C, unless otherwise noted. CH1(VO) : 2V, 50s/div. CH2(EN) : 4V, 50s/div. CH3(SW) : 6V, 50s/div. CH4(IL) : 1A, 50s/div. o CH2 CH1(VO) : 2V, 500s/div. CH2(EN) : 4V, 500s/div. CH3(SW) : 6V, 500s/div. CH4(IL) : 1A, 500s/div. CH2 CH1 CH1 CH3 CH3 CH4 CH4 Figure 4. EN Startup with 2A Load Figure 5. EN Turn-off with 2A Load CH1 CH1 CH2 CH3 CH1(VO) : 2V, 1ms/div. CH2(VIN) : 4V, 1ms/div. CH3(SW) : 6V, 1ms/div. CH4(Io) : 1A, 1ms/div. CH2 CH3 CH1(VO) : 2V, 200s/div. CH2(VIN) : 4V, 200s/div. CH3(SW) : 6V, 200s/div. CH4(Io) : 1A, 200s/div. CH4 CH4 Figure 6. Power-on with 2A Load Figure 7. Power-off with 2A Load CH1(VO) : 5V offset 200mV, 50s/div. CH2(COMP) : 300mV, 50s/div. CH3(SW) : 10V, 50s/div. CH4(Io) : 1A, 50s/div. CH1 Vo = 240mV CH1 CH1(VO) : 5V offset 200mV, 50s/div. CH2(COMP) : 300mV, 50s/div. CH3(SW) : 10V, 50s/div. CH4(Io) : 1A, 50s/div. Vo = 204mV Slew Rate( 2.5A/s) Slew Rate( 2.5A/s) CH4 CH3 CH2 CH4 CH3 CH2 Figure 8. Load Transient Response (0.5A to 1.5A) Figure 9. Load Transient Response (1.5A to 0.5A) (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 www.fairchildsemi.com 6 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Typical Performance Characteristics (Continued) VIN = 12V, VOUT = 5V, L1 = 15H, CIN = 10F, COUT = 22F, TA = +25 C, unless otherwise noted. o f=45kHz CH1 CH2 CH1 CH2 CH4 CH1(VO) : 2V, 20/div. CH2(VIN) : 4V, 20s/div. CH3(SW) : 6V, 20s/div. CH4(IL) : 2A, 20s/div. CH4 CH1(VO) : 2V, 20/div. CH2(VIN) : 4V, 20s/div. CH3(SW) : 6V, 20s/div. CH4(IL) : 2A, 20s/div. Figure 10. Hard-Short at Output (OCP) Figure 11. Overload at Output (OCP) 95 5.0Vo 1 0.5 3.3Vo 90 Efficiency [%] 2.5Vo Vout [%] 85 80 75 70 0 0.5 1 1.5 Load Current [A] 2 1.8Vo 0 -0.5 -1 -1.5 -40 -15 10 35 60 85 Temperature [] Figure 12. Efficiency Curve Figure 13. Normalized Output Voltage vs. Temperature 380 370 Frequency [kHz] 4 360 350 340 330 320 -40 -15 10 35 60 85 Temperature [] Load Current [A] 3.5 3 2.5 2 0 20 40 60 80 100 Duty [%] Figure 14. Oscillator Frequency vs. Temperature (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 Figure 15. Current Limited Level vs. Duty Ratio www.fairchildsemi.com 7 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Functional Description The FAN8303 is a monolithic, non-synchronous, current-mode, step-down regulator with internal power MOSFETs. It achieves 2A continuous output current over a wide input supply range from 5V to 23V with excellent load and line regulation. The output voltage can be regulated as low as 0.6V. The FAN8303 uses current-mode operation that provides fast transient response and eases loop stabilization. The FAN8303 requires a minimum number of readily available standard external components. Inductor Selection A higher inductor value lowers ripple current. The inductor value can be calculated as: L= VOUT fS IL VOUT 1 - VIN (1) where: fs is the switching frequency; VOUT is the output voltage; VIN is the input supply voltage; and IL Is the inductor ripple current. Considering worst case, the equation is changed to: L= VOUT fS I L,MAX 1 - VOUT VIN ,MAX Current Mode PWM Control Loop FAN8303 uses current-mode PWM control scheme. The peak inductor current is modulated in each switching cycle by an internal op-amp output signal to achieve the output voltage regulation. An internal slope compensation circuit is included to avoid sub-harmonic oscillation at duty cycle greater than 50%. Currentmode control provides cycle-by-cycle current limit protection and superior regulation control loop response compared to the traditional voltage-mode control. In normal operation, the high-side MOSFET is turned on at the beginning of each switching cycle, which causes the current in the inductor to build up. The currentcontrol loop senses the inductor current by sensing the voltage across the high-side senseFET during on time. The output of the current-sense amplifier is summed with the slope compensation signal and the combined signal is compared with the error amplifier output to generate the PWM signal. As the inductor current ramps up to the controlled value, the high-side MOSFET is turned off and the inductor current reaches zero through a freewheeling diode. In light-load condition, the high-side switch may be kept off for several cycles to improve efficiency. (2) Input Capacitor Selection To prevent high-frequency switching current passing to the input, the input capacitor impedance at the switching frequency must be less than input source impedance. High-value, small, inexpensive, lower-ESR ceramic capacitors are recommended. 10F ceramic capacitors should be adequate for 2A applications. Output Capacitor Selection A larger output capacitor value keeps the output ripple voltage smaller. The formula of output ripple VOUT is: 1 VOUT IL ESR + 8 COUT fS Short-Circuit Protection The FAN8303 protects output short circuit by switching frequency fold-back. The oscillator frequency is reduced to about 45kHz when the output is shorted to ground. This frequency fold-back allows the inductor current more time to decay to prevent potential run-away condition. The oscillator frequency switches to 370kHz as VOUT rises gradually from 0V back to regulated level. (3) where COUT is the output capacitor and ESR is the equivalent series resistance of the output capacitor. Output Voltage Programming The output voltage is set by a resistor divider, according to the following equation: R2 VOUT = 0.61 + R3 Slope Compensation and Inductor Peak Current The slope compensation provides stability in constant frequency architecture by preventing sub-harmonic oscillations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 50%. (4) Freewheeling Diode An output freewheeling diode carries load current when the high-side switch is turned off. Therefore, use a Schottky diode to reduce loss due to diode forward voltage and recovery time. The diode should have at least 2A current rating and a reverse blocking voltage greater than the maximum input voltage. The diode should be close to the SW node to keep traces short and reduce ringing. Maximum Load Current at Low VIN The FAN8303 is able to operate with input supply voltage as low as 5V, although the maximum allowable output current is reduced as a function of duty cycle (see Figure 15). Additionally, at this low input voltage; if the duty cycle is greater than 50%, slope compensation reduces allowable output current. (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 8 www.fairchildsemi.com FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Soft-Start A capacitor, CSS, connected between the SS pin and GND helps control the rate of rise on the output voltage. When EN is HIGH and VIN is within the operating range, a trimmed bias current charges the capacitor connected to the SS pin, causing the voltage to rise. The time it takes this voltage to reach 0.6V and the PWM output to reach regulation is given by: tRISE ( ms ) 0.1 * CSS The system crossover frequency (fC), where the control loop has unity gain, is recommended for setting the 1/10th of switching frequency. Generally, higher fC means faster response to load transients, but can result in instability if not properly compensated. The first step of the compensation design is choosing the compensation resistor (RC) to set the crossover frequency by the following equation: RC = 2 COUT fC VOUT GCS GEA VFB (5) (10) where CSS is in nF. Loop Compensation The goal of the compensation design is to shape the converter frequency response to achieve high DC gain and fast transient, while maintaining loop stability. FAN8303 employs peak current-mode control for fast transient response and to help simplify the loop to a one-pole and one-zero system. The system pole is calculated by the equation: fP1 = 1 2 COUT RL where VFB is reference voltage and GCS is the current sense gain, which is roughly the output current divided by the voltage at COMP (2A/V). The next step is choosing the compensation capacitor (CC) to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, fZ2, to below one fourth of the crossover frequency provides sufficient phase margin. Determine the (CC) value by the following equation: CC = 2 (6) RC fC (11) where RL is the load resistor value (VOUT/IOUT). The system zero is due to the output capacitor and its ESR system zero is calculated by following equation: fz1 = 1 2 COUT ESR Determine if the second compensation capacitor (CA) is required. It is required if the ESR zero of the output capacitor is located at less than half of the switching frequency. 1 f (7) The characteristics of the control system are controlled by a series capacitor and resistor network connected to the COMP pin to set the pole and zero. The pole is calculated by the following equation: fp 2 = GEA 2 CC AVEA If required, add the second compensation capacitor (CA) to set the pole fP3 at the location of the ESR zero. Determine the (CA) value by the equation: CA = COUT ESR RC (13) SW (8) FAN8303 _ PWM modulator VO where: GEA is the error amplifier transconductance (380A/V); AVEA is the error amplifier voltage gain (400V/V); and CC is the compensation capacitor. Zero is due to the compensation capacitor (CC) and resistor (RC) calculated by the following equation: f z2 = 1 2 CC RC FB + 0.6V COMP RC CA CC (9) Figure 16. Block Diagram of Compensation where RC is compensation resistor. (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 www.fairchildsemi.com 9 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Design example Assume the VIN voltage is 12V with a 10% tolerance. The maximum load current is 2A and the output voltage is set to 2.5V at 2A maximum load. Calculate the inductor value from the following formula: VOUT L= fOSC I L,MAX 1 - VOUT VIN ,MAX Layout Consideration As with all switching power supplies, careful attention to PCB layout is important to the design. A few design rules should be implemented to ensure good layout: Keep the high-current traces and load connections as short as possible. Place the input capacitor, the inductor, the freewheeling diode, and the output capacitor as close as possible to the IC terminals. Keep the loop area between the SW node, freewheeling diode, inductor, and output capacitor as small as possible. Minimizing ground loops reduces EMI issues. Route high-dV/dt signals, such as SW node, away from the error amplifier input/output pins. Keep components connected to these pins close to the pins. To effectively remove heat from the MOSFETs, use wide land areas with appropriate thermal vias. (14) Substituting VOUT=2.5V, VIN,MAX=12V, IL,MAX=0.4A, and fS = 370kHz in the formula gives: 2.5 2.5 L= 1 - = 13 H 370kHz(0.4 A ) 12 (15) A 15H inductor is chosen for this application. If the VOUT voltage is 2.5V, choose R2=18k(1%), and R3 can be calculated from: 0.6 R3 = 18k = 5.68k 2.5 - 0.6 (16) Choose R3=5.6k (1%). In this application, with the desired crossover frequency at 30kHz, RC value is calculated as follows: RC = 2 22F 30kHz 2.5V 2 A / V 380 A / V 0.6V (17) If RC=22.72k , choose 22k for the design. If RC=22k , use the following equation to get CC: CC = 2 22k 30kHz (18) CC= 0.965nF, choose 1nF for the design. Table 1. Recommended Compensation Values (VIN=12V) VO 1.8V 2.5V 3.3V 5V L 10H 15H 15H 22H COUT 22F MLCC R2 R3 9k 5.6k 4k 2.45k RC 16k 22k 27k 43k CC 1.5nF 1nF 820pF 560pF 18k Figure 17. Recommended PCB Layout (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev. 1.0.0 www.fairchildsemi.com 10 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator Physical Dimensions 5.00 4.80 3.81 8 5 B A 0.65 6.20 5.80 4.00 3.80 1 4 1.75 5.60 PIN ONE INDICATOR (0.33) 1.27 0.25 M CBA 1.27 LAND PATTERN RECOMMENDATION 0.25 0.10 1.75 MAX C 0.10 0.51 0.33 0.50 x 45 0.25 C SEE DETAIL A 0.25 0.19 OPTION A - BEVEL EDGE R0.10 R0.10 GAGE PLANE 0.36 OPTION B - NO BEVEL EDGE NOTES: UNLESS OTHERWISE SPECIFIED A) THIS PACKAGE CONFORMS TO JEDEC MS-012, VARIATION AA, ISSUE C, B) ALL DIMENSIONS ARE IN MILLIMETERS. C) DIMENSIONS DO NOT INCLUDE MOLD FLASH OR BURRS. D) LANDPATTERN STANDARD: SOIC127P600X175-8M. E) DRAWING FILENAME: M08AREV13 8 0 0.90 0.406 SEATING PLANE (1.04) DETAIL A SCALE: 2:1 Figure 18. 8-Lead, Small Outline Integrated Circuit (SOIC-8) Dimensions Symbol A A1 b c D E e F H L Min. 1.346 0.101 Millimeter Typ. Max. 1.752 0.254 Min. 0.053 0.004 Inch Typ. Max. 0.069 0.010 0.406 0.203 4.648 3.810 1.270 0.381X45 5.791 0.406 0 6.197 1.270 8 0.228 0.016 0 4.978 3.987 0.183 0.150 0.016 0.008 0.196 0.157 0.050 0.015X45 0.244 0.050 8 Package drawings are provided as a service to customers considering Fairchild components. Drawings may change in any manner without notice. Please note the revision and/or date on the drawing and contact a Fairchild Semiconductor representative to verify or obtain the most recent revision. Package specifications do not expand the terms of Fairchild's worldwide terms and conditions, specifically the warranty therein, which covers Fairchild products. Always visit Fairchild Semiconductor's online packaging area for the most recent package drawings: http://www.fairchildsemi.com/packaging/. (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev.1.0.0 www.fairchildsemi.com 11 FAN8303 -- 2A 23V Non-Synchronous Step-Down DC/DC Regulator (c) 2008 Fairchild Semiconductor Corporation FAN8303 * Rev.1.0.0 www.fairchildsemi.com 12 |
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