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| WIDE INPUT 2A STEP DOWN CONVERTER FSP3126 FEATURES 2A Output Current Up to 95% Efficiency 4.75V to 20V Input Range 8A Shutdown Supply Current 410kHz Switching Frequency Adjustable Output Voltage Cycle-by-Cycle Current Limit Protection Thermal Shutdown Protection Frequency Foldback at Short Circuit Stability with Wide Range of Capacitors, Including Low ESR Ceramic Capacitors SOP8L Package GENERAL DESCRIPTION The FSP3126 is a current-mode step-down DC/DC converter that generates up to 2A of output current at 410kHz switching frequency. The device utilizes ISOBCD20 process for operation with input voltages up to 20V. Consuming only 8A in shutdown mode, the FSP3126 is highly efficient with peak operating efficiency at 95%. Protection features include cycle-by-cycle current limit, thermal shutdown, and frequency foldback at short circuit. The FSP3126 is available in a SOP8L package and requires very few external devices for operation. Typical Application TFT LCD Monitors Portable DVDs Car-Powered or Battery-Powered Equipments Set-Top Boxes Telecom Power Supplies DSL and Cable Modems and Routers Termination Supplies PIN ASSIGNMENT (Top View) BS 1 IN SW G 2 3 4 8 7 N/C EN COMP FB FSP3126 6 5 PIN DESCRIPTION Name BS IN SW G FB COMP EN N/C No. 1 2 3 4 5 6 7 8 Description Bootstrap. This pin acts as the positive rail for the high-side switch's gate driver. Connect a 10nF capacitor between BS and SW. Input Supply. Bypass this pin to G with a low ESR capacitor. See Input Capacitor in the Application Information section. Switch Output. Connect this pin to the switching end of the inductor. Ground. Feedback Input. The voltage at this pin is regulated to 1.293V. Connect to the resistor divider between output and ground to set output voltage. Compensation Pin. See Stability Compensation in the Application Information section. Enable Input. When higher than 1.3V, this pin turns the IC on. When lower than 0.7V, this pin turns the IC off. Output voltage is discharged when the IC is off. When left unconnected, EN is pulled up to 4.5V tip with a 2A pull up current. Not Connected. 1/9 2007-5-28 WIDE INPUT 2A STEP DOWN CONVERTER FSP3126 ABSOLUTE MAXIMUM RATINGS Parameter Value Unit IN Supply Voltage -0.3 to 25 V SW Voltage -1 to VIN + 1 V BS Voltage VSW - 0.3 to VSW + 8 V EN, FB, COMP Voltage -0.3 to 6 V Continuous SW Current Internally limited A Junction to Ambient Thermal Resistance (JA) 105 C/W Maximum Power Dissipation 0.76 W Operating Junction Temperature -40 to 150 C Storage Temperature -55 to 150 C Lead Temperature (Soldering, 10 sec) 300 C (Note: Exceeding these limits may damage the device. Exposure to absolute maximum rating conditions for long periods may affect device reliability.) ELECTRICAL CHARACTERISTICS (VIN = 12V, TA= 25C unless otherwise specified.) Parameter Input Voltage Feedback Voltage High-Side Switch On Resistance Low-Side Switch On Resistance SW Leakage Current Limit COMP to Current Limit Transconductance Error Amplifier Transconductance Error Amplifier DC Gain Switching Frequency Short Circuit Switching Frequency Maximum Duty Cycle Minimum Duty Cycle Enable Threshold Voltage Enable Pull Up Current Supply Current in Shutdown IC Supply Current in Operation Thermal Shutdown Temperature Symbol VIN VFB RONH RONL VEN = 0 ILIMT GCOMP GEA AVEA fSW VFB = 0 DMAX VFB = 1.1V VFB = 1.4V Hysteresis = 0.1V Pin pulled up to 4.5V typically when left unconnected VEN = 0 VEN = 3V, VFB = 1.4V Hysteresis = 10C ICOMP = 10A 350 2.4 Test Conditions VOUT = 5V, ILOAD = 0A to 1A 4.75V VIN 20V, VCOMP = 1.5V Min. 7 1.267 Typ. 1.293 0.20 4.7 0 2.85 1.8 550 4000 410 50 90 0.7 1 2 8 0.7 160 20 0 1.3 470 10 Max. 20 1.319 Unit V V A A A/V A/V V/V kHz kHz % % V A A mA C 2/9 2007-5-28 WIDE INPUT 2A STEP DOWN CONVERTER FSP3126 FUNCTIONAL BLOCK DIAGRAM IN ENABLE REGULATOR & REFERENCE EN BS CURRENT SENSE AMPLIFIER COMP ERROR AMPLIFIER + - 1.298V FB + + - PWM COMPARATOR - + - 0.2O HIGH-SIDE POWER SWITCH SW FOLDBACK CONTROL OSCILLATOR & RAMP LOGIC 4.7O LOW-SIDE POWER SWITCH THERMAL SHUTDOWN G FUNCTIONAL DESCRIPTION As seen in the above Figure, Functional Block Diagram, the FSP3126 is a current mode pulse width modulation (PWM) converter. The converter operates as follows: A switching cycle starts when the rising edge of the Oscillator clock output causes the High-Side Power Switch to turn on and the Low-Side Power Switch to turn off. With the SW side of the inductor now connected to IN, the inductor current ramps up to store energy in the magnetic field. The inductor current level is measured by the Current Sense Amplifier and added to the Oscillator ramp signal. If the resulting summation is higher than the COMP voltage, the output of the PWM Comparator goes high. When this happens or when Oscillator clock output goes low, the High-Side Power Switch turns off and the Low-Side Power Switch turns on. At this point, the SW side of the inductor swings to a diode voltage below ground, causing the inductor current to decrease and magnetic energy to be transferred to output. This state continues until the cycle starts again. The High-Side Power Switch is driven by logic using BS as the positive rail. This pin is charged to VSW + 6V when the Low-Side Power Switch turns on. The COMP voltage is the integration of the error between FB input and the internal 1.293V reference. If FB is lower than the reference voltage, COMP tends to go higher to increase current to the output. Current limit happens when COMP reaches its maximum clamp value of 2.55V. The Oscillator normally switches at 410kHz. However, if FB voltage is less than 0.7V, then the switching frequency decreases until it reaches a minimum of 50kHz at VFB = 0.5V. Shutdown control The FSP3126 has an enable input EN for turning the IC on or off. When EN is less than 0.7V, the IC is in 8A low current shutdown mode and output is discharged through the Low-Side Power Switch. When EN is higher than 1.3V, the IC is in normal operation mode. EN is internally pulled up with a 2A current source and can be left unconnected for always-on operation. Note that EN is a low voltage input with a maximum voltage of 6V; it should never be directly connected to IN. Thermal Shutdown The FSP3126 automatically turns off when its junction temperature exceeds 160C. 3/9 2007-5-28 WIDE INPUT 2A STEP DOWN CONVERTER FSP3126 APPLICATION INFORMATION Output Voltage Setting Figure1. Output Voltage Setting Figure 1 shows the connections for setting the output voltage. Select the proper ratio of the two feedback resistors RFB1 and RFB2 based on the output voltage. Typically, use RFB2 10k and determine RFB1 from the following equation: V R FB1 = R FB2 OUT - 1 1.293 V (1) Inductor Selection The inductor maintains a continuous current to the output load. This inductor current has a ripple that is dependent on the inductance value: higher inductance reduces the peak-to-peak ripple current. The trade off for high inductance value is the increase in inductor core size and series resistance, and the reduction in current handling capability. In general, select an inductance value L based on the ripple current requirement: VOUT * ( VIN - VOUT ) L= VIN f SW I OUTMAX K RIPPLE (2) where VIN is the input voltage, VOUT is the output voltage, fSW is the switching frequency, IOUTMAX is the maximum output current, and KRIPPLE is the ripple factor. Typically, choose KRIPPLE = 30% to correspond to the peak-to-peak ripple current being 30% of the maximum output current. With this inductor value, the peak inductor current is IOUT * (1 + KRIPPLE / 2). Make sure that this peak inductor current is less that the 3A current limit. Finally, select the inductor core size so that it does not saturate at 3A. Typical inductor values for various output voltages are shown in Table 1. VOUT 1.5V 1.8V 2.5V 3.3V 5V L 6.8H 6.8H 10H 15H 22H Table 1. Typical Inductor Values Input Capacitor The input capacitor needs to be carefully selected to maintain sufficiently low ripple at the supply input of the converter. A low ESR capacitor is highly recommended. Since large current flows in and out of this capacitor during switching, its ESR also affects efficiency. The input capacitance needs to be higher than 10F. The best choice is the ceramic type; however, low ESR tantalum or electrolytic types may also be used provided that the RMS ripple current rating is higher than 50% of the output current. The input capacitor should be placed close to the IN and G pins of the IC, with the shortest traces possible. In the case of tantalum or electrolytic types, they can be further away if a small parallel 0.1F ceramic capacitor is placed right next to the IC. Output Capacitor The output capacitor also needs to have low ESR to keep low output voltage ripple. The output ripple voltage is: VIN + 28 * f SW 2 LC OUT VRIPPLE = I OUTMAX K RIPPLE R ESR (3) where IOUTMAX is the maximum output current, KRIPPLE is the ripple factor, RESR is the ESR of the output capacitor, fSW is the switching frequency, L is the inductor value, and COUT is the output capacitance. In the case of ceramic output capacitors, RESR is very small and does not contribute to the ripple. Therefore, a lower capacitance value can be used for ceramic capacitors. In the case of tantalum or electrolytic capacitors, the ripple is dominated by RESR multiplied by the ripple current. In that case, the output capacitor is chosen to have sufficiently low ESR. 4/9 2007-5-28 WIDE INPUT 2A STEP DOWN CONVERTER For ceramic output capacitors, typically choose a capacitance of about 22F. For tantalum or electrolytic capacitors, choose a capacitor with less than 50m ESR. Rectifier Diode Use a Schottky diode as the rectifier to conduct current when the High-Side Power Switch is off. The Schottky diode must have a current rating higher than the maximum output current and a reverse voltage rating higher than the maximum input voltage. Stability Compensation FSP3126 Figure 2. Stability Compensation The feedback loop of the IC is stabilized by the components at the COMP pin, as shown in Figure 2. The DC loop gain of the system is determined by the following equation: 1. 3V AVDC = AVEA G COMP I OUT (4) The dominant pole P1 is due to CCOMP: G EA f P1 = 2 AVEA C COMP (5) The second pole P2 is the output pole: I OUT fP 2 = 2VOUT C OUT (6) The first zero Z1 is due to RCOMP and CCOMP: 1 fZ1 = 2R COMP C COMP (7) And finally, the third pole is due to RCOMP and CCOMP2 (if CCOMP2 is used): 1 fP 3 = 2R COMP C COMP 2 (8) The following steps should be used to compensate the IC: STEP1. Set the crossover frequency at 1/10 of the switching frequency via RCOMP: 2 VOUT C OUT f SW R COMP = 10 G EAG COMP * 1. 3V (9) but limit RCOMP to 15k maximum. STEP2. Set the zero fZ1 at 1/4 of the crossover frequency. If RCOMP is less than 15k, the equation for CCOMP is: C COMP = 1 . 8 x 10 -5 R COMP (F ) CCOMP2 is needed only for high ESR output capacitor (10) If RCOMP is limited to 15k, then the actual crossover frequency is 3.4 / (VOUTCOUT). Therefore: CCOMP = 1.2 x 10 -5 VOUT COUT (F ) (11) STEP3. If the output capacitor's ESR is high enough to cause a zero at lower than 4 times the crossover frequency, an additional compensation capacitor CCOMP2 is required. The condition for using CCOMP2 is: 1 . 1 x 10 -6 ,0. 012 * VOUT Min C OUT () R ESRCOUT 5/9 (12) 2007-5-28 WIDE INPUT 2A STEP DOWN CONVERTER FSP3126 And the proper value for CCOMP2 is: C COMP 2 = C OUT R ESRCOUT R COMP (13) Though CCOMP2 is unnecessary when the output capacitor has sufficiently low ESR, a small value CCOMP2 such as 100pF may improve stability against PCB layout parasitic effects. Table 2 shows some calculated results based on the compensation method above. VOUT 2.5V 3.3V 5V 2.5V 3.3V 5V 2.5V 3.3V 5V COUT 22F Ceramic 22F Ceramic 22F Ceramic 47F SP Cap 47F SP Cap 47F SP Cap 470F/6.3V/30m 470F/6.3V/30m 470F/10V/30m RCOMP 8.2k 12k 15k 15k 15k 15k 15k 15k 15k CCOMP 2.2nF 1.5nF 1.5nF 1.5nF 1.8nF 2.7nF 15nF 22nF 27nF CCOMP2 None None None None None None 1nF 1nF None Table 2. Typical Compensation for Different Output Voltages and Output Capacitors Figure 3 shows a sample FSP3126 application circuit generating 2.5V/2A output. Figure3. FSP3126 2.5V/2A Output Application 6/9 2007-5-28 WIDE INPUT 2A STEP DOWN CONVERTER FSP3126 TYPICAL CHARACTERISTICS 7/9 2007-5-28 WIDE INPUT 2A STEP DOWN CONVERTER FSP3126 ORDER INFORMATION FSP3126XXX Package: S: SOP8L Packing: Blank: Tube or Bulk A: Tape & Reel Temperature Grade: D: -40~85 MARKING INFORMATION Logo FSP3126 YYWWXX Part number Internal code Date code: YY: Year (01=2001) WW: Nth week (01~52) 8/9 2007-5-28 WIDE INPUT 2A STEP DOWN CONVERTER FSP3126 PACKAGE INFORMATION E H 7(4) D 7(4) 0.01545 A e A1 B A2 VIEW "A" Symbol A A1 A2 B C D E e H L Min. 1.35 0.10 1.35 0.33 0.19 4.80 3.80 Dimensions In Millimeters Nom. Max. 1.60 1.75 0.25 1.45 1.55 0.41 0.51 0.20 0.25 4.90 5.00 3.90 4.00 1.27TYP. 5.80 5.99 6.30 0.38 0.71 1.27 0 8 Dimensions In Inches Min. Nom. 0.053 0.063 0..004 0.053 0.057 0.013 0.016 0.0075 0.008 0.192 0.196 0.148 0.154 0.050TYP. 0.228 0.236 0.015 0.028 0 Max. 0.069 0.010 0.061 0.020 0.010 0.200 0.160 0.248 0.050 8 9/9 2007-5-28 |
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