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| WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 FEATURES 3A Output Current Up to 95% Efficiency 4.5V to 30V Input Range 6A Shutdown Supply Current 400kHz Switching Frequency Adjustable Output Voltage Cycle-by-Cycle Current Limit Protection Thermal Shutdown Protection Internal Soft-Start Function Frequency Fold Back at Short Circuit Stability with Wide Range of Capacitors, Including Low ESR Ceramic Capacitors SOP8L/EP(Exposed Pad) Package GENERAL DESCRIPTION The FSP3128 is a current-mode step-down DC/DC converter that generates up to 3A output current at 400kHz switching frequency. The device can operate with input voltage up to 30V. Consuming only 6A in shutdown mode, the FSP3128 is highly efficient with peak efficiency at 95% when in operation. Protection features include cycle-by-cycle current limit, thermal shutdown, and frequency fold back at short circuit. The device also includes an internal soft start function to prevent overshoot. The FSP3128 is available in SOP8L/EP package and requires very few external devices for operation. TYPICAL APPLICATION TFT LCD Monitors or Televisions and HDTV Portable DVD Players Car-Powered or Battery-Powered Equipment Set-Top Boxes Telecom Power Supplies DSL and Cable Modems and Routers PIN ASSIGNMENT (Top View) BS 1 IN 2 8 7 6 EP 5 N/C EN COMP FB SW 3 G 4 PIN DESCRIPTION Pin Number Pin Name 1 2 3 4 5 6 7 8 EP BS IN SW G FB COMP EN N/C EP Pin Description Bootstrap. This pin acts as the positive rail for the high-side switch's gate driver. Connect a 10nF between this pin and SW. Input Supply. Bypass this pin to G with a low ESR capacitor. See Input Capacitor in 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 1.222V. Connect to the resistor divider between the output and ground to set output voltage. Compensation Pin. See Compensation Technique in 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. This pin has a small internal pull up current to a high level voltage when pin is not connected. Not Connected. Exposed Pad shown as dashed box. The exposed thermal pad should be connected to board ground plane and pin 4. The ground plane should include a large exposed copper pad under the package for thermal dissipation. The leads and exposed pad should be flush with the board, without offset from the board surface. 2007-1-4 1/10 WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 ABSOLUTE MAXIMUM RATINGS(NOTE) Parameter IN to G EN to G SW to G BS to SW FB, COMP to G Continuous SW Current Junction to Ambient Thermal Resistance (JA) Maximum Power Dissipation Operating Junction Temperature Storage Temperature Lead Temperature (Soldering, 10 sec) Value -0.3 to +34 -0.3 to VIN + 0.3 -1 to VIN + 1 -0.3 to +8 -0.3 to +6 Internally limited 46 1.8 -40 to 150 -55 to 150 300 Unit V V V V V A C/W W C C C Note : Do not exceed these limits to prevent damage to 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 Symbol 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 DMAX DMIN VIN VFB RONH RONL VEN = 0V, VIN = 12V, VSW = 0 ILIM GCOMP GEA AVEA fSW VFB = 0 VFB = 1.1V, PWM mode VFB = 1.4V, PFM mode Hysteresis = 0.1V Pin pulled up to VIN when left unconnected VEN = 0 VEN = 3V, not switching Hysteresis = 10C 0.7 340 ILOAD/ICOMP ICOMP = 10A 3.5 Test Conditions Min. 4.5 1.198 1.222 100 10 0 5 3 550 4000 400 40 90 0 1 2 6 0.85 160 20 2 1.3 460 Typ. Max. 30 1.246 Unit V V m A A A/V A/V V/V KHz KHz % % V A A mA C VOUT = 2.5V, ILOAD = 0A to 3A 10 2/10 2007-1-4 WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 FUNCTIONAL BLOCK DIAGRAM FUNCTIONAL DESCROPTION As seen in the Functional Block Diagram, the FSP3128 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 its 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 the output. This state continues until the cycle starts again. The High-Side Power Switch is driven by logic using the BS bootstrap pin 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 the FB input and the internal 1.222V 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 2.65V. The Oscillator normally switches at 400kHz. However, if the FB voltage is less than 0.7V, then the switching frequency decreases until it reaches a typical value of 40kHz at VFB = 0V. Shutdown Control The FSP3128 has an enable input EN for turning the IC on or off. When EN is less than 0.7V, the IC is in 6A low current shutdown mode and the 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. Thermal Shutdown The FSP3128 automatically turns off when its junction temperature exceeds 160C and then restarts once the temperature falls to 150C. 3/10 2007-1-4 WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 TYPICAL APPLICATION CIRCUIT APPLICATION INFORMATION 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 RFB210K and determine RFB1 from the output voltage: Figure 1 Output Voltage Setting 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 ripple current requirement. 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 = between 20% and 30% to correspond to the peak-to-peak ripple current being a percentage of the maximum output current. With this inductor value (Table 1), the peak inductor current is IOUT * (1 + KRIPPLE / 2). Make sure that this peak inductor current is less that the 5A current limit. Finally, select the inductor core size so that it does not saturate at 5A. VO U T L 1.5V 6.8 H 1.8V 2.5V 3.3V 5V 15 H 6.8 H 6.8 H 8.5 H Table 1: Typical Inductor Values 4/10 2007-1-4 WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 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 a 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 shortest possible traces. 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: where IOUTMAX is the maximum output current, KRIPPLE is the ripple factor, RESR is the ESR resistance of the output capacitor, fSW is the switching frequency, L is the inductor value, 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 type, typically choose a capacitance of about 22F. In the case of tantalum or electrolytic type, the ripple is dominated by RESR multiplied by the ripple current. In that case, the output capacitor is chosen to have sufficiently low due to ESR, typically 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 current rating higher than the maximum output current and the reverse voltage rating higher than the maximum input voltage. Stability Compensation CCOMP2 is needed only for high ESR output capacitor Figure 2: Stability compensation The feedback system of the IC is stabilized by the components at COMP pin, as shown in Figure 2. The DC loop gain of the system is determined by the following equation: The dominant pole P1 is due to CCOMP The second pole P2 is the output pole: The first zero Z1 is due RCOMP and CCOMP 5/10 2007-1-4 WIDE INPUT 3A STEP-DOWN CONVERTER And finally, the third pole is due to RCOMP and CCOMP2 (if CCOMP2 is used): FSP3128 Follow the following steps to compensate the IC: STEP1. Set the cross over frequency at 1/10 of the switching frequency via RCOMP: but limit RCOMP to 15k maximum. STEP2. Set the zero fZ1 at 1/4 of the cross over frequency. If RCOMP is less than 15k, the equation for CCOMP is: If RCOMP is limited to 15k, then the actual cross over frequency is 4.8/(VOUTCOUT). Therefore: STEP3. If the output capacitor's ESR is high enough to cause a zero at lower than 4 times the cross over frequency, an additional compensation capacitor C COMP2 is required. The condition for using CCOMP2 is: And the proper value for CCOMP2 is: Though CCOMP2 is unnecessary when the output capacitor has sufficiently low ESR, a small value CCOMP2 such as 220pF may improve stability against PCB layout parasitic effects. Table 2 shows some calculated results based on the compensation method above. CCOMP2 220pF 220pF 220pF 220pF 220pF 220pF VOUT 1.8V 2.5V 5V 1.8V 2.5V 5V COUT 22F Ceramic 22F Ceramic 22F Ceramic 100F SP CAP 100F SP CAP 100F SP CAP RCOMP 4k 5.6k 12k 15k 15k 15k CCOMP 3.3nF 3.3nF 1.5nF 1.5nF 2.2nF 4.7nF Table 2: Typical Compensation for Different Output voltages and Output Capacitors 6/10 2007-1-4 WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 Figure 3 shows a sample FSP3128 application circuit generating a 2.5V/3A output. Figure 3: FSP3128 2.5V/3A Output Application TYPICAL CHARACTERISTICS (Circuit of Figure 3, Unless other specified) 7/10 2007-1-4 WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 TYPICAL CHARACTERISTICS (CONTINUED) (Circuit of Figure 3, Unless other specified) 8/10 2007-1-4 WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 ORDER INFORMATION FSP3128XXXX Output Voltage: Package: S: SOP8L/EP Blank: ADJ Temperature Grade: Packing: Blank: Tube or Bulk D: -40~85 A: Tape & Reel MARKING INFORMATION Logo FSP3128 YYWWXX Part number: Internal code Date code: YY: Year (01=2001) WW: Nth week (01~52) 9/10 2007-1-4 WIDE INPUT 3A STEP-DOWN CONVERTER FSP3128 PACKAGE INFORMATION E1 7(4) D 7(4) 0.01545 A2 A e Symbol A A1 A2 B C D D1 E E1 e H L Min. 1.35 0.05 1.35 0.33 0.19 4.70 3.202 3.80 2.313 Dimensions In Millimeters Nom. Max. 1.60 1.75 0.15 1.45 1.55 0.41 0.51 0.20 0.25 4.90 5.10 3.402 3.90 4.00 2.513 1.27TYP. 5.80 5.99 6.30 0.38 0.71 1.27 0 8 A1 B VIEW "A" Dimensions In Inches Min. Nom. 0.053 0.063 0..002 0.053 0.057 0.013 0.016 0.0075 0.008 0.185 0.196 0.126 0.148 0.154 0.091 0.050TYP. 0.228 0.236 0.015 0.028 0 D1 E H Max. 0.069 0.006 0.061 0.020 0.010 0.200 0.134 0.160 0.099 0.248 0.050 8 10/10 2007-1-4 |
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