EL7554 ® Data Sheet May 8, 2006 Monolithic 4 Amp DC-DC Step-Down Regulator Features • Integrated MOSFETs The EL7554 is a full-feature synchronous 4A step-down regulator capable of up to 96% efficiency. This device operates from 3V to 6V VIN input supply. With internal CMOS power FETs, the device can operate at up to 100% duty ratio, allowing for output voltage range from 0.8V up to nearly VIN.The adjustable high switching frequency of up to 1MHz enables the use of small components, making the whole converter occupy less than 0.58 square inch with components on one side of the PCB. The EL7554 operates at constant frequency PWM mode, making external synchronization possible. The EL7554 features soft-start and full start-up control, which eliminates the in-rush current and enables users to control the start-up of multiple converters to any configuration with ease. The EL7554 also offers a ±5% voltage margining capability that allows raising and lowering of the supplies derived from the EL7554 to validate the performance and reliability of system cards quickly and easily during manufacturing testing. A junction temperature indicator conveniently monitors the silicon die temperature, saving designers time in the tedious thermal characterization. An easy-to-use simulation tool is available for download and can be used to modify design parameters such as switching frequency, voltage ripple, ambient temperature, as well as view schematics waveforms, efficiency graphs, and complete BOM with Gerber layout. The EL7554 is available in a 28 Ld HTSSOP package and is specified for operation over the -40°C to +85°C temperature range. • 4A continuous output current • Up to 96% efficiency • All ceramic capacitors • Multiple supply start-up tracking • Built-in ±5% voltage margining • 3V to 6V input voltage • 0.58 in2 footprint with components on one side of PCB • Adjustable switching frequency to 1MHz • Oscillator synchronization possible • 100% duty ratio • Junction temperature indicator • Over-temperature protection • Internal soft-start • Variable output voltage down to 0.8V • Power-good indicator • 28 Ld HTSSOP package • Pb-free plus anneal available (RoHS compliant) Applications • Point-of-regulation power supplies • FPGA Core and I/O supplies • DSP, CPU Core, and IO supplies • Logic/Bus supplies Ordering Information PART NUMBER FN7360.4 PART TAPE & MARKING REEL PACKAGE PKG. DWG. # EL7554IRE 7554IRE - 28 Ld HTSSOP MDP0048 EL7554IRE-T7 7554IRE 7” 28 Ld HTSSOP MDP0048 EL7554IRE-T13 7554IRE 13” 28 Ld HTSSOP MDP0048 EL7554IREZ (See Note) 7554IREZ - 28 Ld HTSSOP MDP0048 (Pb-free) EL7554IREZ-T7 (See Note) 7554IREZ 7” 28 Ld HTSSOP MDP0048 (Pb-free) EL7554IREZ-T13 7554IREZ (See Note) 13” 28 Ld HTSSOP MDP0048 (Pb-free) • Portable equipment Related Documentation • Technical Brief 418 - Using the EL7554 Demo Board • Easy to use applications software simulation tool available at www.intersil.com/dc-dc NOTE: Intersil Pb-free plus anneal products employ special Pb-free material sets; molding compounds/die attach materials and 100% matte tin plate termination finish, which are RoHS compliant and compatible with both SnPb and Pb-free soldering operations. Intersil Pb-free products are MSL classified at Pb-free peak reflow temperatures that meet or exceed the Pb-free requirements of IPC/JEDEC J STD-020. 1 CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures. 1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc. Copyright Intersil Americas Inc. 2004-2006. All Rights Reserved All other trademarks mentioned are the property of their respective owners. EL7554 Typical Application Diagram CC R2 10.2K RC 1 COMP SGND 28 2 VREF COSC 27 220pF 0.018µF 2.32K 0.018µF R1 12.7K COSC 3 FB STN 26 4 VO STP 25 5 VTJ EN 24 6 TM PG 23 0.22µF 7 SEL 2.2µH VOUT (1.8V, 4A) 47µF 2 COUT VDD 22 8 LX VIN 21 9 LX VIN 20 10 LX VIN 19 11 LX PGND 18 12 LX PGND 17 13 LX PGND 16 14 NC NC 15 VIN (3V TO 6V) 2x10µF CIN FN7360.4 May 8, 2006 EL7554 Absolute Maximum Ratings (TA = 25°C) VIN, VDD to SGND. . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +6.5V VX to PGND. . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to VIN +0.3V SGND to PGND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . -0.3V to +0.3V COMP, VREF, FB, VO, VTJ, TM, SEL, PG, EN, STP, STN, COSC to SGND . . . . . -0.3V to VDD +0.3V Storage Temperature . . . . . . . . . . . . . . . . . . . . . . . .-65°C to +150°C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . +125°C Ambient Operating Temperature . . . . . . . . . . . . . . . .-40°C to +85°C CAUTION: Stresses above those listed in “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress only rating and operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. IMPORTANT NOTE: All parameters having Min/Max specifications are guaranteed. Typ values are for information purposes only. Unless otherwise noted, all tests are at the specified temperature and are pulsed tests, therefore: TJ = TC = TA DC Electrical Specifications PARAMETER VDD = VIN = 3.3V, TA = TJ = 25°C, COSC = 390pF, Unless Otherwise Specified DESCRIPTION CONDITIONS MIN VIN Input Voltage Range 3 VREF Reference Accuracy 1.24 VREFTC Reference Temperature Coefficient VREFLOAD Reference Load Regulation VRAMP Oscillator Ramp Amplitude IOSC_CHG Oscillator Charge Current IOSC_DIS TYP 1.26 MAX UNIT 6 V 1.28 V 50 0 < IREF < 50µA ppm/°C -1 % 1.15 V 0.1V < VOSC < 1.25V 200 µA Oscillator Discharge Current 0.1V < VOSC < 1.25V 8 mA IVDD VDD Supply Current VEN = 1 (L disconnected) IVDD_OFF VDD Standby Current EN = 0 VDD_OFF VDD for Shutdown VDD_ON VDD for Startup TOT Over-temperature Threshold 135 °C THYS Over-temperature Hysteresis 20 °C ILEAK Internal FET Leakage Current ILMAX Peak Current Limit RDSON1 PFET On Resistance 35 70 mΩ RDSONTC2 NFET On Resistance 30 60 mΩ RDSONTC RDSON Tempco 0.2 mΩ/°C ISTP STP Pin Input Pull-down Current VSTP = VIN/2 2.5 µA ISTN STN Pin Input Pull-up Current VSTN = VIN/2 VPGP Positive Power Good Threshold With respect to target output voltage VPGN Negative Power Good Threshold VPG_HI 2 2.7 5 mA 1 1.5 mA 2.4 2.65 V 2.6 2.95 V EN = 0, LX = 6V (low FET), LX = 0V (high FET) 10 6 -4 µA A 4 µA 6 14 % With respect to target output voltage -14 -6 % Power Good Drive High IPG = 1mA 2.6 VPG_LO Power Good Drive Low IPG = -1mA VOVP Output Over-voltage Protection VFB Output Initial Accuracy ILOAD = 0A VFB_LINE Output Line Regulation VIN = 3.3V, ΔVIN = 10%, ILOAD = 0A GMEA Error Amplifier Transconductance VCC = 0.65V VFB_TC Output Temperature Stability 0°C < TA < 85°C, ILOAD = 3A FS Switching Frequency IFB Feedback Input Pull-up Current 3 2.5 V 0.5 10 0.79 85 VFB = 0V % 0.8 0.81 V 0.2 0.5 % 125 165 µs ±1 300 V % 370 440 kHz 100 200 nA FN7360.4 May 8, 2006 EL7554 DC Electrical Specifications PARAMETER VDD = VIN = 3.3V, TA = TJ = 25°C, COSC = 390pF, Unless Otherwise Specified DESCRIPTION VEN_HI EN Input High Level VEN_LO EN Input Low Level IEN Enable Pull-up Current TM, SEL_HI Input High Level TM, SEL_LO Input Low Level CONDITIONS MIN TYP MAX UNIT 2.6 V 1 VEN = 0 -4 V -2.5 µA 2.6 V 1 V Pin Descriptions PIN NUMBER PIN NAME 1 COMP Error amplifier output; place loop compensation components here 2 VREF Bandgap reference bypass capacitor; typically 0.01µF to 0.047µF to SGND 3 FB Voltage feedback input; connected to external resistor divider between VOUT and SGND for adjustable output; also used for speed-up capacitor connection 4 VO Output sense for fixed output; also used for speed-up capacitor connection 5 VTJ Junction temperature monitor output, connected to a 0.01µF - 0.047µF to SGND 6 TM Stress test enable; allows ±5% output movement; needs a pull-down resistor (1K - 100K); connect to SGND if function is not used 7 SEL Positive or negative voltage margining set pin; needs a pull-down resistor (1K - 100K); connect to SGND if function is not used 8, 9, 10, 11, 12, 13 LX Inductor drive pin; high current output whose average voltage equals the regulator output voltage 14, 15 NC Not used 16, 17, 18 PGND 19, 20, 21 VIN Power supply input of the regulator; connected to the drain of the high-side PMOS Power FET 22 VDD Control circuit positive supply; connected to VIN through an internal 20Ω resistor 23 PG Power-good window comparator output; logic 1 when regulator output is within ±10% of target output voltage 24 EN Chip enable, active high; a 2µA internal pull-up current enables the device if the pin is left open; a capacitor can be added at this pin to delay the start of a converter 25 STP Auxilliary supply tracking positive input; tied to regulator output to synchronize start-up with a second supply; leave open for standalone operation; 2µA internal pull-up current 26 STN Auxiliary supply tracking negative input; connect to output of a second supply to synchronize start-up; leave open for standalone operation; 2µA internal pull-up current 27 COSC Oscillator timing capacitor (see performance curves) 28 SGND Control circuit negative supply or signal ground 4 PIN FUNCTION Ground return of the regulator; connected to the source of the low-side synchronous NMOS Power FET FN7360.4 May 8, 2006 EL7554 Block Diagram TM 0.018µF SEL COSC VREF VTJ VDD 2.2nF JUNCTION TEMPERATURE VOLTAGE REFERENCE 220pF OSCILLATOR VDD EN 20Ω 0.22µF VIN STP POWER TRACKING STN PWM CONTROLLER VIN 2x10µF POWER FET 2.2µH DRIVERS VOUT (UP TO 4A) POWER FET 47µF PGND EA CURRENT SENSE COMP VDD RC VREF CC SGND FB R2 + PG VO R1 5 FN7360.4 May 8, 2006 EL7554 Typical Performance Curves VIN = VD = 3.3V, VO = 1.8V, IO = 4A, L = 2.2µH, CIN = 2x10µF, COUT = 47µF, COSC = 220pF, TA = 25°C unless otherwise noted. 1 100 VO=2.5V 0.95 95 0.9 90 EFFICIENCY (%) EFFICIENCY (%) VO=3.3V 0.85 VO=0.8V 0.8 VO=1V 0.75 VO=1.2V VO=1.8V 0.7 VO=2.5V 85 VO=0.8V 80 VO=1V 75 VO=1.2V VO=1.8V 70 0.65 65 0.6 60 0 2 1 3 4 0 2 1 IO (mA) FIGURE 2. EFFICIENCY (VIN = 3.3V) 1.266 1.6 1.264 1.5 1.4 1.258 1.3 VTJ 1.26 1.256 VDD=3.3V 1.2 1.254 VDD=5V VDD=5V 1.1 1.25 1 1.248 1.246 -50 0 100 50 0.9 -50 150 0 4 1200 3.5 1000 VEN_HI 800 2.5 VEN_LOW 3.5 4 4.5 5 5.5 VDD (V) FIGURE 5. VEN_HI & VEN_LOW vs VDD 6 VDD=3.3V 200 1 3 VDD=5V 600 500 2 1.5 150 FIGURE 4. VTJ vs TEMPERATURE FIGURE 3. VREF vs TEMPERATURE 3 100 50 JUNCTION TEMPERATURE JUNCTION TEMPERATURE FS (kHz) VREF VDD=3.3V 1.252 4 IO (mA) FIGURE 1. EFFICIENCY (VIN = 5V) 1.262 3 6 0 100 200 300 400 500 600 700 COSC (pF) FIGURE 6. FS vs COSC FN7360.4 May 8, 2006 EL7554 Typical Performance Curves (Continued) VIN = VD = 3.3V, VO = 1.8V, IO = 4A, L = 2.2µH, CIN = 2x10µF, COUT = 47µF, COSC = 220pF, TA = 25°C unless otherwise noted. 0.8 610 0.6 605 VIN=5V (%) FS (KHz) 0.4 600 0.2 595 0.0 VIN=3.3V 590 -0.2 -0.4 585 0 0.5 1 1.5 2 2.5 3 3.5 1 0 4 2 4 IO (A) IO (A) FIGURE 7. FS vs IO FIGURE 8. LOAD REGULATIONS JEDEC JESD51-7 HIGH EFFECTIVE THERMAL CONDUCTIVITY TEST BOARD CONDITION: 2 3 4 5 6 7 8 8 P2 /W C 0° 1 =3 25 2.0 SO 30 A 35 2.5 θJ 40 3.0 TS 28 Ld HTSSOP THERMAL PAD SOLDERED TO 2-LAYER PCB WITH 0.039" THICKNESS AND 1 OZ. COPPER ON BOTH SIDES 45 3.5 H ALLOWABLE POWER DISSIPATION (W) 50 θJA (°C/W) 3 1.5 1.0 0.5 0 9 0 25 PCB AREA (in2) 50 75 85 100 125 150 AMBIENT TEMPERATURE (°C) FIGURE 10. PACKAGE POWER DISSIPATION vs AMBIENT TEMPERATURE JEDEC JESD51-3 LOW EFFECTIVE THERMAL CONDUCTIVITY TEST BOARD 1.00 0.90 0.80 H 8 P2 W / S O °C TS 10 =1 0.70 A θJ ALLOWABLE POWER DISSIPATION (W) FIGURE 9. HTSSOP THERMAL RESISTANCE vs PCB AREA (NO AIR FLOW) 0.60 0.50 0.40 0.30 0.20 0.10 0 0 25 50 75 85 100 125 150 AMBIENT TEMPERATURE (°C) FIGURE 11. PACKAGE POWER DISSIPATION vs AMBIENT TEMPERATURE 7 FN7360.4 May 8, 2006 EL7554 Waveforms VIN = VD = 3.3V, VO = 1.8V, IO = 4A, L = 2.2µH, CIN = 2x10µF, COUT = 47µF, COSC = 220pF, TA = 25°C unless otherwise noted. VIN (2V/DIV) ΔVIN (100mV/DIV) IIN (1A/DIV) VO (1V/DIV) VLX (2V/DIV) PG (2V/DIV) ΔVO (10mV/DIV) 1µs/DIV 0.5ms/DIV FIGURE 13. STEADY-STATE OPERATION FIGURE 12. START-UP 3A 1.0A VEN IO IIN (2A/DIV) ΔVO (100mV/DIV) VO (2V/DIV) 100µs/DIV 50µs/DIV FIGURE 14. SHUT-DOWN FIGURE 15. TRANSIENT RESPONSE PG TM VO (2V/DIV) SEL ΔVO (200mV/DIV) VLX (5V/DIV) 0.5ms/DIV 1ms/DIV FIGURE 16. VOLTAGE MARGINING 8 FIGURE 17. OVER-VOLTAGE SHUT-DOWN FN7360.4 May 8, 2006 EL7554 Waveforms (Continued) VIN = VD = 3.3V, VO = 1.8V, IO = 4A, L = 2.2µH, CIN = 2x10µF, COUT = 47µF, COSC = 220pF, TA = 25°C unless otherwise noted. VIN (2V/DIV) VIN (5V/DIV) IIN (2A/DIV) VO1=2.5V VO (1V/DIV) VO2=1.8V CIN = 100µF, COUT = 150µF CIN = 100µF, COUT = 150µF 5ms/DIV 2ms/DIV FIGURE 19. TRACKING START-UP FIGURE 18. ADJUSTABLE START-UP Detailed Description The EL7554 is a full-feature synchronous 6A step-down regulator capable of up to 96% efficiency. This device operates from 3V to 6V VIN input supply. With internal CMOS power FETs, the device can operate at up to 100% duty ratio, allowing for output voltage range from 0.8V up to nearly VIN.The adjustable high switching frequency of up to 1MHz enables the use of small components, making the whole converter occupy less than 0.58 square inch with components on one side of the PCB. The EL7554 operates at constant frequency PWM mode, making external synchronization possible. Patented on-chip resistorless current-sensing enables current mode control, which provides over-current protection, and excellent step load response. The EL7554 features soft-start and full start-up control, which eliminate the in-rush current and enables users to control the start-up of multiple converters to any configuration with ease. The EL7554 also offers a ±5% voltage margining capability that allows raising and lowering of the supplies derived from the EL7554 to validate the performance and reliability of system cards quickly and easily during manufacturing testing. A junction temperature indicator conveniently monitors the silicon die temperature, saving designers time in the tedious thermal characterization. Start-Up The EL7554 employs a special soft-start to suppress the inrush current (see Figure 12). The start-up process takes about 2ms and begins when the input voltage reaches about 2.8V and EN pin voltage 2V. When EN is released from LOW, or the converter comes out of thermal shut-down mode, the soft-start process repeats. When the input voltage ramps up too slowly, slight over- current at the input can occur. Connecting a small capacitor at EN will delay the start-up. The delay time TD can be calculated by: V EN_HI T D = C EN × -------------------I EN where: • CEN is the capacitance at EN pin • VEN_HI is the EN input high level (function of VDD voltage, see Figure 5) • IEN is the EN pin pull-up current, nominal 2.5µA If a slower than 2ms soft start-up is needed, please refer to Full Start-Up Control section. Steady-State Operation The converter always operates at fixed frequency continuous-conduction mode. For fast transient response, peak current control method is employed. The inductor current is sensed from the upper PFET. This current signal, the slope compensation, and the compensated error signal are fed to the PWM comparator to generate the PWM signal for the internal power switches. When the upper PFET is on, the low-side NFET is off and input voltage charges the inductor. When PFET is off, the NFET is on and energy stored in the inductor is dumped to the output to maintain constant output voltage. Therefore, the LX waveform is always a stable square waveform (see Figure 13) with peak close to VIN. So LX is a good indication that the converter is operating properly. 100% Duty Ratio EL7554 uses CMOS as internal synchronous power switches. The upper switch is a PMOS and the lower switch an NMOS. This not only saves a boot capacitor, it also allows 100% turn-on of the upper PFET switch, achieving 9 FN7360.4 May 8, 2006 EL7554 VO close to VIN. The maximum achievable VO is: 100pF V O = V IN – ( R L + R DSON1 ) × I O EL7554 COSC Where RL is the DC resistance on the inductor and RDSON1 is the PFET on-resistance, nominal 35mΩ at room temperature with tempco of 0.2mΩ/°C. Output Voltage Selection The output voltage can be as high as the input voltage minus the PMOS and inductor voltage drops. Use R1 and R2 to set the output voltage according to the following formula: R ⎞ ⎛ V O = 0.8 × ⎜ 1 + ------1-⎟ R ⎝ 2⎠ Standard values of R1 and R2 are listed in Table 1. TABLE 1. VO (V) R1 (kΩ) R2 (kΩ) 0.8 2 Open 1 2.49 10 1.2 4.99 10 1.5 10 11.5 1.8 12.7 10.2 2.5 21.5 10 3.3 36 11.5 FIGURE 20. EXTERNAL SYNC CIRCUIT Thermal Protection and Junction Temperature Indicator An internal temperature sensor continuously monitors the junction temperature. In the event that the junction temperature exceeds 135°C, the regulator is in a fault condition and will shut down. When the temperature falls back below 110°C, the regulator goes through the soft-start procedure again. The VTJ pin is an accurate indicator of the internal silicon junction temperature TJ, which can be determined by the following formula. This saves engineering time. 1.2 – V TJ T J = 75 + -----------------------0.00384 where VTJ is the voltage at VTJ pin. Under-Voltage Lockout (UVLO) When VDD falls bellow 2.5V, the regulator shuts down. When VDD rises above 2.8V, converter goes through soft-start process again. Power Good Indicator (PG) and Over-Voltage Protection Voltage Margining The EL7554 has built-in 5% load stress test (commonly called voltage margining) function. Combinations of TM and SEL set the margins shown in Table 2. When this function is not used, both pins should be connected to SGND, either directly or through a 10kΩ resister. Figure 16 shows this feature. TABLE 2. CONDITION EXTERNAL SYNC SOURCE TM SEL VO Normal 0 X Nominal High Margin 1 1 Nominal + 5% Low Margin 1 0 Nominal - 5% Switching Frequency The regulator operates from 200kHz to 1MHz. The switching frequency is generated by a relaxation comparator and adjusted by a COSC. The triangle waveform has 95% duty ratio and runs from 0.2V to 1.2V. Please refer to Figure 6 for a specific frequency. When external synchronization is required, use the following circuit for connection. Always choose the converter selfswitching frequency 20% lower than the sync frequency to accommodate component variations. 10 When the output reaches 10% of the preset voltage, the PG pin outputs a HI signal as shown in the start-up waveform (Figure 12). If the output voltage is higher than 10% of the preset value for any reason, PG will go low and the regulator will shut down. In addition to the indication power is good, the PG pin can be used for multiple regulators’ start-up control as described in the next section. Full Start-Up Control The EL7554 offers full start-up control. The core of this control is a start-up comparator in front of the main PWM controller. The STP and STN are the inputs to the comparator, whose HI output forces the PWM comparator to skip switching cycles. The user can choose any of the following control configurations: 1. ADJUSTABLE SOFT-START In this configuration, the ramp-up time is adjustable to any time longer than the building soft-start time of 2ms. The approximate ramp-up time, TST, is: ⎛ VO ⎞ T ST = RC ⎜ ---------⎟ ⎝ V IN⎠ Figure 18 shows the waveforms. FN7360.4 May 8, 2006 EL7554 goes HI, where VREF is the regulator reference voltage. VREF=1.26. + VO STN C STP R 200K EL7554 0.1µF VO VREF VIN TST RB + VO2 RA VO1 EL7554 FIGURE 21. ADJUSTABLE START-UP VIN EL7554 In this application, CIN and COUT may be increased to reduce input/output ripple because the pulse skipping nature of the method. VIN VREF(1+RB/RA) VO1 2. CASCADE START-UP VO2 In this configuration, EN pin of Regulator 2 is connected to the PG pin of Regulator 1 (Figure 22). VO2 will only start after VO1 is good. FIGURE 24. OFFSET START-UP TRACKING Component Selection INPUT CAPACITOR EN VO2 PG VO1 VIN EL7554 EL7554 The main functions of the input capacitor(s) are to maintain the input voltage steady and to filter out the pulse current passing through the upper switch. The root-mean-square value of this current is: V O × ( V IN – V O ) I IN,RMS = ----------------------------------------------- × I O ≈ 1/2 ( I O ) V IN VO1 VO2 for a wide range of VIN and VO. FIGURE 22. CASCADE START-UP 3. LINEAR START-UP In the linear start-up tracking configuration, the regulator with lower output voltage, VO2, tracks the one with higher output voltage, VO1. The waveform is shown in Figure 19. C + VO2 + STN STP R VO1 EL7554 VIN EL7554 VIN For long-term reliability, the input capacitor or combination of capacitors must have the current rating higher than IIN,RMS. Use X5R or X7R type ceramic capacitors, or SPCAP or POSCAP types of Polymer capacitors for their high current handling capability. INDUCTOR The NFET positive current limit is set at about 5A. For optimal operation, the peak-to-peak inductor current ripple ΔIL should be less than 1A. The following equation gives the inductance value: ( V IN – V O ) × V O L = ------------------------------------------V IN × ΔI L × F S The peak current the inductor sees is: VO1 VO2 FIGURE 23. LINEAR START-UP TRACKING ΔI I LPK = I O + --------L 2 When inductor is chosen, make sure the inductor can handle this peak current and the average current of IO. 4. OFFSET START-UP OUTPUT CAPACITOR Compared with the cascade start-up, this configuration allows Regulator 2 to begin the start-up process when VO1 reaches a particular value of VREF*(1+RB/RA) before PG If there is no holding time requirement for output; output voltage ripple and transient response are the main deciding factors in choosing the output capacitor. Initially, choose the 11 FN7360.4 May 8, 2006 EL7554 output capacitor with the ESR to satisfy the output ripple ΔVO requirement: ΔV O = ΔI L × ESR Design Example A 5V to 1.8V converter at 4A is needed. 1. Choose the input capacitor When output has a step load change ΔIO, the initial voltage drop is ESR*ΔIO. Then VO will drop even further before the loop has the chance to respond. The higher the output capacitance, the lower the voltage drop is. Also, higher loop bandwidth will generate less voltage drop. Experiment with the transient response (see Figure 15) to determine the final values of output capacitance. Like the input capacitor, it is recommended to use X5R or X7R type of ceramic capacitors, or SPCAP or POSCAP type of Polymer capacitors for the low ESR and high capacitance. Generally, the AC current rating of the output capacitor is not a concern because the RMS current is only 1/√12 of ΔIL. This is easily satisfied. LOOP COMPENSATION Current mode converter forces the inductor current proportional to the error signal, thus gets rid of the 2nd order effect formed by the inductor and output capacitor. The PWM comparator and the inductor form an equivalent transconductance amplifier. So, a simple Type 1 compensator is good enough to generate a high bandwidth stable converter. The compensation capacitor and resister are decided by: V FB × GM PWM × GM EA C C = ---------------------------------------------------------------π × F C × I OUT The input capacitor or combination of capacitors has to be able to take about 1/2 of the output current, e.g., 2A. TDK’s C3216X5RIA106M is rated at 2.7A, 6.3V, meeting the above criteria using 2 generators less input voltage ripple. 2. Choose the inductor. Set the converter switching frequency at 600kHz: ( V IN – V O ) × V O L = ------------------------------------------V IN × ΔI L × F S ΔIL = 1A yields 1.72µH. Leave some margin and choose L = 2.2µH. TDK RLF7030-2R2M5R4 has the required current rating. 3. Choose the output capacitor L = 2.2µH yields about 0.9A inductor ripple current. 47µF ceramic capacitor has less than 5mΩ of ESR easily satisfying by the requirement. ESR is not the only factor deciding the output capacitance. As discussed earlier, output voltage droops less with more capacitance when converter is in load transient. Multiple iterations may be needed before final components are chosen. 4. Loop compensation 50kHz is the intended crossover frequency. With the conditions RC and CC are calculated as: RC = 2.32kΩ and CC = 0.018pF For convenience, Table 3 lists the compensation values for frequently used output voltages. C OUT R C = 2 × R OUT × ---------------CC TABLE 3. COMPENSATION VALUES where: • GMPWM is the transconductance of the PWM comparator, GMPWM = 120s V OUT R OUT = --------------I OUT VO (V) RC (kΩ) CC (µF) 3.3 4.22 0.018 2.5 3.24 0.018 1.8 2.32 0.018 • VOUT output voltage 1.5 1.91 0.018 • IOUT output current 1.2 1.54 0.018 1 1.27 0.018 0.8 1.02 0.018 • COUT is output capacitance • GMEA is the transconductance of the error amplifier, GMEA = 120µs • FC is the intended crossover frequency of the loop. For best performance, set this value to about one-tenth of the switching frequency. 12 FN7360.4 May 8, 2006 EL7554 Thermal Management Layout Considerations The EL7554IRE is packaged in a thermally-efficient HTSSOP-28 package, which utilizes the exposed thermal pad at the bottom to spread heat through PCB metal. The layout is very important for the converter to function properly. Follow these tips for best performance: Therefore: 1. The thermal pad must be soldered to the PCB 2. Maximize the PCB area 3. If a multiple layer PCB is used, thermal vias (13 to 25 mil) must be placed underneath the thermal pad to connect to ground plane(s). Do not place thermal reliefs on the vias. Figure 25 shows a typical connection. The thermal resistance for this package is as low as 26°C/W for 2 layer PCB of 0.39" thickness (see Figure 9). The actual junction temperature can be measured at VTJ pin. The thermal performance of the IC is heavily dependent on the layout of the PCB. The user should exercise care during the design phase to ensure the IC will operate within the recommended environmental conditions. COMPONENT SIDE CONNECTION 1. Separate the Power Ground ( ) and Signal Ground ( ); connect them only at one point right at the SGND pin 2. Place the input capacitor(s) as close to VIN and PGND pins as possible 3. Make as small as possible the loop from LX pins to L to CO to PGND pins 4. Place R1 and R2 pins as close to the FB pin as possible 5. Maximize the copper area around the PGND pins; do not place thermal relief around them 6. Thermal pad should be soldered to PCB. Place several via holes under the chip to the ground plane to help heat dissipation The demo board is a good example of layout based on this outline. Please refer to the EL7554 Application Brief. GROUND PLANE CONNECTION FIGURE 25. PCB LAYOUT - 28 Ld HTSSOP PACKAGE 13 FN7360.4 May 8, 2006 EL7554 Package Outline Drawing NOTE: The package drawing shown here may not be the latest version. To check the latest revision, please refer to the Intersil website at http://www.intersil.com/design/packages/index.asp All Intersil U.S. products are manufactured, assembled and tested utilizing ISO9000 quality systems. Intersil Corporation’s quality certifications can be viewed at www.intersil.com/design/quality Intersil products are sold by description only. Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to verify that data sheets are current before placing orders. Information furnished by Intersil is believed to be accurate and reliable. However, no responsibility is assumed by Intersil or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Intersil or its subsidiaries. For information regarding Intersil Corporation and its products, see www.intersil.com 14 FN7360.4 May 8, 2006