FAN5354_11

FAN5354 — 3MHz, 3A Synchronous Buck Regulator March 2011 FAN5354 3MHz, 3A Synchronous Buck Regulator Features 3MHz Fixed-Frequency Operation Best-in-Class Load Transient 3A Output Current Capability 2.7V to 5.5V Input Voltage Range Adjustable Output Voltage: 0.8 to VIN•0.9 PFM Mode for High Efficiency in Light Load (Forced PWM Available on MODE Pin) Minimum PFM Frequency Avoids Audible Noise 270 A Typical Quiescent Current in PFM Mode External Frequency Synchronization Low Ripple Light-Load PFM Mode with Forced PWM Control Power Good Output Internal Soft-Start Input Under-Voltage Lockout (UVLO) Thermal Shutdown and Overload Protection 12-Lead 3x3.5mm MLP Description The FAN5354 is a step-down switching voltage regulator that delivers an adjustable output from an input voltage supply of 2.7V to 5.5V. Using a proprietary architecture with synchronous rectification, the FAN5354 is capable of delivering 3A at over 85% efficiency, while maintaining a very high efficiency of over 80% at load currents as low as 2mA. The regulator operates at a nominal fixed frequency of 3MHz, which reduces the value of the external components to 470nH for the output inductor and 10 F for the output capacitor. Additional output capacitance can be added to improve regulation during load transients without affecting stability and inductance up to 1.2 H may be used with additional output capacitance. At moderate and light loads, pulse frequency modulation (PFM) is used to operate the device in power-save mode with a typical quiescent current of 270 A. Even with such a low quiescent current, the part exhibits excellent transient response during large load swings. At higher loads, the system automatically switches to fixed-frequency control, operating at 3MHz. In shutdown mode, the supply current drops below 1 A, reducing power consumption. PFM mode can be disabled if constant frequency is desired. To avoid audible noise, the regulator limits its minimum PFM frequency. The FAN5354 is available in 12-lead 3x3.5mm MLP package. Applications Set-Top Box Hard Disk Drive Communications Cards DSP Power Figure 1. Typical Application Ordering Information Part Number FAN5354MPX Temperature Range -40 to 85°C Package MLP-12, 3x3.5mm Packing Method Tape and Reel © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com FAN5354 — 3MHz, 3A Synchronous Buck Regulator Table 1. Recommended External Components for 3A Maximum Load Current Component Description Vendor IHLP1616ABER47M01 (Vishay) SD12-R47-R (Coiltronics) VLC5020T-R47N (TDK) (TDK) LQH55PNR47NT0 (Murata) GRM21BR60J106M (Murata) C2012X5R0J106M (TDK) GRM155R71E103K (Murata) C1005X7R1E103K (TDK) GRM188R60J475K (Murata) C1608X5R0J475K (TDK) Any Parameter L DCR Typ. 0.47 20 Units µH mΩ L1 470nH Nominal COUT CIN CIN1 CVCC R3 (1) 2 Pieces 10µF, 6.3V, X5R, 0805 10µF, 6.3V, X5R, 0805 10nF, 25V, X7R, 0402 4.7µF, 6.3V, X5R, 0603 Resistor: 1Ω 0402 C 10.0 µF C C R 10 4.7 1 nF µF Ω Note: 1. R3 is optional and improves IC power supply noise rejection. See Layout recommendations for more information. Pin Configuration FB 1 VOUT 2 PGND 3 PGND 4 SW 5 SW 6 P1 (GND) 12 MODE 11 PGOOD 10 EN 9 VCC 8 PVIN 7 PVIN Figure 2. 12-Pin, 3x3.5mm MLP (Top View) Pin Definitions Pin # 1 2 3, 4 5, 6 P1 7, 8 9 10 11 12 Name FB VOUT PGND SW GND PVIN VCC EN PGOOD MODE Description FB. Connect to resistor divider. The IC regulates this pin to 0.8V. VOUT. Sense pin for VOUT. Connect to COUT. Power Ground. Low-side MOSFET is referenced to this pin. CIN and COUT should be returned with a minimal path to these pins. Switching Node. Connect to inductor. Ground. All signals are referenced to this pin. Power Input Voltage. Connect to input power source. Connect to CIN with minimal path. IC Bias Supply. Connect to input power source. Use a separate bypass capacitor CVCC from this pin to the P1 GND terminal between pins 1 and 12. Enable. The device is in shutdown mode when this pin is LOW. Do not leave this pin floating. Power Good. This open-drain pin pulls LOW if the output falls out of regulation or is in soft-start. MODE / Sync. A logic 0 allows the IC to automatically switch to PFM during light loads. When held HIGH, the IC to stays in PWM mode. The regulator also synchronizes its switching frequency to the frequency provided on this pin. Do not leave this pin floating. Note: 2. P1 is the bottom heat-sink pad. Ground plane should flow through pins 3, 4, and P1 and can be extended through pin 11 if PGOOD’s function is not required, and through pin 12 if MODE is to be grounded, to improve IC cooling. © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 2 FAN5354 — 3MHz, 3A Synchronous Buck 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. Symbol Parameter VIN SW, PVIN, VCC Pins Other Pins VINOV_SLEW Maximum Slew Rate of VIN Above 6.5V when PWM is Switching RPGOOD ESD TJ TSTG TL Pull-Up Resistance from PGOOD to VCC Electrostatic Discharge Protection Level Junction Temperature Storage Temperature Lead Soldering Temperature, 10 Seconds Human Body Model per JESD22-A114 Charged Device Model per JESD22-C101 IC Not Switching IC Switching Min. -0.3 -0.3 -0.3 1 2 2 –40 –65 Max. 7.0 6.5 VCC + 0.3 15 (3) Units V V V/ms KΩ KV +150 +150 +260 °C °C °C Note: 3. Lesser of 7V or VCC+0.3V. 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 VCC, VIN VOUT IOUT L CIN COUT TA TJ Parameter Supply Voltage Range Output Voltage Range Output Current Inductor Input Capacitor Output Capacitor Operating Ambient Temperature Operating Junction Temperature Min. 2.7 0.8 0 Typ. Max. 5.5 90% Duty Cycle 3 Units V V A H F F 0.47 10 20 -40 -40 +85 +125 °C °C Thermal Properties Symbol θJA Parameter Junction-to-Ambient Thermal Resistance (4) Min. Typ. 46 Max. Units °C/W Note: 4. Junction-to-ambient thermal resistance is a function of application and board layout. This data is measured with four-layer 1s2p boards in accordance to JESD51- JEDEC standard. Special attention must be paid not to exceed junction temperature TJ(max) at a given ambient temperate TA. © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 3 FAN5354 — 3MHz, 3A Synchronous Buck Regulator Electrical Characteristics Minimum and maximum values are at VIN=2.7V to 5.5V, TA=-40°C to +85°C, unless otherwise noted. Typical values are at TA=25°C, VIN=5V. Symbol Parameter Power Supplies IQ I SD VUVLO Quiescent Current Shutdown Supply Current Under-Voltage Lockout Threshold Conditions ILOAD=0, MODE=0 ILOAD=0, MODE=1 (Forced PWM) EN=GND VIN Rising VIN Falling Min. Typ. Max. Units 270 14 0.1 2.83 2.10 2.30 530 1.05 0.4 100 3.0 2.95 2.40 µA mA µA V V mV V V mV 1.00 1 0.01 1.00 µA mA µA V V % %/A %/V mV m m 5.50 A °C °C V V 3.3 3.3 36 250 420 MHz MHz kHz µs µs V/ms VUVHYST Under-Voltage Lockout Hysteresis Logic Pins VIH VIL VLHYST IIN IOUTL IOUTH HIGH-Level Input Voltage LOW-Level Input Voltage Logic Input Hysteresis Voltage Input Bias Current PGOOD Pull-Down Current PGOOD HIGH Leakage Current Input Tied to GND or VIN VPGOOD=0.4V VPGOOD=VIN TA=25°C At VOUT Pin W.R.T. Calculated Value, ILOAD=500mA IOUT(DC)=1 to 3A 2.7V ≤ VIN ≤ 5.5V, IOUT(DC)=1.5A ILOAD Step 0.1A to 1.5A, tr=tf=100ns, VOUT=1.2V 0.01 VOUT Regulation VREF VREG ∆VOUT ∆ILOAD ∆VOUT ∆VIN Output Reference DC Accuracy Measured at FB Pin VOUT DC Accuracy Load Regulation Line Regulation Transient Response 0.792 0.800 0.808 0.788 0.800 0.812 1.6 –0.03 0.01 +40 60 40 3.75 4.55 150 20 +1.6 Power Switch and Protection RDS(ON)P P-Channel MOSFET On Resistance RDS(ON)N N-Channel MOSFET On Resistance ILIMPK TLIMIT THYST VSDWN P-MOS Peak Current Limit Thermal Shutdown Thermal Shutdown Hysteresis Input OVP Shutdown Rising Threshold Falling Threshold 5.50 2.7 External Square-Wave, 30% to 70% Duty Cycle TA = 25°C, VIN = 5.0V RLOAD > 5Ω, to VOUT=1.2V RLOAD > 5Ω, to VOUT=1.8V 2.7 17 6.2 5.85 3.0 3.0 26 210 340 10 Frequency Control fSW fSYNC Soft-Start tSS VSLEW Regulator Enable to Regulated VOUT Soft-Start VREF Slew Rate Oscillator Frequency MODE Pin Synchronization Range fPFM(MIN) Minimum PFM Frequency © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 4 FAN5354 — 3MHz, 3A Synchronous Buck Regulator Typical Characteristics Unless otherwise specified, VIN=5V, VOUT=1.2V, circuit of Figure 1, and components per Table 1. 100% 90% 80% 70% Efficiency Efficiency 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% VIN = 3.3V, Mode = 0 VIN = 3.3V, Mode = 1 VIN = 5V, Mode = 0 VIN = 5V, Mode = 1 60% 50% 40% 30% 20% 10% 0% 1 10 100 1000 10000 I LOAD Output Current (mA) VIN = 3.3V, Mode=0 VIN = 3.3V, Mode=1 VIN = 5V, Mode=0 VIN = 5V, Mode=1 1 10 100 1000 10000 I LOAD Output Current (mA) Figure 3. Efficiency vs. ILOAD at VOUT=1.2V Figure 4. Efficiency vs. ILOAD at VOUT=1.8V 100% 90% 80% 70% Efficiency 60% 50% 40% 30% 20% 10% 0% 1 10 100 1000 10000 I LOAD Output Current (mA) VIN = 3.3V, Mode = 0 VIN = 3.3V, Mode = 1 VIN = 5V, Mode = 0 VIN = 5V, Mode = 1 100% 90% 80% 70% Efficiency 60% 50% 40% 30% 20% 10% 0% 1 10 100 1000 10000 I LOAD Output Current (mA) VIN = 4.2V, Mode = 0 VIN = 4.2V, Mode = 1 VIN = 5V, Mode = 0 VIN = 5V, Mode = 1 Figure 5. Efficiency vs. ILOAD at VOUT=2.5V Figure 6. Efficiency vs. ILOAD at VOUT=3.3V 1 PFM, No load supply Current( A) 400 375 350 VOUT = 3.3 0.9 0.8 Supply Current (µ A) 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 2.7 3.2 3.7 4.2 Input Voltage(V) 4.7 5.2 85°C 25°C -40°C 325 300 VOUT = 1.2 275 250 225 200 2.7 3.2 3.7 4.2 4.7 5.2 85°C 25°C –40°C Input Voltage(V) Figure 7. Shutdown Supply Current vs VIN, EN=0 Figure 8. Quiescent Current in PFM vs. VIN, No Load © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 5 FAN5354 — 3MHz, 3A Synchronous Buck Regulator Typical Characteristics Unless otherwise specified, VIN=5V, VOUT=1.2V, circuit of Figure 1, and components per Table 1. 1000 900 800 Load Current (mA) 1.2VOUT boundary 1.2VOUT boundary 3.3VOUT boundary 3.3VOUT boundary Always PWM 3.5 3.0 Switching Frequency (Mhz) 2.5 2.0 1.5 1.0 0.5 0 0.5 1 1.5 Load Current (A) 2 2.5 3 VIN = 4.1V VIN = 4.0V VIN = 3.9V VIN = 3.8V 700 600 500 400 300 200 Hysteresis Always PFM 100 0 2.7 3.2 3.7 4.2 Input Voltage(V) 4.7 5.2 Figure 9. PFM/PWM Mode-Change Boundaries Figure 10. Effect of tOFF Minimum on Reducing Switching Frequency at Large Duty Cycles, VOUT=3.3V 16 14 VOUT ripple (mVAC p-p) 12 10 8 6 5VIN, 1.2VOUT PFM PWM 90 80 70 60 50 1.2VOUT,1.5A load PSRR Attenuation (dB) 4 2 0 0.1 1 3.3VIN, 1.2VOUT 5VIN,3.3VOUT 40 30 20 3.3VOUT, 0.5A load in PFM 3.3VOUT, 1.5A load 10 100 1000 10000 0.01 0.1 1 Frequency (KHz) 10 100 Load Current(mA) Figure 11. Output Voltage Ripple vs. Load Current (See explanation on page 12) Figure 12. Power Supply Rejection Ratio (PSRR) VOUT IL Figure 13. PFM-to-PWM Mode Transition, Slowly Increasing Load Current, 2 s/div. Figure 14 PWM-to-PFM Mode Transition, Slowly Decreasing Load Current, 2 s/div. © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 6 FAN5354 — 3MHz, 3A Synchronous Buck Regulator Typical Characteristics Unless otherwise specified, VIN=5V, VOUT=1.2V, circuit of Figure 1, and components per Table 1. 31.0 Switching Frequency (Khz) 30.0 TJ = 85°C 29.0 28.0 27.0 26.0 25.0 24.0 2.5 3 3.5 4 4.5 5 TJ = 25°C TJ = –40°C 5.5 6 Input Voltage (V) Figure 15. PFM frequency, ILOAD = 0 Load Transient Response (Figure 16 – Figure 19). ILOAD tR = tF = 100ns Figure 16. MODE=0, 100mA to 1.5A to 100mA, 5 s/div. Figure 17. 500mA to 3A to 500mA, 5 s/div. VOUT IL I load Figure 18. MODE=1, 100mA to 1.5A to 100mA, 5 s/div. Figure 19. 24mA to 500mA to 24mA, MODE=0, 5 s/div. © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 7 FAN5354 — 3MHz, 3A Synchronous Buck Regulator Typical Characteristics Unless otherwise specified, VIN=5V, VOUT=1.2V, circuit of Figure 1, and components per Table 1. VEN VOUT VIN=VEN VOUT VPG VPG Isupply Isupply Figure 20. Soft-Start, EN Voltage Raised After VIN=5V, ILOAD=0, 100µs/div. Figure 21. Soft-Start, EN Pin Tied to VCC ILOAD=0, 1ms/div. Figure 22. Soft-Start, EN Pin Raised After VIN=5V RLOAD=400mΩ, COUT=100µF, 100µs/div. Figure 23. Soft-Start, EN Pin Tied to VCC RLOAD=400mΩ, COUT=100µF, 1ms/div. Figure 24. Line Transient Response in PWM Mode, 10µs/div. Figure 25. Line Transient Response in PFM Mode, 10µs/div. © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 8 FAN5354 — 3MHz, 3A Synchronous Buck Regulator Typical Characteristics Unless otherwise specified, VIN=5V, VOUT=1.2V, circuit of Figure 1, and components per Table 1. Circuit Protection Response VOUT VOUT IL IL VPG Figure 27. VOUT to GND Short Circuit, 5µs/div. VPG Figure 26. VOUT to GND Short Circuit, 200µs/div. VEN VOUT VOUT IL VPG IL VPG Figure 28. Over-Current at Startup, RLOAD=200mΩ, 50µs/div. Figure 29. Progressive Overload, 200µs/div. © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 9 FAN5354 — 3MHz, 3A Synchronous Buck Regulator Operation Description The FAN5354 is a step-down switching voltage regulator that delivers an adjustable output from an input voltage supply of 2.7V to 5.5V. Using a proprietary architecture with synchronous rectification, the FAN5354 is capable of delivering 3A at over 80% efficiency. The regulator operates at a nominal frequency of 3MHz at full load, which reduces the value of the external components to 470nH for the output inductor and 20 F for the output capacitor. High efficiency is maintained at light load with single-pulse PFM mode. limits the COUT capacitance when a heavy load is applied during the startup. For a typical FAN5354 starting with a resistive load: COUT MAX ( µF ) ≈ 400 −100 • I LOAD( A ) where I LOAD = VOUT (3) RLOAD Control Scheme The FAN5354 uses a proprietary non-linear, fixed-frequency PWM modulator to deliver a fast load transient response, while maintaining a constant switching frequency over a wide range of operating conditions. The regulator performance is independent of the output capacitor ESR, allowing for the use of ceramic output capacitors. Although this type of operation normally results in a switching frequency that varies with input voltage and load current, an internal frequency loop holds the switching frequency constant over a large range of input voltages and load currents. For very light loads, the FAN5354 operates in discontinuous current (DCM) single-pulse PFM mode, which produces low output ripple compared with other PFM architectures. Transition between PWM and PFM is seamless, with a glitch of less than 18mV at VOUT during the transition between DCM and CCM modes. The regulator limits minimum PFM frequency to typically26Khz. PFM mode can be disabled by holding the MODE pin HIGH. The IC synchronizes to the MODE pin frequency. When synchronizing to the MODE pin, PFM mode is disabled. Synchronous rectification is inhibited during soft-start, allowing the IC to start into a pre-charged load. MODE Pin – External Frequency Synchronization Logic 1 on this pin forces the IC to stay in PWM mode. A logic 0 allows the IC to automatically switch to PFM during light loads. If the MODE pin is toggled, the converter synchronizes its switching frequency to the frequency on the mode pin (fMODE). The MODE pin is internally buffered with a Schmitt trigger, which allows the MODE pin to be driven with slow rise and fall times. An asymmetric duty cycle for frequency synchronization is permitted as long as the minimum time below VIL(MAX) or above VIH(MAX) is 100ns. PGOOD Pin The PGOOD pin is an open-drain that pin indicates that the IC is in regulation when its state is open. PGOOD pulls LOW under the following conditions: 1. 2. 3. The IC has operated in cycle-by-cycle current limit for eight or more consecutive PWM cycles. The circuit is disabled, either after a fault occurs, or when EN is LOW. The IC is performing a soft-start. Setting the output voltage The output voltage is set by the R1, R2, and VREF (0.8V): R1 VOUT − VREF = R2 VREF R1 must be set at or below 100K ; therefore: R2 = (1) Under-Voltage Lockout W hen EN is HIGH, the under-voltage lockout keeps the part from operating until the input supply voltage rises high enough to properly operate. This ensures no misbehavior of the regulator during startup or shutdown. (VOUT − 0.8 ) R1 • 0.8 (2) For example, for VOUT=1.2V, R1=100K , R2=200K . Input Over-Voltage Protection (OVP) W hen VIN exceeds VSDWN (about 6.2V) the IC stops switching to protect the circuitry from internal spikes above 6.5V. An internal 40µs filter prevents the circuit from shutting down due to noise spikes. For the circuit to fully protect the internal circuitry, the VIN slew rate above 6.2V must be limited to no more than 15V/ms when the IC is switching. The IC protects itself if VIN overshoots to 7V during initial power-up as long as the VIN transition from 0 to 7V occurs in less than 10µs (10% to 90%). Enable and Soft Start W hen the EN pin is LOW, the IC is shut down, all internal circuits are off, and the part draws very little current. Raising EN above its threshold voltage activates the part and starts the soft-start cycle. During soft-start, the modulator’s internal reference is ramped slowly to minimize any large surge currents on the input and prevents any overshoot of the output voltage. If large values of output capacitance are used, the regulator may fail to start. If VOUT fails to achieve regulation within 320µs from the beginning of soft-start, the regulator shuts down and waits 1200µs before attempting a restart. If the regulator is at its current limit for more than about 60µs, the regulator shuts down before restarting 1200µs later. This © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 Current Limiting A heavy load or short circuit on the output causes the current in the inductor to increase until a maximum current threshold is reached in the high-side switch. Upon reaching this point, the high-side switch turns off, preventing high currents from causing damage. 16 consecutive PWM cycles in current limit www.fairchildsemi.com 10 FAN5354 — 3MHz, 3A Synchronous Buck Regulator cause the regulator to shut down and stay off for about 1200µs before attempting a restart. In the event of a short circuit, the soft-start circuit attempts to restart and produces an over-current fault after about 50µs, which results in a duty cycle of less than 10%, providing current into a short circuit. Efficiency is affected by the inductor DCR and inductance value. Decreasing the inductor value for a given physical size typically decreases the DCR; but since ∆I increases, the RMS current increases, as do core and skin-effect losses. IRMS = IOUT(DC) + 2 ∆I2 12 (7) Thermal Shutdown W hen the die temperature increases, due to a high load condition and/or a high ambient temperature, the output switching is disabled until the temperature on the die has fallen sufficiently. The junction temperature at which the thermal shutdown activates is nominally 150°C with a 20°C hysteresis. The increased RMS current produces higher losses through the RDS(ON) of the IC MOSFETs as well as the inductor ESR. Increasing the inductor value produces lower RMS currents, but degrades transient response. For a given physical inductor size, increased inductance usually results in an inductor with lower saturation current. Table 2 shows the effects of inductance higher or lower than the recommended 470nH on regulator performance. Minimum Off-Time Effect on Switching Frequency tON(MIN) and tOFF(MIN) are both 45ns. This imposes constraints on the maximum VOUT that the FAN5354 can provide, VIN while still maintaining a fixed switching frequency in PWM mode. While regulation is unaffected, the switching frequency will drop when the regulator cannot provide sufficient duty cycle at 3 MHz to maintain regulation. The calculation for switching frequency is given below Table 2. Effects of Increasing the Inductor Value (from 470nH Recommended) on Regulator Performance IMAX(LOAD) Increase ∆VOUT (EQ. 8) Decrease Transient Response Degraded   1 1  fSW = min  ,  t SW(MAX) 333.3ns    where  VOUT + IOUT • R OFF   t SW (MAX) = 45ns • 1 +  V −I  IN OUT • R ON − VOUT   Inductor Current Rating The FAN5354’s current limit circuit can allow a peak current of 5.5A to flow through L1 under worst-case conditions. If it is possible for the load to draw that much continuous current, the inductor should be capable of sustaining that current or failing in a safe manner. For space-constrained applications, a lower current rating for L1 can be used. The FAN5354 may still protect these inductors in the event of a short circuit, but may not be able to protect the inductor from failure if the load is able to draw higher currents than the DC rating of the inductor. (4) ROFF = RDSON _ N + DCRL RON = RDSON _ P + DCRL Application Information Selecting the Inductor The output inductor must meet both the required inductance and the energy handling capability of the application. The inductor value affects the average current limit, the output voltage ripple, and the efficiency. The ripple current (∆I) of the regulator is: ∆I ≈ VOUT  VIN − VOUT • VIN  L • fSW      (5) Output Capacitor and VOUT Ripple Note: Table 1 suggests 0805 capacitors, but 0603 capacitors may be used if space is at a premium. Due to voltage effects, the 0603 capacitors have a lower in-circuit capacitance than the 0805 package, which can degrade transient response and output ripple. Increasing COUT has no effect on loop stability and can therefore be increased to reduce output voltage ripple or to improve transient response. Output voltage ripple, ∆VOUT, is:   1 ∆VOUT = ∆I •  + ESR  8•C  • fSW OUT   (8) The maximum average load current, IMAX(LOAD) is related to the peak current limit, ILIM(PK), by the ripple current as: ∆I (6) 2 The FAN5354 is optimized for operation with L=470nH, but is stable with inductances up to 1.2µH (nominal). The inductor should be rated to maintain at least 80% of its value at ILIM(PK). Failure to do so lowers the amount of DC current the IC can deliver. IMAX(LOAD) = ILIM(PK ) − © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 where COUT is the effective output capacitance. The capacitance of COUT decreases at higher output voltages, which results in higher ∆VOUT . If COUT is greater than 100µF, the regulator may fail to start under load. If an inductor value greater than 1.0µH is used, at least 30µF of COUT should be used to ensure stability. www.fairchildsemi.com 11 FAN5354 — 3MHz, 3A Synchronous Buck Regulator As can be seen in Figure 11 the lowest ∆VOUT is obtained when the IC is in PWM mode and, therefore, operating at 3Mhz. In PFM mode, fSW is reduced, causing ∆VOUT to increase. At extremely light loads, the output ripple decreases, as the minimum frequency circuit becomes active and the effective tON (high-side on-time) decreases. Layout Recommendations The layout recommendations below highlight various topcopper planes by using different colors. It includes COUT3 to demonstrate how to add COUT capacitance to reduce ripple and transient excursions. The inductor in this example is the TDK VLC5020T-R47N. VCC and VIN should be connected together by a thin trace some distance from the IC, or through a resistor (shown as R3 below), to isolate the switching spikes on PVIN from the IC’s bias supply on VCC. If PCB area is at a premium, the connection between PVIN and VCC can be made on another PCB layer through vias. The via impedance provides some filtering for the high-frequency spikes generated on PVIN. PGND and AGND connect through the thermal pad of the IC. Extending the PGND and AGND planes improves IC cooling. The IC analog ground (AGND) is bonded to P1 between pins 1 and 12. Large AC ground currents should return to pins 3 and 4 (PGND) either through the copper under P1 between pins 6 and 7 or through a direct trace from pins 3 and 4 (as shown for COUT1-COUT3). EN and PGOOD connect through vias to the system control logic. CIN1 is an optional device used to provide a lower impedance path for high-frequency switching edges/spikes, which helps to reduce SW node and VIN ringing. CIN should be placed as close as possible between PGND and VIN as shown below. PGND connection back to inner planes should be accomplished as series of vias distributed among the COUT return track and CIN return plane between pins 6 and 7. ESL Effects The ESL (Equivalent Series Inductance) of the output capacitor network should be kept low to minimize the square wave component of output ripple that results from the division ratio COUT ESL and the output inductor (LOUT). The square wave component due to the ESL can be estimated as: ∆VOUT ( SQ ) ≈ VIN • ESLCOUT L1 (9) A good practice to minimize this ripple is to use multiple output capacitors to achieve the desired COUT value. For example, to obtain COUT=20µF, a single 22µF 0805 would produce twice the square wave ripple of 2 x 10µF 0805. To minimize ESL, try to use capacitors with the lowest ratio of length to width. 0805s have lower ESL than 1206s. If low output ripple is a chief concern, some vendors produce 0508 or 0612 capacitors with ultra-low ESL. Placing additional small value capacitors near the load also reduces the highfrequency ripple components. Input Capacitor The 10µF ceramic input capacitor should be placed as close as possible between the VIN pin and PGND to minimize the parasitic inductance. If a long wire is used to bring power to the IC, additional “bulk” capacitance (electrolytic or tantalum) should be placed between CIN and the power source lead to reduce under-damped ringing that can occur between the inductance of the power source leads and CIN. The effective CIN capacitance value decreases as VIN increases due to DC bias effects. This has no significant impact on regulator performance. AGND 0402 VOUT COUT3 COUT2 COUT1 0402 1 2 12 CVCC 10µ F 0805 10µF 0805 10µ F 0805 FAN5354 P1 (GND) 11 10 9 VCC 8 7 10µ F 0805 CIN 0603 PGND 3 4 5 R3 0402 L1 6 VIN SW 0.47µ H 5 x 5 mm CIN1 0402 PGND Figure 30. 3A Layout Recommendation © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 12 FAN5354 — 3MHz, 3A Synchronous Buck Regulator Physical Dimensions Figure 31. 12-lead 3 x 3.5mm MLP 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/. © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 13 FAN5354 — 3MHz, 3A Synchronous Buck Regulator © 2009 Fairchild Semiconductor Corporation FAN5354 • Rev. 1.0.5 www.fairchildsemi.com 14