MP4350 2.5A, 4MHz, 20V Step-Down Converter The Future of Analog IC Technology DESCRIPTION FEATURES The MP4350 is a high frequency step-down switching regulator with an integrated internal high-side high voltage power MOSFET. It provides 2.5A output with current mode control for fast loop response and easy compensation. • • • • The wide 4.5V to 20V input range accommodates a variety of step-down applications, including those in an automotive input environment. A 100µA operational quiescent current allows use in battery-powered applications. • • • • • • High power conversion efficiency over a wide load range is achieved by scaling down the switching frequency at light load condition to reduce the switching and gate driving losses. 100µA Quiescent Current Wide 4.5V to 20V Operating Input Range 150mΩ Internal Power MOSFET Up to 4MHz Programmable Switching Frequency Ceramic Capacitor Stable Internal Soft-Start Internal Set Current Limit without a Current Sensing Resistor Up to 95% Efficiency Output Adjustable from 0.8V to 16V Available in a 10-Pin QFN (3mm x 3mm) Package APPLICATIONS • • • The frequency foldback helps prevent inductor current runaway during startup and thermal shutdown provides reliable, fault tolerant operation. DSL Power Set Top Boxes Battery Powered Systems “MPS” and “The Future of Analog IC Technology” are Registered Trademarks of Monolithic Power Systems, Inc. By switching at 4MHz, the MP4350 is able to prevent switching related EMI (Electromagnetic Interference) noise problems, such as those found in AM radio and ADSL applications. The MP4350 is available in a small 3mm x 3mm 10-pin QFN package. TYPICAL APPLICATION C4 100nF 100 10 VIN BST SW 1,2 D1 EN 3 7 EN MP4350 COMP FREQ GND 5 MP4350 Rev. 1.0 5/26/2008 FB 5 4 C3 220pF C6 NS VIN=5V 90 VOUT 3.3V EFFICIENCY (%) VIN 8,9 Efficiency vs Load Current 80 VIN=12V 70 60 50 40 30 VOUT=3.3V 0 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 0.5 1.0 1.5 2.0 LOAD CURRENT (A) 2.5 1 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER ABSOLUTE MAXIMUM RATINGS (1) PACKAGE REFERENCE SW 1 10 BST SW 2 9 VIN EN 3 8 VIN Supply Voltage (VIN) .................... –0.3V to +22V Switch Voltage (VSW) ........... –0.3V to VIN + 0.3V BST to SW..................................... –0.3V to +6V All Other Pins................................. –0.3V to +6V Junction Temperature...............................150°C Lead Temperature ....................................260°C Storage Temperature ..............–65°C to +150°C COMP 4 7 FREQ Recommended Operating Conditions FB 5 6 GND Supply Voltage VIN ........................... 4.5V to 20V Output Voltage VOUT ...................... +0.8V to 16V Operating Temperature .............–40°C to +85°C TOP VIEW EXPOSED PAD ON BACKSIDE Thermal Resistance * Part Number* Package Temperature MP4350DQ QFN10 (3x3) –40°C to +85°C For Tape & Reel, add suffix –Z (eg. MP4350DQ–Z) For RoHS compliant packaging, add suffix –LF (eg. MP4350DQ–LF–Z) (3) θJA (2) θJC QFN10 (3mm x 3mm) ............. 50 ...... 12... °C/W Notes: 1) Exceeding these ratings may damage the device. 2) The device is not guaranteed to function outside of its operating conditions. 3) Measured on approximately 1” square of 1 oz copper. ELECTRICAL CHARACTERISTICS VIN = 12V, VEN = 2.5V, VCOMP = 1.4V, TA= +25°C, unless otherwise noted. Parameter Feedback Voltage Upper Switch On Resistance Upper Switch Leakage Current Limit Error Amp Voltage Gain (4) Error Amp Transconductance Error Amp Min Source current Error Amp Min Sink current VIN UVLO Threshold VIN UVLO Hysteresis Soft-Start Time (4) Oscillator Frequency Shutdown Supply Current Quiescent Supply Current Thermal Shutdown Thermal Shutdown Hysteresis Minimum Off Time (4) Minimum On Time (4) EN Up Threshold EN Down Threshold Symbol Condition VFB 4.5V < VIN < 20V RDS(ON) VBST – VSW = 5V VEN = 0V, VSW = 0V, VIN = 36V Duty Cycle = 50% ICOMP = ±3µA VFB = 0.7V VFB = 0.9V Min 0.776 2.9 40 2.7 0V < VFB < 0.8V RFREQ = 45kΩ RFREQ = 18kΩ VEN = 0V No load, VFB = 0.9V 1.6 3.2 1.35 1.15 Typ 0.8 150 1 3.5 200 60 5 –5 3.0 0.35 1.5 2 4 12 100 150 15 100 100 1.5 1.2 Max 0.824 80 3.3 2.4 4.8 18 125 1.65 1.25 Units V mΩ µA A V/V µA/V µA µA V V ms MHz MHz µA µA °C °C ns ns V V Note: 4) Guaranteed by design. MP4350 Rev. 1.0 5/26/2008 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 2 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER PIN FUNCTIONS Pin # Name Description Switch Node. This is the output from the high-side switch. A low forward drop Schottky diode to SW ground is required. The diode must be close to the SW pins to reduce switching spikes. Enable Input. Pulling this pin below the specified threshold shuts the chip down. Pulling it up EN above the specified threshold or leaving it floating enables the chip. Compensation. This node is the output of the error amplifier. Control loop frequency COMP compensation is applied to this pin. Feedback. This is the input to the error amplifier. An external resistive divider connected FB between the output and GND which scales down VOUT equal to the internal +0.8V reference. Ground. It should be connected as close as possible to the output capacitor to shorten the high GND current switch paths. Switching Frequency Program Input. Connect a resistor from this pin to ground to set the FREQ switching frequency. Input Supply. This supplies power to all the internal control circuitry, including the bootstrap VIN regulator and the high-side switch. A decoupling capacitor to ground must be placed close to this pin to minimize switching spikes. Bootstrap. This is the positive power supply for the internal floating high-side MOSFET driver. BST Connect a bypass capacitor between this pin and SW pin. 1, 2 3 4 5 6 7 8, 9 10 TYPICAL PERFORMANCE CHARACTERISTICS VIN = 12V, C1 = 10µF, C2 = 22µF, L = 10µH and TA = +25°C, unless otherwise noted. 100 VIN=5V 80 VIN=12V 70 60 50 40 VIN=12V 90 EFFICIENCY (%) EFFICIENCY (%) 90 Efficiency vs Load Current 80 70 60 50 VOUT=2.5V 0 0.5 1.0 1.5 2.0 LOAD CURRENT (A) 2.5 40 VOUT=5V 0 0.5 1.0 1.5 2.0 LOAD CURRENT (A) Steady State Steady State IOUT = 0.1A IOUT = 1A 2.5 OSCILLATIONG EFFICIENCY (KHz) 100 Efficiency vs Load Current Oscillating Frequency vs Rfreq 4000 3500 3000 2500 2000 1500 1000 500 0 10 IOUT = 2A VOUT AC Coupled 20mV/div. VOUT AC Coupled 10mV/div. VSW 10V/div. VSW 10V/div. VSW 10V/div. IL 1A/div. IL 1A/div. MP4350 Rev. 1.0 5/26/2008 1000 Steady State VOUT AC Coupled 10mV/div. IL 1A/div. 100 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 3 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER TYPICAL PERFORMANCE CHARACTERISTICS (continued) VIN = 12V, C1 = 10µF, C2 = 22µF, L = 10µH and TA = +25°C, unless otherwise noted. Startup Shutdown Startup IOUT = 0.1A IOUT = 0.1A IOUT = 1A VEN 5V/div. VEN 5V/div. VEN 5V/div. VOUT 2V/div. VSW 10V/div. VOUT 2V/div. VOUT 2V/div. VSW 10V/div. VSW 10V/div. IL 1A/div. IL 1A/div. IL 1A/div. 1ms/div. 1ms/div. Shutdown Startup IOUT = 1A IOUT = 2A 1ms/div. Shutdown IOUT = 2A VEN 5V/div. VEN 5V/div. VEN 5V/div. VOUT 2V/div. VOUT 2V/div. VOUT 2V/div. VSW 10V/div. VSW 10V/div. VSW 10V/div. IL 1A/div. IL 2A/div. IL 2A/div. 1ms/div. Short Circuit Entry Short Circuit Recovery IOUT = 0.1A to Short IOUT = Short to 0.1A VOUT 2V/div. VOUT 2V/div. IL 1A/div. IL 1A/div. MP4350 Rev. 1.0 5/26/2008 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 4 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER OPERATION The MP4350 is a variable frequency, non-synchronous, step-down switching regulator with an integrated high-side high voltage power MOSFET. It provides a single highly efficient solution with current mode control for fast loop response and easy compensation. It features a wide input voltage range, internal soft-start control and precision current limiting. Its very low operational quiescent current makes it suitable for battery powered applications. PWM Control At moderate to high output current, the MP4350 operates in a fixed frequency, peak current control mode to regulate the output voltage. A PWM cycle is initiated by the internal clock. The power MOSFET is turned on and remains on until its current reaches the value set by the COMP voltage. When the power switch is off, it remains off for at least 100ns before the next cycle starts. If, in one PWM period, the current in the power MOSFET does not reach the COMP set current value, the power MOSFET remains on, saving a turn-off operation. VIN VIN EN REFERENCE UVLO/ THERMAL SHUTDOWN 5V + -2.6V INTERNAL REGULATORS + -BST SW VOUT 1.5ms SS -- ISW SS + ISW Level Shift FB SW Gm Error Amp SS 0V8 -- COMP + OSCILLATOR CLK VOUT COMP GND FREQ Figure 1—Functional Block Diagram MP4350 Rev. 1.0 5/26/2008 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 5 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER Error Amplifier The error amplifier compares the FB pin voltage with the internal reference (REF) and outputs a current proportional to the difference between the two. This output current is then used to charge the external compensation network to form the COMP voltage, which is used to control the power MOSFET current. During operation, the minimum COMP voltage is clamped to 0.9V and its maximum is clamped to 2.0V. COMP is internally pulled down to GND in shutdown mode. COMP should not be pulled up beyond 2.6V. Internal Regulator Most of the internal circuitries are powered from the 2.6V internal regulator. This regulator takes the VIN input and operates in the full VIN range. When VIN is greater than 3.0V, the output of the regulator is in full regulation. When VIN is lower than 3.0V, the output decreases. Enable Control The MP4350 has a dedicated enable control pin (EN). With high enough input voltage, the chip can be enabled and disabled by EN, which has positive logic. Its falling threshold is a precision 1.2V, and its rising threshold is 1.5V (300mV higher). When floating, EN is pulled up to about 3.0V by an internal 1µA current source so it is enabled. To pull it down, 1µA current capability is needed. Under-Voltage Lockout (UVLO) Under-voltage lockout (UVLO) is implemented to protect the chip from operating at insufficient supply voltage. The UVLO rising threshold is about 3.0V while its falling threshold is a consistent 2.6V. Internal Soft-Start The soft-start is implemented to prevent the converter output voltage from overshooting during startup. When the chip starts, the internal circuitry generates a soft-start voltage (SS) ramping up from 0V to 2.6V. When it is lower than the internal reference (REF), SS overrides REF so the error amplifier uses SS as the reference. When SS is higher than REF, REF regains control. Thermal Shutdown Thermal shutdown is implemented to prevent the chip from operating at exceedingly high temperatures. When the silicon die temperature is higher than its upper threshold, it shuts down the whole chip. When the temperature is lower than its lower threshold, the chip is enabled again. Floating Driver and Bootstrap Charging The floating power MOSFET driver is powered by an external bootstrap capacitor. This floating driver has its own UVLO protection. This UVLO’s rising threshold is 2.2V with a threshold of 150mV. When EN is pulled down below 1.2V, the chip is put into the lowest shutdown current mode. When EN is higher than zero but lower than its rising threshold, the chip is still in shutdown mode but the shutdown current increases slightly. MP4350 Rev. 1.0 5/26/2008 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 6 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER The bootstrap capacitor is charged and regulated to about 5V by the dedicated internal bootstrap regulator. When the voltage between the BST and SW nodes is lower than its regulation, a PMOS pass transistor connected from VIN to BST is turned on. The charging current path is from VIN, BST and then to SW. External circuit should provide enough voltage headroom to facilitate the charging. As long as VIN is sufficiently higher than SW, the bootstrap capacitor can be charged. When the power MOSFET is ON, VIN is about equal to SW so the bootstrap capacitor cannot be charged. When the external diode is on, the difference between VIN and SW is largest, thus making it the best period to charge. When there is no current in the inductor, SW equals the output voltage VOUT so the difference between VIN and VOUT can be used to charge the bootstrap capacitor. At higher duty cycle operation condition, the time period available to the bootstrap charging is less so the bootstrap capacitor may not be sufficiently charged. In case the internal circuit does not have sufficient voltage and the bootstrap capacitor is not charged, extra external circuitry can be used to ensure the bootstrap voltage is in the normal operational region. Refer to External Bootstrap Diode in Application section. The DC quiescent current of the floating driver is about 20µA. Make sure the bleeding current at the SW node is higher than this value, such that: IO + VO > 20µA (R1 + R2) Current Comparator and Current Limit The power MOSFET current is accurately sensed via a current sense MOSFET. It is then fed to the high speed current comparator for the current mode control purpose. The current comparator takes this sensed current as one of its inputs. When the power MOSFET is turned on, the comparator is first blanked till the end of the turn-on transition to avoid noise issues. The comparator then compares the power switch current with the COMP voltage. When the sensed current is higher than the COMP voltage, the comparator output is low, turning off the power MOSFET. The cycle-by-cycle maximum current of the internal power MOSFET is internally limited. Startup and Shutdown If both VIN and EN are higher than their appropriate thresholds, the chip starts. The reference block starts first, generating stable reference voltage and currents, and then the internal regulator is enabled. The regulator provides stable supply for the remaining circuitries. While the internal supply rail is up, an internal timer holds the power MOSFET OFF for about 50µs to blank the startup glitches. When the internal soft-start block is enabled, it first holds its SS output low to ensure the remaining circuitries are ready and then slowly ramps up. Three events can shut down the chip: EN low, VIN low and thermal shutdown. In the shutdown procedure, the power MOSFET is turned off first to avoid any fault triggering. The COMP voltage and the internal supply rail are then pulled down. Programmable Oscillator The MP4350 oscillating frequency is set by an external resistor, Rfreq from the FREQ pin to ground. The value of Rfreq can be calculated from: R freq (KΩ) = MP4350 Rev. 1.0 5/26/2008 180000 f s (KHz)1.1 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 7 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER APPLICATION INFORMATION COMPONENT SELECTION Setting the Output Voltage The output voltage is set using a resistive voltage divider from the output voltage to FB pin. The voltage divider divides the output voltage down to the feedback voltage by the ratio: VFB = VOUT R2 R1 + R2 Thus the output voltage is: VOUT = VFB (R1 + R2) R2 About 20µA current from high side BS circuitry can be seen at the output when the MP4350 is at no load. In order to absorb this small amount of current, keep R2 under 40KΩ. A typical value for R2 can be 40.2kΩ. With this value, R1 can be determined by: R1 = 50.25 × ( VOUT − 0.8)(kΩ) For example, for a 3.3V output voltage, R2 is 40.2kΩ, and R1 is 127kΩ. Inductor The inductor is required to supply constant current to the output load while being driven by the switched input voltage. A larger value inductor will result in less ripple current that will result in lower output ripple voltage. However, the larger value inductor will have a larger physical size, higher series resistance, and/or lower saturation current. MP4350 Rev. 1.0 5/26/2008 A good rule for determining the inductance to use is to allow the peak-to-peak ripple current in the inductor to be approximately 30% of the maximum switch current limit. Also, make sure that the peak inductor current is below the maximum switch current limit. The inductance value can be calculated by: L1 = ⎛ ⎞ VOUT V × ⎜1 − OUT ⎟⎟ fS × ∆IL ⎜⎝ VIN ⎠ Where VOUT is the output voltage, VIN is the input voltage, fS is the switching frequency, and ∆IL is the peak-to-peak inductor ripple current. Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by: ILP = ILOAD + ⎛ ⎞ VOUT V × ⎜⎜1 − OUT ⎟⎟ 2 × fS × L1 ⎝ VIN ⎠ Where ILOAD is the load current. Table 1 lists a number of suitable inductors from various manufacturers. The choice of which style inductor to use mainly depends on the price vs. size requirements and any EMI requirement. www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 8 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER Table 1—Inductor Selection Guide Inductance (µH) Max DCR (Ω) Current Rating (A) Dimensions L x W x H (mm3) 7447789002 2.2 0.019 4 7.3x7.3x3.2 7447789003 3.3 0.024 3.42 7.3x7.3x3.2 7447789004 4.7 0.033 2.9 7.3x7.3x3.2 744066100 10 0.035 3.6 10x10x3.8 744771115 15 0.025 3.75 12x12x6 744771122 22 0.031 3.37 12x12x6 RLF7030T-2R2 2.2 0.012 5.4 7.3x6.8x3.2 RLF7030T-3R3 3.3 0.02 4.1 7.3x6.8x3.2 RLF7030T-4R7 4.7 0.031 3.4 7.3x6.8x3.2 Part Number Wurth Electronics TDK SLF10145T-100 10 0.0364 3 10.1x10.1x4.5 SLF12565T-150M4R2 15 0.0237 4.2 12.5x12.5x6.5 SLF12565T-220M3R5 22 0.0316 3.5 12.5x12.5x6.5 FDV0630-2R2M 2.2 0.021 5.3 7.7x7x3 FDV0630-3R3M 3.3 0.031 4.3 7.7x7x3 FDV0630-4R7M 4.7 0.049 3.3 7.7x7x3 919AS-100M 10 0.0265 4.3 10.3x10.3x4.5 919AS-160M 16 0.0492 3.3 10.3x10.3x4.5 919AS-220M 22 0.0776 3 10.3x10.3x4.5 Toko Output Rectifier Diode The output rectifier diode supplies the current to the inductor when the high-side switch is off. To reduce losses due to the diode forward voltage and recovery times, use a Schottky diode. Choose a diode whose maximum reverse voltage rating is greater than the maximum input voltage, and whose current rating is greater than the maximum load current. Table 2 lists example Schottky diodes and manufacturers. Input Capacitor The input current to the step-down converter is discontinuous, therefore a capacitor is required to supply the AC current to the step-down converter while maintaining the DC input voltage. Use low ESR capacitors for the best performance. Ceramic capacitors are preferred, but tantalum or low-ESR electrolytic capacitors may also suffice. For simplification, choose the input capacitor with RMS current rating greater than half of the maximum load current. Table 2—Diode Selection Guide Diodes B320A-13-F CMSH3-20MA MP4350 Rev. 1.0 5/26/2008 Voltage/ Current Rating 20V, 3A 20V, 3A Manufacturer Diodes Inc. Central Semi www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 9 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER The input capacitor (C1) can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1µF, should be placed as close to the IC as possible. When using ceramic capacitors, make sure that they have enough capacitance to provide sufficient charge to prevent excessive voltage ripple at input. The input voltage ripple caused by capacitance can be estimated by: ∆VIN = ⎛ ILOAD V V × OUT × ⎜1 − OUT fS × C1 VIN ⎜⎝ VIN ⎞ ⎟⎟ ⎠ Output Capacitor The output capacitor (C2) is required to maintain the DC output voltage. Ceramic, tantalum, or low ESR electrolytic capacitors are recommended. Low ESR capacitors are preferred to keep the output voltage ripple low. The output voltage ripple can be estimated by: ∆VOUT = VOUT ⎛ V × ⎜⎜1 − OUT fS × L ⎝ VIN ⎞ ⎞ ⎛ 1 ⎟ ⎟⎟ × ⎜ R ESR + ⎜ 8 × f S × C2 ⎟⎠ ⎠ ⎝ Where L is the inductor value and RESR is the equivalent series resistance (ESR) value of the output capacitor. In the case of ceramic capacitors, the impedance at the switching frequency is dominated by the capacitance. The output voltage ripple is mainly caused by the capacitance. For simplification, the output voltage ripple can be estimated by: ∆VOUT = ⎛ ⎞ V × ⎜⎜1 − OUT ⎟⎟ VIN ⎠ × L × C2 ⎝ VOUT 8 × fS 2 In the case of tantalum or electrolytic capacitors, the ESR dominates the impedance at the switching frequency. For simplification, the output ripple can be approximated to: ∆VOUT = VOUT ⎛ V × ⎜1 − OUT f S × L ⎜⎝ VIN ⎞ ⎟⎟ × R ESR ⎠ The characteristics of the output capacitor also affect the stability of the regulation system. The MP4350 can be optimized for a wide range of capacitance and ESR values. MP4350 Rev. 1.0 5/26/2008 Compensation Components MP4350 employs current mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the COMP pin. COMP pin is the output of the internal error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. The DC gain of the voltage feedback loop is given by: A VDC = R LOAD × G CS × A VEA × VFB VOUT Where AVEA is the error amplifier voltage gain, 200V/V; GCS is the current sense transconductance, 3.7A/V; RLOAD is the load resistor value. The system has two poles of importance. One is due to the compensation capacitor (C3), the output resistor of error amplifier. The other is due to the output capacitor and the load resistor. These poles are located at: fP1 = GEA 2π × C3 × A VEA fP2 = 1 2π × C2 × R LOAD Where, GEA is the transconductance, 60µA/V. error amplifier The system has one zero of importance, due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at: f Z1 = 1 2π × C3 × R3 The system may have another zero of importance, if the output capacitor has a large capacitance and/or a high ESR value. The zero, due to the ESR and capacitance of the output capacitor, is located at: fESR = 1 2π × C2 × R ESR www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 10 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER In this case (as shown in Figure 2), a third pole set by the compensation capacitor (C6) and the compensation resistor (R3) is used to compensate the effect of the ESR zero on the loop gain. This pole is located at: f P3 = 1 2π × C6 × R3 R3 = 2π × C2 × f C VOUT × G EA × G CS VFB Where fC is the desired crossover frequency. The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The system crossover frequency where the feedback loop has the unity gain is important. Lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause system unstable. A good rule of thumb is to set the crossover frequency to approximately one-tenth of the switching frequency. The Table 3 lists the typical values of compensation components for some standard output voltages with various output capacitors and inductors. The values of the compensation components have been optimized for fast transient responses and good stability at given conditions. Table 3—Compensation Values for Typical Output Voltage/Capacitor Combinations VOUT (V) L (µH) C2 (µF) R3 (kΩ) C3 (pF) C6 1.8 4.7 47 105 100 None 2.5 4.7 - 6.8 22 54.9 220 None 3.3 6.8 -10 22 58.1 220 None 5 15 - 22 22 100 150 None 12 22 - 33 22 147 150 None To optimize the compensation components for conditions not listed in Table 3, the following procedure can be used. MP4350 Rev. 1.0 5/26/2008 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation: 2. Choose the compensation capacitor (C3) to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, fZ1, below one forth of the crossover frequency provides sufficient phase margin. Determine the C3 value by the following equation: C3 > 4 2π × R3 × f C 3. Determine if the second compensation capacitor (C6) is required. It is required if the ESR zero of the output capacitor is located at less than half of the switching frequency, or the following relationship is valid: f 1 < S 2π × C2 × R ESR 2 If this is the case, then add the second compensation capacitor (C6) to set the pole fP3 at the location of the ESR zero. Determine the C6 value by the equation: C6 = C2 × R ESR R3 High Frequency Operation The switching frequency of MP4350 can be programmed up to 4MHz by an external resistor. Please pay attention to the following if the switching frequency is above 2MHz. The minimum on time of MP4350 is about 80ns (typ). Pulse skipping operation can be seen more easily at higher switching frequency due to the minimum on time. Recommended operating voltage is 12V or below, and 24V or below at 2MHz. Refer to Figure 2 below for detailed information. www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 11 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER 30 Recommended VIN (max) vs Switching Frequency VIN (MAX) (V) 25 20 15 VOUT=3.3V 10 VOUT=2.5V 5 1500 2000 2500 3000 3500 4000 fs (KHz) Figure 2—Recommend Max VIN vs. fs Since the internal bootstrap circuitry has higher impedance, which may not be adequate to charge the bootstrap capacitor during each (1-D)×Ts charging period, an external bootstrap charging diode is strongly recommended if the switching frequency is above 2MHz (see External Bootstrap Diode section for detailed implementation information). With higher switching frequencies, the inductive reactance (XL) of capacitor comes to dominate, so that the ESL of input/output capacitor determines the input/output ripple voltage at higher switching frequency. As a result of that, high frequency ceramic capacitor is strongly recommended as input decoupling capacitor and output filtering capacitor for such high frequency operation. Layout becomes more important when the device switches at higher frequency. It is essential to place the input decoupling capacitor, catch diode and the MP4350 (Vin pin, SW pin and PGND) as close as possible, with traces that are very short and fairly wide. This can help to greatly reduce the voltage spike on SW node, and lower the EMI noise level as well. Try to run the feedback trace as far from the inductor and noisy power traces as possible. It is often a good idea to run the feedback trace on the side of the PCB opposite of the inductor with a ground plane separating the two. The compensation components should be placed closed to the MP4350. MP4350 Rev. 1.0 5/26/2008 Do not place the compensation components close to or under high dv/dt SW node, or inside the high di/dt power loop. If you have to do so, the proper ground plane must be in place to isolate those. Switching loss is expected to be increased at high switching frequency. To help to improve the thermal conduction, a grid of thermal vias can be created right under the exposed pad. It is recommended that they be small (15mil barrel diameter) so that the hole is essentially filled up during the plating process, thus aiding conduction to the other side. Too large a hole can cause ‘solder wicking’ problems during the reflow soldering process. The pitch (distance between the centers) of several such thermal vias in an area is typically 40mil. Please refer to the layout example on EV4460 datasheet. External Bootstrap Diode It is recommended that an external bootstrap diode be added when the input voltage is no greater than 5V or the 5V rail is available in the system. This helps improve the efficiency of the regulator. The bootstrap diode can be a low cost one such as IN4148 or BAT54. 5V BS MP4350 SW Figure 3—External Bootstrap Diode This diode is also recommended for high duty cycle operation (when VOUT /VIN >65%) or low VIN (<5Vin) applications. At no load or light load, the converter may operate in pulse skipping mode in order to maintain the output voltage in regulation. Thus there is less time to refresh the BS voltage. In order to have enough gate voltage under such operating conditions, the difference of VIN –VOUT should be greater than 3V. For example, if the VOUT is set to 3.3V, the VIN needs to be higher than 3.3V+3V=6.3V to maintain enough BS voltage at no load or light load. To meet this requirement, EN pin can be used to program the input UVLO voltage to Vout+3V. www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 12 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER TYPICAL APPLICATION CIRCUITS C4 100nF 10 VIN 8,9 6V - 36V VIN BST SW 1,2 VOUT 1.8V D1 EN 3 7 EN FB MP4460 COMP FREQ 5 4 C3 100pF GND 6 C6 NS Figure 4—1.8V Output Typical Application Schematic C4 100nF 10 VIN 10V - 20V 8,9 VIN BST SW 1,2 VOUT 5V D1 EN 3 7 EN MP4350 FB COMP FREQ GND 6 5 4 C3 150pF C6 NS Figure 5—5V Output Typical Application Schematic MP4350 Rev. 1.0 5/26/2008 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 13 MP4350 – 2.5A, 4MHz, 20V STEP-DOWN CONVERTER PACKAGE INFORMATION QFN10 (3mm x 3mm) 2.90 3.10 0.30 0.50 PIN 1 ID MARKING 0.18 0.30 2.90 3.10 PIN 1 ID INDEX AREA 1.45 1.75 PIN 1 ID SEE DETAIL A 10 1 2.25 2.55 0.50 BSC 5 6 TOP VIEW BOTTOM VIEW PIN 1 ID OPTION A R0.20 TYP. PIN 1 ID OPTION B R0.20 TYP. 0.80 1.00 0.20 REF 0.00 0.05 SIDE VIEW DETAIL A NOTE: 2.90 0.70 1) ALL DIMENSIONS ARE IN MILLIMETERS. 2) EXPOSED PADDLE SIZE DOES NOT INCLUDE MOLD FLASH. 3) LEAD COPLANARITY SHALL BE 0.10 MILLIMETER MAX. 4) DRAWING CONFORMS TO JEDEC MO-229, VARIATION VEED-5. 5) DRAWING IS NOT TO SCALE. 1.70 0.25 2.50 0.50 RECOMMENDED LAND PATTERN NOTICE: The information in this document is subject to change without notice. Users should warrant and guarantee that third party Intellectual Property rights are not infringed upon when integrating MPS products into any application. MPS will not assume any legal responsibility for any said applications. MP4350 Rev. 1.0 5/26/2008 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2008 MPS. All Rights Reserved. 14