MP1586 3A, 28V, 50kHz to 400kHz Step-Down Converter The Future of Analog IC Technology DESCRIPTION FEATURES The MP1586 is a high frequency step-down switching regulator with an integrated internal high-side high voltage power MOSFET. It provides 3A output with current mode control for fast loop response and easy compensation. • • The wide 4.5V to 28V input range accommodates a variety of step-down applications, including those in an automotive input environment. A 115µA 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. APPLICATIONS The frequency foldback helps prevent inductor current runaway during startup and thermal shutdown provides reliable, fault tolerant operation. • • • • • • • Wide 4.5V to 28V Operating Input Range Programmable Switching Frequency from 50kHz to 400kHz High-Efficiency Pulse Skipping Mode for Light Load Ceramic Capacitor Stable Internal Soft-Start Internally Set Current Limit without a Current Sensing Resistor Available in SOIC8E Package. High Voltage Power Conversion Automotive Systems Industrial Power Systems Distributed Power Systems Battery Powered Systems “MPS” and “The Future of Analog IC Technology” are Registered Trademarks of Monolithic Power Systems, Inc. The MP1586 is available in a thermally enhanced SOIC8E package. TYPICAL APPLICATION Efficiency vs. Load Current C4 100nF 100 VOUT=5V,FSW=250kHz 8 VIN BST SW 1 D1 EN 2 6 EN MP1586 FB COMP FREQ GND 5 4 3 C3 820pF C6 NS VIN=8V 90 VOUT 5V EFFICIENCY (%) VIN 7 VIN=12V 80 70 VIN=24V 60 50 40 0.01 0.1 1 10 LOAD CURRENT(A) MP1586 Rev. 0.91 1/8/2010 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 1 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER ORDERING INFORMATION Part Number* Package Top Marking Free Air Temperature (TA) MP1586EN SOIC8E MP1586EN –20°C to +85°C * For Tape & Reel, add suffix –Z (e.g. MP1586EN–Z); For RoHS Compliant Packaging, add suffix –LF. (e.g. MP1586EN–LF–Z) PACKAGE REFERENCE TOP VIEW SW 1 8 BST EN 2 7 VIN COMP 3 6 FREQ FB 4 5 GND (3) ABSOLUTE MAXIMUM RATINGS (1) Recommended Operating Conditions Supply Voltage (VIN).....................–0.3V to +30V Switch Voltage (VSW).............–0.3V to 30 + 0.3V BST to SW .....................................–0.3V to +6V All Other Pins .................................–0.3V to +6V Continuous Power Dissipation (TA = +25°C)(2) ............................................................. 2.5W Junction Temperature ...............................150°C Lead Temperature ....................................260°C Storage Temperature.............. –65°C to +150°C Supply Voltage VIN ........................... 4.5V to 28V Output Voltage VOUT ........................ 0.8V to 25V Operating Junct. Temp. (TJ) ....–20°C to +125°C MP1586 Rev. 0.91 1/8/2010 Thermal Resistance (4) θJA θJC SOIC8E .................................. 50 ...... 10... °C/W Notes: 1) Exceeding these ratings may damage the device. 2) The maximum allowable power dissipation is a function of the maximum junction temperature TJ(MAX), the junction-toambient thermal resistance θJA, and the ambient temperature TA. The maximum allowable continuous power dissipation at any ambient temperature is calculated by PD(MAX)=(TJ(MAX)TA)/ θJA. Exceeding the maximum allowable power dissipation will cause excessive die temperature, and the regulator will go into thermal shutdown. Internal thermal shutdown circuitry protects the device from permanent damage. 3) The device is not guaranteed to function outside of its operating conditions. 4) Measured on JESD51-7, 4-layer PCB. www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 2 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER ELECTRICAL CHARACTERISTICS VIN = 12V, VEN = 2.5V, VCOMP = 1.4V, TA= +25°C, unless otherwise noted. Parameter Symbol Condition Feedback Voltage Upper Switch On Resistance(5) Upper Switch Leakage Current Limit COMP to Current Sense Transconductance(5) Error Amp Voltage Gain (5) Error Amp Transconductance Error Amp Min Source current Error Amp Min Sink current VIN UVLO Threshold VIN UVLO Hysteresis Soft-Start Time (5) VFB RDS(ON) 4.5V < VIN < 28V VBST – VSW = 5V VEN = 0V, VSW = 0V, VIN = 28V Min Typ Max Units 0.776 0.8 150 1 4.7 0.824 V mΩ μA A 4.0 GCS 9 ICOMP = ±3µA VFB = 0.7V VFB = 0.9V 40 2.7 0V < VFB < 0.8V Oscillator Frequency RFREQ =100kΩ Shutdown Supply Current Quiescent Supply Current VEN = 0V No load, VFB = 0.9V 200 Thermal Shutdown Thermal Shutdown Hysteresis (5) Minimum Off Time Minimum On Time (5) EN Up Threshold EN Hysteresis 1.35 200 60 5 –5 3.0 0.35 1.5 A/V 80 3.3 V/V µA/V µA µA V V ms 250 300 kHz 12 115 20 145 µA µA 150 °C 15 °C ns ns V mV 100 100 1.5 300 1.65 Note: 5) Guaranteed by design. MP1586 Rev. 0.91 1/8/2010 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 3 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER PIN FUNCTIONS SOIC Pin # Name 1 SW 2 EN 3 COMP 4 FB 5 6 7 8 Description Switch Node. This is the output from the high-side switch. A low forward drop Schottky diode to 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 above the specified threshold or leaving it floating enables the chip. Compensation. This node is the output of the error amplifier. Control loop frequency compensation is applied to this pin. Feedback. This is the input to the error amplifier. The output voltage is set by a resistive divider connected between the output and GND which scales down VOUT equal to the internal +0.8V reference. GND Ground. It should be connected as close as possible to the output capacitor to shorten the high Exposed current switch paths. Connect exposed pad to GND plane for optimal thermal performance. Pad 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, both BS regulators and the VIN 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. MP1586 Rev. 0.91 1/8/2010 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 4 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER TYPICAL PERFORMANCE CHARACTERISTICS VIN = 12V, VOUT=5V, FSW=250kHz, C1 = 10µF, C2 = 2×22µF, L1= 22µH and TA = +25°C, unless otherwise noted. Efficiency vs. Efficiency vs. Load Current Load Current 100 VOUT=2.5V, FSW=250kHz 100 VIN=5V VIN=8V 90 80 EFFICIENCY (%) EFFICIENCY (%) 90 VOUT=3.3V, FSW=250kHz VIN=12V 70 VIN=24V 60 50 VIN=12V 80 70 VIN=24V 60 50 40 0.01 0.1 1 10 40 0.01 0.1 1 10 LOAD CURRENT(A) LOAD CURRENT(A) OSCILLATING FREQUENCY (kHz) Oscillating Frequency vs. RFREQ 450 400 350 300 250 200 150 100 50 0 10 MP1586 Rev. 0.91 1/8/2010 100 1000 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 5 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER TYPICAL PERFORMANCE CHARACTERISTICS (continued) VIN = 12V, VOUT=5V, FSW=250kHz, C1 = 10µF, C2 = 2×22µF, L1 = 22µH, and TA = +25°C, unless otherwise noted. Steady State Steady State Steady State IOUT =0.1A IOUT =1A IOUT =3A VOUT/AC 10mV/div. VOUT/AC 10mV/div. VOUT/AC 10mV/div. VSW 10V/div. VSW 10V/div. VSW 10V/div. IL 1A/div. IL 1A/div. IL 2A/div. EN Startup EN Startup EN Startup IOUT =0.1A IOUT =1A IOUT =3A VOUT 2V/div. VOUT 2V/div. VOUT 2V/div. VEN 5V/div. VSW 10V/div. VEN 5V/div. VSW 10V/div. VEN 5V/div. VSW 10V/div. IL 1A/div. IL 1A/div. IL 2A/div. EN Shut Down EN Shut Down EN Shut Down IOUT =0.1A IOUT =1A IOUT =3A VOUT 2V/div. VEN 5V/div. VSW 10V/div. IL 1A/div. MP1586 Rev. 0.91 1/8/2010 VOUT 2V/div. VEN 5V/div. VSW 10V/div. IL 1A/div. VOUT 2V/div. VEN 5V/div. VSW 10V/div. IL 2A/div. www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 6 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER BLOCK DIAGRAM VIN VIN EN REFERENCE UVLO/ THERMAL SHUTDOWN 5V + -2.6V INTERNAL REGULATORS + -BST SW 1.5ms SS VOUT SS -- ISW + ISW Level Shift FB SW Gm Error Amp SS 0V8 -- COMP + OSCILLATOR CLK VOUT COMP GND FREQ Figure 1—Function Block Diagram OPERATION The MP1586 is a non-synchronous, step-down switching regulator with an integrated high-side high voltage power MOSFET. It provides a 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. MP1586 Rev. 0.91 1/8/2010 PWM Control At moderate to high output current, the MP1586 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. www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 7 MP1586 – 3A, 28V, 50kHz TO 400kHz 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 MP1586 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. 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. 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. MP1586 Rev. 0.91 1/8/2010 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 hysteresis of 150mV. 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. www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 8 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER 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. MP1586 Rev. 0.91 1/8/2010 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, 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 MP1586 oscillating frequency is set by an external resistor, Rfreq from the FREQ pin to ground. The value of Rfreq can be calculated from: 180000 R freq (kΩ ) = 1.1 ⎡⎣3.7 × fs ( kHz ) ⎤⎦ www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 9 MP1586 – 3A, 28V, 50kHz TO 400kHz 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 MP1586 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. MP1586 Rev. 0.91 1/8/2010 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 × ΔIL ⎜⎝ VIN ⎠ Where VOUT is the output voltage, VIN is the input voltage, fS is the switching frequency, and ΔIL 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. © 2010 MPS. All Rights Reserved. 10 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER Table 1—Inductor Selection Guide Inductance (µH) Max DCR (Ω) Current Rating (A) Dimensions L x W x H (mm3) 744066100 10 0.035 3.6 10x10x3.8 744770115 15 0.027 5 12x12x8 744770122 22 0.043 4.1 12x12x8 SLF12565T-100M4R8 10 0.0202 4.8 12.5x12.5x6.5 SLF12565T-150M4R2 15 0.0237 4.2 12.5x12.5x6.5 SLF12565T-220M3R5 22 0.0316 3.5 12.5x12.5x6.5 919AS-100M 10 0.0265 4.3 10.3x10.3x4.5 931BS-150M 15 0.0158 4.6 12.3x12.3x8 931BS-220M 22 0.0283 3.9 12.3x12.3x8 Part Number Wurth Electronics TDK 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. Table 2—Diode Selection Guide Diodes B340A-13-F CMSH3-40MA Voltage/ Current Rating 40V, 3A 40V, 3A Manufacturer Diodes Inc. Central Semi 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. 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: ΔVIN = MP1586 Rev. 0.91 1/8/2010 ⎛ V ILOAD V × OUT × ⎜1 − OUT VIN fS × C1 VIN ⎜⎝ www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. ⎞ ⎟ ⎟ ⎠ 11 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER 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: ΔVOUT = 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: ΔVOUT = ⎞ ⎛ 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: ΔVOUT = 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 MP1586 can be optimized for a wide range of capacitance and ESR values. Compensation Components MP1586 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 × MP1586 Rev. 0.91 1/8/2010 VFB VOUT Where AVEA is the error amplifier voltage gain, is the current sense 200V/V; GCS transconductance, 9A/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 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 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 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 12 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER the crossover frequency to approximately onetenth of the switching frequency. f 1 < S 2π × C2 × R ESR 2 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. 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: Table 3—Compensation Values for Typical Output Voltage/Capacitor Combinations 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. VOUT (V) L (µH) C2 (µF) R3 (kΩ) C3 (pF) C6 1.8 10 2×22 24 820 None 2.5 15 2×22 36 680 None 3.3 15 2×22 47 680 None 5 22 2×22 47 820 None 12 33 2×22 100 820 None C6 = C2 × R ESR R3 5V BS To optimize the compensation components, the following procedure can be used. 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation: R3 = 2π × C2 × f C VOUT × G EA × G CS VFB Where fC is the desired crossover frequency. 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 > MP1586 SW Figure 2—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. 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: MP1586 Rev. 0.91 1/8/2010 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 13 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER TYPICAL APPLICATION CIRCUITS C4 100nF 8 VIN EN 7 2 6 VIN EN BST SW FB MP1586 COMP FREQ L1 10uH 1 C2 2x22uF 6.3V 4 VOUT 1.8V 3 C3 820pF GND 5 C6 NS Figure 3—1.8V Output Typical Application Schematic C4 100nF 8 VIN 7 VIN BST SW L1 15uH 1 VOUT 3.3V D1 EN 2 6 EN MP1586 FB COMP FREQ GND 5 4 3 C3 680pF C6 NS Figure 4—3.3V Output Typical Application Schematic MP1586 Rev. 0.91 1/8/2010 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 14 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER PCB LAYOUT GUIDE PCB layout is very important to achieve stable operation. It is highly recommended to duplicate EVB layout for optimum performance. If change is necessary, please follow these guidelines and take Figure 5 for reference. 1) 2) Keep the path of switching current short and minimize the loop area formed by Input cap, high-side MOSFET and external switching diode. Bypass ceramic capacitors are suggested to be put close to the VIN Pin. 3) Ensure all feedback connections are short and direct. Place the feedback resistors and compensation components as close to the chip as possible. 4) Route SW away from sensitive analog areas such as FB. 5) Connect IN, SW, and especially GND respectively to a large copper area to cool the chip to improve thermal performance and long-term reliability. C4 VIN VIN C1 BST L1 VOUT SW D1 R5 EN EN MP1586 C2 FB R1 R2 R4 COMP FREQ GND R6 C3 R3 MP1586 Typical Application Circuit Top Layer Bottom Layer Figure 5―MP1586 Typical Application Circuit and PCB Layout Guide MP1586 Rev. 0.9 1/8/2010 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 15 MP1586 – 3A, 28V, 50kHz TO 400kHz STEP-DOWN CONVERTER PACKAGE INFORMATION SOIC8E (EXPOSED PAD) 0.189(4.80) 0.197(5.00) 0.124(3.15) 0.136(3.45) 8 5 0.150(3.80) 0.157(4.00) PIN 1 ID 1 0.228(5.80) 0.244(6.20) 0.089(2.26) 0.101(2.56) 4 TOP VIEW BOTTOM VIEW SEE DETAIL "A" 0.051(1.30) 0.067(1.70) SEATING PLANE 0.000(0.00) 0.006(0.15) 0.013(0.33) 0.020(0.51) 0.0075(0.19) 0.0098(0.25) SIDE VIEW 0.050(1.27) BSC FRONT VIEW 0.010(0.25) x 45o 0.020(0.50) GAUGE PLANE 0.010(0.25) BSC 0.050(1.27) 0.024(0.61) 0o-8o 0.016(0.41) 0.050(1.27) 0.063(1.60) DETAIL "A" 0.103(2.62) 0.138(3.51) RECOMMENDED LAND PATTERN 0.213(5.40) NOTE: 1) CONTROL DIMENSION IS IN INCHES. DIMENSION IN BRACKET IS IN MILLIMETERS. 2) PACKAGE LENGTH DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. 3) PACKAGE WIDTH DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. 4) LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.004" INCHES MAX. 5) DRAWING CONFORMS TO JEDEC MS-012, VARIATION BA. 6) DRAWING IS NOT TO SCALE. 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. MP1586 Rev. 0.91 1/8/2010 www.MonolithicPower.com MPS Proprietary Information. Unauthorized Photocopy and Duplication Prohibited. © 2010 MPS. All Rights Reserved. 16