MP2378DS-LF

TM MP2378 2A, 30V, 385kHz Step-Down Converter The Future of Analog IC Technology TM DESCRIPTION FEATURES The MP2378 is a monolithic step-down switch mode converter with a built-in internal power MOSFET. It achieves 2A continuous output current over a wide input supply range with excellent load and line regulation. x x x x x x x x x x x x Current mode operation provides fast transient response and eases loop stabilization. Fault condition protection includes cycle-by-cycle current limiting and thermal shutdown. In shutdown mode the regulator draws 23ȝA of supply current. Programmable soft-start minimizes the inrush supply current and the output overshoot at initial startup. APPLICATIONS The MP2378 requires a minimum number of readily available standard external components. x x x x EVALUATION BOARD REFERENCE Board Number Dimensions EV2378DS-00A 2.3”X x 1.4”Y x 0.5”Z 2A Output Current 0.28ȍ Internal Power MOSFET Switch Stable with Low ESR Output Ceramic Capacitors 95% Efficiency 23ȝA Shutdown Mode Fixed 385kHz Frequency Thermal Shutdown Cycle-by-Cycle Over Current Protection Wide 4.75V to 30V Operating Input Range Output Adjustable from 0.92V to 16V Programmable Under Voltage Lockout Available in a SO8 Package Distributed Power Systems Battery Charger DSL Modems Pre-Regulator for Linear Regulators All MPS parts are lead-free and adhere to the RoHS directive. For MPS green status, please visit MPS website under Quality Assurance. “MPS” and “The Future of Analog IC Technology” are Trademarks of Monolithic Power Systems, Inc. TYPICAL APPLICATION Efficiency vs Output Current INPUT 4.75V - 30V 95 C5 10nF OPEN = AUTOMATIC STARTUP SW EN MP2378 SS GND C4 10nF 90 BS FB COMP C3 3.9nF D1 B220A VOUT 3.3V/2A EFFICIENCY (%) IN 5.0V 3.3V 85 2.5V 80 75 70 65 60 0 0.5 1.0 1.5 2.0 2.5 OUTPUT CURRENT (A) MP2378_EC01 MP2378 Rev. 1.31 1/23/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 1 TM MP2378 – 2A, 30V, 385kHz STEP-DOWN CONVERTER ABSOLUTE MAXIMUM RATINGS (1) PACKAGE REFERENCE Supply Voltage (VIN) .................................... 32V Switch Node Voltage (VSW).......................... 33V Bootstrap Voltage (VBS) ....................... VSW + 6V Feedback Voltage (VFB) ................. –0.3V to +6V Enable/UVLO Voltage (VEN)........... –0.3V to +6V Comp Voltage (VCOMP) ................... –0.3V to +6V SS Voltage (VSS)............................ –0.3V to +6V Junction Temperature.............................+150qC Lead Temperature ..................................+260qC Storage Temperature ..............–65°C to +150qC TOP VIEW BS 1 8 SS IN 2 7 EN SW 3 6 COMP GND 4 5 FB Recommended Operating Conditions (2) Supply Voltage (VIN) ...................... 4.75V to 30V Operating Temperature.................–40qC to +85qC MP2378_PD01_SOIC8 Thermal Resistance (3) șJA șJC SOIC8.................................... 105 ..... 50... qC/W * Part Number* Package Temperature MP2378DS SOIC8 –40qC to +85qC For Tape & Reel, add suffix –Z (eg. MP2378DS–Z) For Lead Free, add suffix –LF (eg. MP2378DS––LF–Z) 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, TA = +25qqC, unless otherwise noted. Parameter Symbol Condition Feedback Voltage Upper Switch On Resistance Lower Switch On Resistance Upper Switch Leakage Current Limit (4) Current Sense Transconductance Output Current to Comp Pin Voltage Error Amplifier Voltage Gain Error Amplifier Transconductance Oscillator Frequency Short Circuit Frequency Soft-Start Pin Equivalent Output Resistance Maximum Duty Cycle Minimum On Time Enable Threshold Enable Pull Up Current MP2378 Rev. 1.31 1/23/2014 VFB 4.75V d VIN d 30V Min Typ Max Units 0.892 0.920 0.948 V 2.6 0.28 6 0 3.4 RDS(ON)1 RDS(ON)2 VEN = 0V, VSW = 0V 10 ȍ ȍ ȝA A GCS 1.95 A/V AVEA 400 V/V GEA 'IC = r10ȝA fS 650 950 1250 ȝA/V 340 385 270 450 KHz KHz VFB = 0V DMAX tON VFB = 0.8V ICC > 100ȝA VEN = 0V 0.7 9 kȍ 90 100 1.0 1.0 % ns V ȝA www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 1.3 2 TM MP2378 – 2A, 30V, 385kHz STEP-DOWN CONVERTER ELECTRICAL CHARACTERISTICS (continued) VIN = 12V, TA = +25qC, unless otherwise noted. Parameter Under Voltage Lockout Threshold Rising Under Voltage Lockout Threshold Hysteresis Symbol Condition Min Typ Max Units 2.37 2.50 2.62 V 210 mV Supply Current (Shutdown) VEN d 0.4V 23 36 ȝA Supply Current (Quiescent) VEN t 3V 1.1 1.3 mA Thermal Shutdown 160 qC Note: 4) Slope compensation changes current limit above 40% duty cycle. PIN FUNCTIONS QFN10 SOIC8 Pin # Pin # 1 2 1 3 4 2 5 6 3 4 7 5 8 6 9 7 10 8 MP2378 Rev. 1.31 1/23/2014 Name Description NC BS No Connect. Bootstrap. This capacitor (C5) is needed to drive the power switch’s gate above the supply voltage. It is connected between the SW and BS pins to form a floating supply across the power switch driver. The voltage across C5 is about 5V and is supplied by the internal +5V supply when the SW pin voltage is low. NC No Connect. IN Supply Voltage. The MP2378 operates from a +4.75V to +30V unregulated input. C1 is needed to prevent large voltage spikes from appearing at the input. SW Switch. This connects the inductor to either IN through M1 or to GND through M2. GND Ground. This pin is the voltage reference for the regulated output voltage. For this reason care must be taken in its layout. This node should be placed outside of the D1 to C1 ground path to prevent switching current spikes from inducing voltage noise into the part. FB Feedback. An external resistor divider from the output to GND, tapped to the FB pin, sets the output voltage. To prevent current limit runaway during a short circuit fault condition the frequency foldback comparator lowers the oscillator frequency when the FB voltage is below 400mV. COMP Compensation. This node is the output of the transconductance error amplifier and the input to the current comparator. Frequency compensation is done at this node by connecting a series R-C to ground. See the compensation section for exact details. EN Enable/UVLO. A voltage greater than 2.62V enables operation. Leave EN unconnected if unused. An Under Voltage Lockout (UVLO) function can be implemented by the addition of a resistor divider from VIN to GND. For complete low current shutdown the EN pin voltage needs to be less than 700mV. SS Soft-Start. Connect SS to an external capacitor to program the soft-start. If unused, leave it open. www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 3 TM MP2378 – 2A, 30V, 385kHz STEP-DOWN CONVERTER OPERATION MP2378 reverts to its initial M1 off, M2 on state. If the Current Sense Amplifier plus Slope Compensation signal does not exceed the COMP voltage, then the falling edge of the CLK resets the Flip-Flop. The MP2378 is a current mode regulator. That is, the COMP pin voltage is proportional to the peak inductor current. At the beginning of a cycle: the upper transistor M1 is off; the lower transistor M2 is on (see Figure 1); the COMP pin voltage is higher than the current sense amplifier output; and the current comparator’s output is low. The rising edge of the 385kHz CLK signal sets the RS Flip-Flop. Its output turns off M2 and turns on M1 thus connecting the SW pin and inductor to the input supply. The increasing inductor current is sensed and amplified by the Current Sense Amplifier. Ramp compensation is summed to Current Sense Amplifier output and compared to the Error Amplifier output by the Current Comparator. When the Current Sense Amplifier plus Slope Compensation signal exceeds the COMP pin voltage, the RS Flip-Flop is reset and the The output of the Error Amplifier integrates the voltage difference between the feedback and the 0.92V bandgap reference. The polarity is such that a FB pin voltage lower than 0.92V increases the COMP pin voltage. Since the COMP pin voltage is proportional to the peak inductor current an increase in its voltage increases current delivered to the output. The lower 6ȍ switch ensures that the bootstrap capacitor voltage is charged during light load conditions. External Schottky Diode D1 carries the inductor current when M1 is off. IN CURRENT SENSE AMPLIFIER INTERNAL REGULATORS OSCILLATOR 270KHz/ 385KHz SLOPE COMP 1.0V EN -2.50V/ 2.29V + FREQUENCY FOLDBACK COMPARATOR 5V -BS CLK + -- + + SHUTDOWN COMPARATOR -- S Q R Q CURRENT COMPARATOR SW LOCKOUT COMPARATOR -- + -- 0.4V 0.92V FB + SS 1.8V GND ERROR AMPLIFIER COMP MP2378_BD01 Figure 1—Functional Block Diagram MP2378 Rev. 1.31 1/23/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 4 TM MP2378 – 2A, 30V, 385kHz STEP-DOWN CONVERTER APPLICATION INFORMATION ILP 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 Where VFB is the feedback voltage and VOUT is the output voltage. Thus the output voltage is: VOUT 0.92 u R1  R2 R2 A typical value for R2 can be as high as 100kȍ, but a typical value is 10kȍ. Using that value, R1 is determined by: R1 10.87 u ( VOUT  0.92) For example, for a 3.3V output voltage, R2 is 10kȍ, and R1 is 25.8kȍ. 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. 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: L § · VOUT V u ¨¨1  OUT ¸¸ fS u ǻIL © VIN ¹ Where fS is the switching frequency, ǻIL is the peak-to-peak inductor ripple current and VIN is the input voltage. Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by: MP2378 Rev. 1.31 1/23/2014 ILOAD  § VOUT V u ¨1  OUT 2 u f S u L ¨© VIN · ¸¸ ¹ Where ILOAD is the load current. 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. 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. Since the input capacitor absorbs the input switching current it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by: I C1 ILOAD u VOUT §¨ VOUT u 1 VIN ¨© VIN · ¸ ¸ ¹ The worst-case condition occurs at VIN = 2VOUT, where: IC1 ILOAD 2 For simplification, choose the input capacitor whose RMS current rating greater than half of the maximum load current. The input capacitor can be electrolytic, tantalum or ceramic. When using electrolytic or tantalum capacitors, a small, high quality ceramic capacitor, i.e. 0.1ȝF, 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: www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 5 TM 'VIN MP2378 – 2A, 30V, 385kHz STEP-DOWN CONVERTER § ILOAD V V u OUT u ¨1  OUT fS u C1 VIN ¨© VIN The DC gain of the voltage feedback loop is given by: · ¸¸ ¹ Where C1 is the input capacitance value. Output Capacitor The output capacitor 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 u ¨1  OUT f S u L ¨© VIN · · § 1 ¸ ¸¸ u ¨ R ESR  ¨ 8 u f S u C2 ¸¹ ¹ © Where L is the inductor value, RESR is the equivalent series resistance (ESR) value of the output capacitor and C2 is the output capacitance value. 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: ǻVOUT § V u ¨¨1  OUT VIN u L u C2 © VOUT 8 u fS 2 · ¸¸ ¹ 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: ǻVOUT V VOUT § u ¨1  OUT f S u L ¨© VIN · ¸¸ u R ESR ¹ The characteristics of the output capacitor also affect the stability of the regulation system. The MP2378 can be optimized for a wide range of capacitance and ESR values. Compensation Components The MP2378 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 transconductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of the control system. MP2378 Rev. 1.31 1/23/2014 A VDC R LOAD u G CS u A VEA u VFB VOUT Where RLOAD is the load resistor value, GCS is the current sense transconductance and AVEA is the error amplifier voltage gain. The system has two poles of importance. One is due to the compensation capacitor (C3) and the output resistor of error amplifier, and the other is due to the output capacitor and the load resistor. These poles are located at: fP1 fP2 GEA 2S u C3 u A VEA 1 2S u C2 u R LOAD Where GEA is transconductance. the 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 2S u C3 u 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 2S u C2 u R ESR In this case, a third pole set by compensation capacitor (C6) and compensation resistor (R3) is used compensate the effect of the ESR zero on loop gain. This pole is located at: f P3 the the to the 1 2S u C6 u 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. www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 6 TM MP2378 – 2A, 30V, 385kHz STEP-DOWN CONVERTER Lower crossover frequencies result in slower line and load transient responses, while higher crossover frequencies could cause the system to become unstable. A good rule of thumb is to set the crossover frequency to below one-tenth of the switching frequency. To optimize the compensation components, the following procedure can be used: PCB Layout Guide PCB layout is very important to achieve stable operation. Please follow these guidelines and take Figure2 and 3 for references. 1) Keep the path of switching current short and minimize the loop area formed by Input cap, high-side MOSFET and schottky diode. 1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation: 2) Keep the connection of schottky diode between SW pin and input power ground as short and wide as possible. 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. For single layer, do not solder exposed pad of the IC. R3 2S u C2 u f C VOUT u G EA u G CS VFB Where fC is the desired crossover frequency, which is typically less than one tenth of the switching frequency. 2. Choose the compensation capacitor (C3) to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, fZ1, to below one forth of the crossover frequency provides sufficient phase margin. Determine the C3 value by the following equation: C3 ! 2 S u R3 u f C Where R3 is the compensation resistor value. 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 2S u C2 u 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 MP2378 Rev. 1.31 1/23/2014 Figure2ʊPCB Layout for Single Layer C2 u R ESR R3 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 7 TM MP2378 – 2A, 30V, 385kHz STEP-DOWN CONVERTER FB 5 4 GND 6 COMP SW 3 EN 7 IN 2 SS/REF 8 R2 1 BS R1 Top Layer Bottom Layer Figure3ʊPCB Layout for Double Layer External Bootstrap Diode An external bootstrap diode may enhance the efficiency of the regulator, the applicable conditions of external BST diode are: z VOUT=5V or 3.3V; VOUT z Duty cycle is high: D ! 65% VIN In these cases, an external BST diode is recommended from the output of the voltage regulator to BST pin, as shown in Fig.4. External BST Diode IN4148 BST MP2378 SW CBST L 5V or 3.3V COUT Figure 4—External Bootstrap Diode The recommended external BST diode is IN4148, and the BST cap is 0.1~1μF. MP2378 Rev. 1.31 1/23/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 8 TM MP2378 – 2A, 30V, 385kHz STEP-DOWN CONVERTER PACKAGE INFORMATION SOIC8 NOTICE: The information in this document is subject to change without notice. Please contact MPS for current specifications. 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. MP2378 Rev. 1.31 1/23/2014 www.MonolithicPower.com MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. © 2014 MPS. All Rights Reserved. 9