MP A, 28V, 1.4MHz Step-Down Converter

Transcription

1 The Future of Analog IC Technology MP8373 3A, 8,.MHz Step-Down Converter DESCRIPTION The MP8373 is a.mhz step-down regulator with a built-in power MOSFET. It achieves 3A continuous output current over a wide input supply range with excellent load and line regulation. Current mode operation provides fast transient response and eases loop stabilization. Fault condition protection includes cycle-bycycle current limiting and thermal shutdown. Adjustable soft-start reduces the stress on the input source at turn-on. In shutdown mode, the regulator draws 0μA of supply current. The MP8373 is available in an 8-pin SOIC package with an exposed pad, and requires a minimum number of readily available external components to complete a 3A step-down DC to DC converter solution. EALUATION BOARD REFERENCE Board Number Dimensions E8373DN-00A.0 x.9 x 0. FEATURES 3A Continuous Output Current, A Peak Output Current Programmable Soft-Start 00mΩ Internal Power MOSFET Switch Stable with Low ESR Output Ceramic Capacitors Up to 9% Efficiency 0μA Shutdown Mode Fixed.MHz Frequency Thermal Shutdown Cycle-by-Cycle Over Current Protection Wide.75 to 8 Operating Input Range Output is Adjustable From 0.9 to Under oltage Lockout APPLICATIONS Distributed Power Systems Battery Chargers Pre-Regulator for Linear Regulators MPS and The Future of Analog IC Technology are Registered Trademarks of Monolithic Power Systems, Inc. TYPICAL APPLICATION PUT OPEN = AUTOMATIC STARTUP C 7 EN 8 C SS GND MP D C3 R3 C5 R L R C PUT 3.3 3A EFFICIENCY (%) Efficiency Curve =5 =3.3 = LOAD CURRENT (A) MP8373 Rev..0 /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

2 MP8373 3A, 8,.MHz STEP-DOWN CONERTER PACKAGE REFERENCE GND 3 EXPOSED PAD CONNECT TO P TOP IEW SS EN AOLUTE MAXIMUM RATGS () Supply oltage to +30 Switch oltage to Boost oltage to + 6 All Other Pins to +6 Junction Temperature...50C Lead Temperature...60C Storage Temperature... 65C to +50C Recommended Operating Conditions () Input oltage to 8 Ambient Operating Temp... 0C to +85C Part Number* Package Temperature MP8373DN SOIC8N 0C to +85C * For Tape & Reel, add suffix Z (eg. MP8373DN Z) For RoHS compliant packaging, add suffix LF (eg. MP8373DN LF Z) Thermal Resistance (3) θ JA θ JC SOIC8N C/W Notes: ) Exceeding these ratings may damage the device. ) The device is not guaranteed to function outside of its operating conditions. 3) Measured on approximately square of oz copper. ELECTRICAL CHARACTERISTICS =, T A = +5C, unless otherwise noted. Parameters Symbol Condition Min Typ Max Units Shutdown Supply Current EN = µa Supply Current EN = 3, =..3.5 ma Feedback oltage.75 8, < Error Amplifier oltage Gain A EA 00 / Error Amplifier Transconductance G EA I = 0μA µa/ High-Side Switch-On Resistance R DS(ON) 00 mω Low-Side Switch-On Resistance R DS(ON) 0 Ω High-Side Switch Leakage Current EN = 0, = µa Short Circuit Current Limit 6.5 A Current Sense to Transconductance G CS 6.0 A/ Oscillation Frequency f S. MHz Short Circuit Oscillation Frequency = 0 80 KHz Maximum Duty Cycle D MAX = % Minimum On Time T ON 30 ns EN Threshold oltage Enable Pull Up Current EN = µa Under oltage Lockout Threshold Rising Under oltage Lockout Threshold Hysteresis 0 m Soft-Start Period C SS = 0.µF 0 ms Thermal Shutdown 60 C MP8373 Rev..0 /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

3 MP8373 3A, 8,.MHz STEP-DOWN CONERTER P FUNCTIONS Pin # Name Description High-Side Gate Drive Boost Input. supplies the drive for the high-side N-Channel MOSFET switch. Connect a 0nF or greater capacitor from to to power the high side switch. Power Input. supplies the power to the IC, as well as the step-down converter switches. Drive with a.75 to 8 power source. Bypass to GND with a suitably large capacitor to eliminate noise on the input to the IC. See Input Capacitor 3 Power Switching Output. is the switching node that supplies power to the output. Connect the output LC filter from to the output load. Note that a capacitor is required from to to power the high-side switch. GND Ground. Connect the exposed pad on backside to Pin. 5 Feedback Input. senses the output voltage to regulate said voltage. Drive with a resistive voltage divider from the output voltage. The feedback threshold is 0.9. See Setting the Output oltage 6 Compensation Node. is used to compensate the regulation control loop. Connect a series RC network from to GND to compensate the regulation control loop. In some cases, an additional capacitor from to GND is required. See Compensation 7 EN 8 SS Enable Input. EN is a digital input that turns the regulator on or off. Drive EN higher than.9 to turn on the regulator, lower than 0.9 to turn it off. For automatic startup, leave EN unconnected. Soft-Start Control Input. SS controls the soft start period. Connect a capacitor from SS to GND to set the soft-start period. A 0.μF capacitor sets the soft-start period to 0ms. ELECTRICAL CHARACTERISTICS =, C = 0µF, C = 7µF, L =.7µH and T A = +5C, unless otherwise noted. 90 Efficiency vs Load Current 8.0 Peak Current vs Duty Cycle 0.95 Feedback oltage vs Die Temperature EFFICIENCY (%) = 5 =.5 = LOAD CURRENT (A) PEAK CURRENT (A) DUTY CYCLE (%) FEEDBACK OLTAGE () TEMPERATURE ( O C) MP8373 Rev /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

4 MP8373 3A, 8,.MHz STEP-DOWN CONERTER ELECTRICAL CHARACTERISTICS (continued) =, C = 0µF, C = 7µF, L =.7µH and T A = +5C, unless otherwise noted. Steady State Operation =.8, I =.5A Steady State Operation =.8, I = 3A AC Coupled 0m/div. AC Coupled 0m/div. AC Coupled 50m/div. 0/div. 0/div. I DUCTOR A/div. I DUCTOR 00ns/div. I DUCTOR 00ns/div. I LOAD A/div. Startup Through Enable = 3.3, I =.5A (Resistance Load) Startup Through Enable = 3.3, I = 3A (Resistance Load) Shutdown Through Enable = 3.3, I =.5A (Resistance Load) EN 5/div. EN 5/div. EN 5/div. /div. /div. /div. 0/div. 0/div. 0/div. I DUCTOR ms/div. I DUCTOR ms/div. I DUCTOR Shutdown Through Enable = 3.3, I = 3A (Resistance Load) EN 5/div. /div. 0/div. I DUCTOR MP8373 Rev..0 /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

5 MP8373 3A, 8,.MHz STEP-DOWN CONERTER OPERATION The MP8373 is a current-mode step-down regulator. It regulates input voltages from.75 to 8 down to an output voltage as low as 0.9, and is able to supply up to 3A of load current. The MP8373 uses current-mode control to regulate the output voltage. The output voltage is measured at through a resistive voltage divider and amplified through the internal error amplifier. The output current of the transconductance error amplifier is presented at where a network compensates the regulation control system. The voltage at is compared to the switch current measured internally to control the output voltage. The converter uses an internal N-Channel MOSFET switch to step-down the input voltage to the regulated output voltage. Since the MOSFET requires a gate voltage greater than the input voltage, a boost capacitor connected between and drives the gate. The capacitor is internally charged while is low. An internal 0Ω switch from to GND is used to insure that is pulled to GND when is low to fully charge the.capacitor. TERNAL REGULATORS CURRENT SENSE AMPLIFIER + 5 OSCILLATOR 80KHz/.MHz SLOPE CLK + + S Q EN 7. SHUTDOWN ARATOR LOCK ARATOR R Q CURRENT ARATOR 3.60/ GND FREQUENCY FOLDBACK ARATOR ERROR AMPLIFIER SS Figure Functional Block Diagram MP8373 Rev /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

6 MP8373 3A, 8,.MHz STEP-DOWN CONERTER APPLICATION FORMATION ONENT SELECTION (Refer to Figure 3) Setting the Output oltage The output voltage is set using a resistive voltage divider from the output voltage to pin. The voltage divider divides the output voltage down to the feedback voltage by the ratio: R R R Where is the feedback voltage and is the output voltage. Thus the output voltage is: R R 0.9 R A typical value for R can be as high as 00kΩ, but a typical value is 0kΩ. Using that value, R is determined by: R 0.87 ( 0.9)(k) 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: Choose an inductor that will not saturate under the maximum inductor peak current. The peak inductor current can be calculated by: I LP ILOAD fs L Where I LOAD is the load current. Table 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. Table Inductor Selection Guide endor/ Model Sumida Core Type Package Dimensions Core (mm) Material W L H CR75 Open Ferrite CDH7 Open Ferrite CDRH5D8 Shielded Ferrite CDRH5D8 Shielded Ferrite CDRH6D8 Shielded Ferrite CDRH0R Shielded Ferrite Toko D53LC Type A Shielded Ferrite D75C Shielded Ferrite D0C Shielded Ferrite D0FL Open Ferrite Coilcraft DO3308 Open Ferrite DO336 Open Ferrite L f ΔI S L Where is the input voltage, f S is the.mhz switching frequency and ΔI L is the peak-to-peak inductor ripple current. MP8373 Rev /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

7 MP8373 3A, 8,.MHz STEP-DOWN CONERTER 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 lists example Schottky diodes and manufacturers. Table Diode Selection Guide oltage/current Manufacture Diode Rating SK33 30, 3A Diodes Inc. SK3 0, 3A Diodes Inc. B330 30, 3A Diodes Inc. B30 0, 3A Diodes Inc. MBRS330 30, 3A On Semiconductor MBRS30 0, 3A On Semiconductor 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 (C) absorbs the input switching current, it requires an adequate ripple current rating. The RMS current in the input capacitor can be estimated by: I C I LOAD The worst-case condition occurs at =, where: ILOAD IC 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.μF, 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: ILOAD fs C Output Capacitor The output capacitor (C) 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: RESR f S L 8 fs C Where L is the inductor value and R ESR 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: Δ 8 fs L C 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: Δ f S L R ESR The characteristics of the output capacitor also affect the stability of the regulation system. The MP8373 can be optimized for a wide range of capacitance and ESR values. MP8373 Rev /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

8 MP8373 3A, 8,.MHz STEP-DOWN CONERTER Compensation Components MP8373 employs current mode control for easy compensation and fast transient response. The system stability and transient response are controlled through the pin. 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. The DC gain of the voltage feedback loop is given by: A DC R LOAD G CS A EA Where A EA is the error amplifier voltage gain, G CS is the current sense transconductance and R LOAD is the load resistor value. 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: f f P P GEA C3 A C R EA LOAD Where G EA is the error amplifier transconductance, 530μA/. The system has one zero of importance, due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at: f Z 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: f ESR C R ESR In this case, 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 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 the crossover frequency to approximately one-tenth of the switching frequency or lower. The switching frequency for the MP8373 is.mhz, so the desired crossover frequency is equal to or less than 0KHz. 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 alues for Typical Output oltage/capacitor Combinations () L (µh) C (µf, Ceramic) R3 (kω) C3 (nf) C None None None x 0.7 None x.. None MP8373 Rev /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

9 MP8373 3A, 8,.MHz STEP-DOWN CONERTER To optimize the compensation components for conditions not listed in Table 3, the following procedure can be used.. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation: R3 C f G G EA C CS Where f C is the desired crossover frequency.. Choose the compensation capacitor (C3) to achieve the desired phase margin. For applications with typical inductor values, setting the compensation zero, f Z, below one forth of the crossover frequency provides sufficient phase margin. Determine the C3 value by the following equation: C3 R3 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.mhz switching frequency, or the following relationship is valid: C R f C f S ESR If this is the case, then add the second compensation capacitor (C6) to set the pole f P3 at the location of the ESR zero. Determine the C6 value by the equation: C RESR C6 R3 External Bootstrap Diode It is recommended that an external bootstrap diode be added when the system has a 5 fixed input or the power supply generates a 5 output. This helps improve the efficiency of the regulator. The bootstrap diode can be a low cost one such as 8 or BAT5. MP nF Figure External Bootstrap Diode This diode is also recommended for high duty cycle operation (when >65%) and high output voltage ( >) applications. MP8373 Rev /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

10 MP8373 3A, 8,.MHz STEP-DOWN CONERTER TYPICAL APPLICATION CIRCUITS PUT.75 to 8 OPEN = AUTOMATIC STARTUP 7 8 EN SS GND MP8373 C6 OPEN C3.7nF C5 0nF D B330A PUT.5 3A Figure 3.5 Output Typical Application Schematic PUT.75 to 8 OPEN = AUTOMATIC STARTUP 7 8 EN SS GND MP8373 C6 OPEN C3 5.6nF C5 0nF D B330A PUT 3.3 3A Figure 3.3 Output Typical Application Schematic MP8373 Rev /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.

11 MP8373 3A, 8,.MHz STEP-DOWN CONERTER PACKAGE FORMATION SOIC8N (EXPOSED PAD) 0.89(.80) 0.97(5.00) (3.5) 0.36(3.5) P ID 0.50(3.80) 0.57(.00) 0.8(5.80) 0.(6.0) 0.089(.6) 0.0(.56) TOP IEW BOTTOM IEW SEE DETAIL "A" 0.03(0.33) 0.00(0.5) 0.05(.30) 0.067(.70) SEATG PLANE 0.000(0.00) 0.006(0.5) 0.050(.7) C SIDE IEW (0.9) (0.5) FRONT IEW 0.00(0.5) 0.00(0.50) x 5 o GAUGE PLANE 0.00(0.5) C 0.0(0.6) 0.063(.60) 0.050(.7) 0 o -8 o 0.06(0.) 0.050(.7) DETAIL "A" 0.38(3.5) 0.03(.6) RECOMMENDED LAND PATTERN 0.3(5.0) NOTE: ) CONTROL DIMENSION IS CHES. DIMENSION BRACKET IS MILLIMETERS. ) PACKAGE LENGTH DOES NOT CLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. 3) PACKAGE WIDTH DOES NOT CLUDE TERLEAD FLASH OR PROTRUSIONS. ) LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMG) SHALL BE 0.00" CHES MAX. 5) DRAWG CONFORMS TO JEDEC MS-0, ARIATION BA. 6) DRAWG 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. MP8373 Rev..0 /8/0 MPS Proprietary Information. Patent Protected. Unauthorized Photocopy and Duplication Prohibited. 0 MPS. All Rights Reserved.