Design Note – DN06075/D 12V Input, 5A DC-DC PWM Buck Controller + FET w/Ceramic Capacitors on the Output Device NCP3011 Application Test & Medical Equipment Input Voltage Output Voltage Output Current Topology 9-18V 1.8V 0.01-5A Buck Table 1: Buck Power Supply Characteristic Input Voltage Output Voltage Output Current Oscillator Frequency Output Voltage Ripple Min 9 .01 350 Circuit Description Typ 12 1.8 3 400 10 Max 18 5 450 Unit V V A kHz mVpk-pk Key Features This circuit is proposed for a wide varying +12V input (9V-18V) where there is a need to step-down the voltage to +5V @ 5A. The requirement specified a low output voltage ripple and all ceramic output capacitors for low noise environments such as medical devices and automated test equipment. Input capacitance can also be optimized for all ceramic but in this demo board configuration two electrolytic capacitors are used. Target efficiency is >85%. The PCB for the NCP3011 is a 2-layer board for use in applications up to 50W. The synchronous buck converter uses voltage mode control, which can be compensated externally with a transconductance amplifier and type-III compensation which enables ceramic capacitors on the output. The soft start time is fixed. The NCP3011 demonstration board is a flexible design allowing the use of electrolytic capacitors or ceramic capacitors. It also allows the use of SO8-FL or D-PAK MOSFETs. High Efficiency Adjustable Current Limit Output Overvoltage and Output Undervoltage protection Short Circuit Protection Fixed Switching Frequency Rev 0, January 2011 Theory of Operation One feature of the NCP3011 controller is that when one follows a few simple design rules, the transconductance amplifier can be employed as a voltage feedback Error Amplifier. Theoretically, a transconductance amplifier is an equivalent voltage controlled current source. It multiplies the difference of input voltage with a certain gain and generates a current into the output node. It features high output impedance and it is stable by most of the output compensation components. The goal of the design is to provide a loop gain function with a high bandwidth (high zero-crossover frequency) and adequate phase margin. As a result, fast load response and good steady state output can be achieved. • • • Used when loop bandwidth is beyond the LC resonance and ceramic capacitors are used Rc1 > 2/gm (required) 1/gm > Rf1//Rf2//Rf3 (desired) • When conditions met (especially Rc1 > 2/gm), behavior is similar to a voltage amp Rev 0, January 2011 • • • • Measured Loop Response PM = 52 o f_cross = 24.6 kHz f0 ~ 3-4 kHz • Predicted Loop Response (utilizing CompCalc design tool: http://www.onsemi.com/pub/Collateral/COMPCALC.ZIP ) PM = 67.9 o f_cross = 25.1 kHz f0 ~ 3.4 kHz • • • Rev 0, January 2011 • • • • Measured Compensator Response Max Phase Boost: 128o @15 kHz fz1/z2 : ~ 2-5 kHz fp1/p2 : ~ 50-70 kHz • Predicted Compensator Response (utilizing CompCalc design tool: http://www.onsemi.com/pub/Collateral/COMPCALC.ZIP ) Max Phase Boost: 125o @ ~20 kHz fz1/z2 : ~ 3-5 kHz fp1/p2 : ~ 200 kHz • • • Note: CompCalc files with these values pre-loaded are available from the NCP3011 web page under “Design & Development Tools” Rev 0, January 2011 Performance The following figures show typical performance of the NCP3011 demonstration boards. Efficiency vs Output Current and Input Voltage 100 Efficiency (%) 90 80 Vin = 12 V 70 60 50 40 30 20 0 1 2 3 4 5 Iout (A) Figure 1: NCP3011 Efficiency at 12V with a 1.8V Output Voltage Load Regulation vs Input Voltage 1.9 1.89 V-Out (V) 1.88 1.87 1.86 NCP3011 Vin = 11 ‐ 14 V Vout = 1.87 V 1.85 1.84 0 0.5 1 Vin = 11 V 1.5 2 I-Out (A) Vin = 12 V 2.5 3 Vin = 13 V 3.5 Vin = 14 V Figure 2: NCP3011 Load Regulation Rev 0, January 2011 4 4.5 5 ON Semiconductor Schematics Figure 4: NCP3011 Schematic Rev 0 - January, 2011 ON Semiconductor Table 2: NCP3011 BOM (1.8Voutput) Qty 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Reference Value C1 470uF C1A 470uF C2 22uF 25V C3 22uF 25V C4 1uF C5 0.1uF C6 1uF C7 27pF C8 2.7nF C9 0.1 uF C10 1.5 nF C11 100uF*2 C12 100uF*2 C13 DNP C14 0.1uF C15 DNP C16 DNP C17 1.8nF C18 DNP L1 5.6 uH Part Number Description C4532X7R1E226M C4532X7R1E226M Ceramic Ceramic Ceramic Ceramic Ceramic Ceramic Ceramic Ceramic Ceramic Ceramic Ceramic C3225X5R0J107M C3225X5R0J107M Ceramic Chip Capacitor Ceramic Chip Capacitor Ceramic Chip Capacitor Ceramic Chip Capacitor Ceramic Chip Capacitor SMT Inductor PCB DECAL ECA_12.5 ECA_12.5 2220CAP 2220CAP 1206CAP 1206CAP 1206CAP 0603CAP 0603CAP 0805CAP 0805CAP 1210CAP 1210CAP ECA_12.5 0805CAP 0805CAP 0805CAP 0603CAP 0603CAP MSS1260 Chip Capacitor Chip Capacitor Chip Capacitor Chip Capacitor Chip Capacitor Chip Capacitor Chip Capacitor Chip Capacitor Chip Capacitor Chip Capacitor Chip Capacitor 1 Q1 NTMS4873NF HSFET Dual Use Footprint; SOPFL and DPAK MOSFET COMBO2_SO8FL-DPAK 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 NTMS4873NF 13.3k 10k 22.1k 499 DNP 12k 0R0 0R0 10k 1.18 0R0 22.1k 0R0 0R0 DNP 22.1k DNP 20R 28 V 400 kHz DNP BAT54T1G Dual Use Footprint; SOPFL and DPAK MOSFET SMT Resistor SMT Resistor SMT Resistor SMT Resistor SMT Resistor Resistor SMT Resistor Resistor SMT Resistor Resistor Resistor SMT Resistor SMT Resistor SMT Resistor SMT Resistor SMT Resistor SMT Resistor SMT Resistor Synchronous PWM Controller Schottky Barrier Diodes Schottky Barrier Diodes COMBO2_SO8FL-DPAK 0603RES 0603RES 0603RES 0603RES 0603RES 1206RES 0603RES 1206RES 0603RES 1206RES 1206RES 0603RES 0603RES 0603RES 0603RES 0603RES 0603RES 0603RES TSSOP-14 SOD_123_BAT54T1 SOD_123_BAT54T1 Q2 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15 R16 R17 R18 U1 D1 D2 LSFET CRCW12060000Z0EA CRCW06030000Z0EA CRCW12068R06FNEA CRCW060310K0FKEA CRCW12060000Z0EA CRCW060322K1FKEA CRCW060310K0FKEA CRCW06030000Z0EA CRCW0603750RFKEA CRCW060320R0FKEA NCP3011 Sync Diode Rev 0 - January, 2011 ON Semiconductor Vout 1.0V 1.8V 2.5V 3.3V 5.0V C7 27pF 68pF 27pF 27pF 27pF C8 2.7nF 8.2nF 2.7nF 2.7nF 2.7nF C10 1.5nF 1.5nF 1.5nF 1.5nF 1.5nF C17 1.8nF 3.9nF 1.8nF 2.7nF 1.8nF L1 5.6uH 5.6uH 5.6uH 5.6uH 6.8uH R1 15.8k 9.76k 15.8k 10.5k 15.4k R2 52.3k 7.32k 7.15k 3.24k 2.87k R3 22.1k 10k 22.1k 22.1k 22.1k R4 599 182 499 340 499 R10 1.18 1.18 1.18 1.18 1.18 C11 100uF*2 100uF*2 100uF*2 100uF*3 47uF*3 C12 100uF*2 100uF*2 100uF*2 100uF*2 47uF*3 Table 3: Component Recommendations for Different Output Voltages © 2011 ON Semiconductor. Disclaimer: ON Semiconductor is providing this design note “AS IS” and does not assume any liability arising from its use; nor does ON Semiconductor convey any license to its or any third party’s intellectual property rights. This document is provided only to assist customers in evaluation of the referenced circuit implementation and the recipient assumes all liability and risk associated with its use, including, but not limited to, compliance with all regulatory standards. ON Semiconductor may change any of its products at any time, without notice. Design note created by Tim Kaske and Jim Hill, e-mail: [email protected] ; [email protected] Rev 0 - January, 2011