MIC4722 3mm x 3mm 2.7MHz 3A PWM Buck Regulator General Description Features The Micrel MIC4722 is a high efficiency PWM buck (stepdown) regulator that provides up to 3A of output current. The MIC4722 operates at 2.7MHz that makes it suitable for next generation XDSL modems requiring operating frequencies in excess of 2.4MHz and has proprietary internal compensation that allows a closed loop bandwidth of over 200KHz. The low on-resistance internal p-channel MOSFET of the MIC4722 allows efficiencies over 92%, reduces external components count and eliminates the need for an expensive current sense resistor. The MIC4722 operates from 2.7V to 5.5V input and the output can be adjusted down to 1V. The devices can operate with a maximum duty cycle of 100% for use in lowdropout conditions. The MIC4722 is available in the exposed pad 12-pin 3mm x 3mm MLF® package with a junction operating range from –40°C to +125°C. • • • • • • • • • • • • • 2.7 to 5.5V supply voltage 2.7MHz PWM mode Output current to 3A >92% efficiency 100% maximum duty cycle Adjustable output voltage option down to 1V Ultra-fast transient response Ultra-small external components Stable with a 0.47µH inductor and a 4.7µF output capacitor Fully integrated 3A MOSFET switch Micropower shutdown Thermal shutdown and current limit protection Pb-free 12-pin 3mm x 3mm MLF® package –40°C to +125°C junction temperature range Applications • • • • • 5V or 3.3V Point of Load Conversion Telecom/Networking Equipment Set Top Boxes Storage Equipment Video Cards Typical Application MIC4722 3A 2.7MHz Buck Regulator MLF and MicroLeadFrame are registered trademarks of Amkor Technology, Inc. Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com September 2007 M9999-090507-B Micrel, Inc. MIC4722 Ordering Information Part Number Voltage MIC4722YML Junction Temp. Range Adj. Package –40° to +125°C 12-Pin 3x3 MLF Lead Finish ® Pb-Free Note: MLF® is a GREEN RoHS compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free. Pin Configuration SW 1 12 SW VIN 2 11 VIN PGND 3 10 PGND SGND 4 9 PGOOD BIAS 5 FB 6 8 EN EP 7 NC 12-Pin 3mm x 3mm MLF® (ML) Pin Description Pin Number Pin Name Pin Function 1,12 SW Switch (Output): Internal power P-Channel MOSFET output switch 2,11 VIN Supply Voltage (Input): Supply voltage for the source of the internal P-channel MOSFET and driver. 3,10 PGND Power Ground. Provides the ground return path for the high-side drive current. 4 SGND Signal (Analog) Ground. Provides return path for control circuitry and internal reference. 5 BIAS 6 FB Feedback. Input to the error amplifier, connect to the external resistor divider network to set the output voltage. 7 NC No Connect. Not internally connected to die. This pin can be tied to any other pin if desired. 8 EN Enable (Input). Logic level low, will shutdown the device, reducing the current draw to less than 5µA. 9 PGOOD EP GND Requires bypass capacitor to GND. September 2007 Internal circuit bias supply. Must be bypassed with a 0.1µF ceramic capacitor to SGND. Power Good. Open drain output that is pulled to ground when the output voltage is within +/- 7.5% of the set regulation voltage Connect to ground. 2 M9999-090507-B Micrel, Inc. MIC4722 Absolute Maximum Ratings(1) Operating Ratings(2) Supply Voltage (VIN) .......................................................+6V Output Switch Voltage (VSW). .........................................+6V Output Switch Current (SW).............................................11A Logic Input Voltage (VEN) .................................. –0.3V to VIN Storage Temperature (Ts) .........................–60°C to +150°C Junction Temperature ................................................ 150°C EDS Rating(3) .................................................................. 2kV Supply voltage (VIN) ..................................... +2.7V to +5.5V Logic Input Voltage (VEN) ....................................... 0V to VIN Junction Temperature (TJ) ........................ –40°C to +125°C Junction Thermal Resistance 3x3 MLF-12 (θJA) ...............................................60°C/W Electrical Characteristics(4) VIN = VEN = 3.6V; L = 0.47µH; COUT = 4.7µF; TA = 25°C, unless noted. Bold values indicate –40°C< TJ < +125°C. Parameter Condition Min (turn-on) 2.45 Supply Voltage Range Under-Voltage Lockout Threshold Typ Max Units 5.5 V 2.55 2.65 V 2.7 UVLO Hysteresis 100 Quiescent Current VFB = 0.9 * VNOM (not switching) Shutdown Current VEN = 0V [Adjustable] Feedback Voltage ± 1% ILOAD = 100mA ± 2% (over temperature) ILOAD = 100mA Current Limit VFB = 0.9 * VNOM Output Voltage Line Regulation mV 570 900 µA 2 10 µA 0.99 0.98 1 1.01 1.02 V 1 nA 3.5 5 A VOUT > 2V; VIN = VOUT+500mV to 5.5V; ILOAD= 100mA VOUT < 2V; VIN = 2.7V to 5.5V; ILOAD= 100mA 0.07 % Output Voltage Load Regulation 20mA < ILOAD < 3A 0.2 % Maximum Duty Cycle VFB ≤ 0.4V FB pin input current Switch ON-Resistance % 100 ISW = 50mA VFB = 0.7VFB_NOM (High Side Switch) 95 200 300 mΩ Oscillator Frequency 2.4 2.7 3.0 MHz Enable Threshold 0.5 0.85 1.3 V Enable Hysteresis 50 Enable Input Current 0.1 2 µA Power Good Range ±7 ±10 % 145 200 Ω Power Good Resistance IPGOOD = 500µA mV Over-Temperature Shutdown 150 °C Over-Temperature Hysteresis 25 °C Notes: 1. Exceeding the absolute maximum rating may damage the device. 2. The device is not guaranteed to function outside its operating rating. 3. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. 4. Specification for packaged product only. September 2007 3 M9999-090507-B Micrel, Inc. MIC4722 Typical Characteristics 94 4.5VIN 92 90 88 94 5VIN 92 5.5VIN 86 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 90 88 4.5VIN 5VIN 86 84 82 80 5.5VIN 78 76 74 72 70 68 66 64 62 60 0.5 1 1.5 2 2.5 0 OUTPUT CURRENT (A) 3 92 90 3VIN 88 3.3VIN 86 84 82 3.6VIN 80 78 76 74 72 70 68 66 64 0 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 3 3 MIC4722 1VOUT Efficiency 80 0 88 86 90 88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 0 3VIN 3.3VIN 3.6VIN 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 80 0 3 86 84 82 80 78 76 74 72 70 68 66 64 0 3 MIC4722 1.2VOUT Efficiency 4.5VIN 3 MIC4722 1.5VOUT Efficiency 4.5VIN 5VIN 5.5VIN 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 3 MIC4722 1.2VOUT Efficiency 90 88 86 3VIN 84 3.3VIN 82 80 78 76 3.6VIN 74 72 70 68 66 64 62 60 0 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 5VIN 5.5VIN 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 3 MIC4722 1VOUT Efficiency 1.010 3 Load Regulation 82 80 78 5VIN 3VIN 3.3VIN 76 74 72 5.5VIN 66 64 70 68 62 66 60 0 64 0 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) September 2007 MIC4722 2.5VOUT Efficiency 84 82 96 94 92 3VIN 3.3VIN 90 88 86 84 3.6VIN 82 80 78 76 74 72 70 68 66 0 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 84 4.5VIN 72 68 5.5VIN MIC4722 1.8VOUT Efficiency MIC4722 1.5VOUT Efficiency 70 92 90 82 MIC4722 1.8VOUT Efficiency 74 96 94 5VIN 84 78 76 100 98 88 82 80 78 4.5VIN 90 86 84 76 0 MIC4722 2.5VOUT Efficiency OUTPUT VOLTAGE (V) 96 MIC4722 3.3VOUT Efficiency 3 3.6VIN 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 4 3 1.005 1.000 0.995 0.990 0 3.3V VIN = 3.3V 0.5 1 1.5 2 2.5 OUTPUT CURRENT (A) 3 M9999-090507-B Micrel, Inc. MIC4722 Typical Characteristics (cont.) Line Regulation 1.0010 1.010 Feedback Voltage vs. Temperature 3.2 1.0008 1.008 3.1 1.0006 1.0004 1.006 1.004 3.0 1.0002 1.002 1.000 2.8 1.0000 0.9998 Frequency vs. Temperature 2.9 2.7 0.998 2.6 0.9996 0.9994 0.996 0.994 2.5 0.9992 0.992 V = 3.3V IN 0.990 20 40 60 80 TEMPERATURE (°C) 2.3 V = 3.3V IN 2.2 20 40 60 80 TEMPERATURE (°C) 0.9990 2.7 1.2 3.2 3.7 4.2 4.7 5.2 SUPPLY VOLTAGE (V) Feedback Voltage vs. Supply Voltage 800 2.4 Quiescent Current vs. Supply Voltage 120 115 700 1.0 110 600 0.8 0.6 0.4 105 500 100 400 95 300 90 85 200 0.2 0 0 160 VEN = VIN 1 2 3 4 5 SUPPLY VOLTAGE (V) R DSON vs. Temperature 140 80 100 0 0 1.2 R DSON vs. Supply Voltage 75 VEN = VIN 1 2 3 4 5 SUPPLY VOLTAGE (V) Enable Threshold vs. Supply Voltage 6 70 2.7 1.2 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 3.2 3.7 4.2 4.7 5.2 SUPPLY VOLTAGE (V) Enable Threshold vs. Temperature 120 100 80 60 40 20 0 VIN = 3.3V 20 40 60 80 TEMPERATURE (°C) September 2007 0 2.7 3.2 3.7 4.2 4.7 SUPPLY VOLTAGE (V) 5 0 VIN = 3.3V 20 40 60 80 TEMPERATURE (°C) M9999-090507-B Micrel, Inc. MIC4722 Functional Characteristics Discontinuous Current Inductor Current (200mA/div) Inductor Current (500mA/div) Continuous Current VIN = 3.3V VOUT = 1V L = 1µH 0A VIN = 3.3V VOUT = 1V L = 1µH COUT = 4.7µF 0A IOUT = 300mA COUT = 4.7µF Switch Voltage (2V/div) Switch Voltage (2V/div) IOUT = 1A Time (200ns/div) Time (200ns/div) Load Transient Output Voltage AC Coupled (10mV/div) Output Current (2A/div) Output Ripple Switch Voltage (2V/div) Output Voltage (20mV/div) 0A VIN = 3.3V VOUT = 1.8V Time (400µs/div) IOUT = 3A Time (400ns/div) Inductor Current (2A/div) Feedback Voltage (1V/div) Enable Voltage (2V/div) Input Current (1A/div) Start-Up Waveforms September 2007 Time (40µs/div) 6 M9999-090507-B Micrel, Inc. MIC4722 Functional Diagram VIN VIN P-Channel Current Limit BIAS HSD SW SW PWM Control EN Enable and Control Logic Bias, UVLO, Thermal Shutdown Soft Start EA FB 1.0V PGOOD 1.0V PGND SGND MIC4722 Block Diagram September 2007 7 M9999-090507-B Micrel, Inc. MIC4722 Pin Description SW The switch (SW) pin connects directly to the inductor and provides the switching current necessary to operate in PWM mode. Due to the high speed switching on this pin, the switch node should be routed away from sensitive nodes. This pin also connects to the cathode of the free-wheeling diode. VIN Two pins for VIN provide power to the source of the internal P-channel MOSFET along with the current limiting sensing. The VIN operating voltage range is from 2.7V to 5.5V. Due to the high switching speeds, a 10µF capacitor is recommended close to VIN and the power ground (PGND) for each pin for bypassing. Please refer to layout recommendations. PGOOD Power good is an open drain pull down that indicates when the output voltage has reached regulation. For a power good low, the output voltage is within ±10% of the set regulation voltage. For output voltages greater or less than 10%, the PGOOD pin is high. This should be connected to the input supply through a pull up resistor. A delay can be added by placing a capacitor from PGOOD to ground. BIAS The bias (BIAS) provides power to the internal reference and control sections of the MIC4722. A 10Ω resistor from VIN to BIAS and a 0.1µF from BIAS to SGND is required for clean operation. EN The enable pin provides a logic level control of the output. In the off state, supply current of the device is greatly reduced (typically <1µA). Do not drive the enable pin above the supply voltage. PGND Power ground (PGND) is the ground path for the MOSFET drive current. The current loop for the power ground should be as small as possible and separate from the Signal ground (SGND) loop. Refer to the layout considerations for more details. FB The feedback pin (FB) provides the control path to control the output. For adjustable versions, a resistor divider connecting the feedback to the output is used to adjust the desired output voltage. The output voltage is calculated as follows: SGND Signal ground (SGND) is the ground path for the biasing and control circuitry. The current loop for the signal ground should be separate from the power ground (PGND) loop. Refer to the layout considerations for more details. ⎛ R1 ⎞ + 1⎟ VOUT = VREF × ⎜ ⎝ R2 ⎠ where VREF is equal to 1.0V. A feedforward capacitor is recommended for most designs using the adjustable output voltage option. To reduce current draw, a 10K feedback resistor is recommended from the output to the FB pin (R1). Also, a feedforward capacitor should be connected between the output and feedback (across R1). The large resistor value and the parasitic capacitance of the FB pin can cause a high frequency pole that can reduce the overall system phase margin. By placing a feedforward capacitor, these effects can be significantly reduced. Feedforward capacitance (CFF) can be calculated as follows: C FF = 1 2π × R1 × 200kHz September 2007 8 M9999-090507-B Micrel, Inc. MIC4722 current flows from the input supply through the inductor and to the output. The inductor current is Application Information The MIC4722 is a 3A PWM non-synchronous buck regulator. By switching an input voltage supply, and filtering the switched voltage through an Inductor and capacitor, a regulated DC voltage is obtained. Figure 1 shows a simplified example of a non-synchronous buck converter. Figure 1. Example of non-synchronous buck converter For a non-synchronous buck converter, there are two modes of operation; continuous and discontinuous. Continuous or discontinuous refer to the inductor current. If current is continuously flowing through the inductor throughout the switching cycle, it is in continuous operation. If the inductor current drops to zero during the off time, it is in discontinuous operation. Critically continuous is the point where any decrease in output current will cause it to enter discontinuous operation. The critically continuous load current can be calculated as follows; IOUT Figure 3. On-Time charged at the rate; (VIN − VOUT ) L To determine the total on-time, or time at which the inductor charges, the duty cycle needs to be calculated. The duty cycle can be calculated as; 2⎤ ⎡ V ⎢ VOUT − OUT ⎥ VIN ⎥⎦ ⎢⎣ = 2.7MHz × 2 × L D= VOUT VIN and the On time is; Continuous or discontinuous operation determines how we calculate peak inductor current. TON = Continuous Operation Figure 2 illustrates the switch voltage and inductor current during continuous operation. D 2.7MHz Therefore, peak to peak ripple current is; (V IN − VOUT ) × VOUT Ipk −pk = VIN 2.7MHz × L Since the average peak to peak current is equal to the load current. The actual peak (or highest current the inductor will see in a steady state condition) is equal to the output current plus ½ the peak-to-peak current. (VIN − VOUT ) × VOUT Ipk = IOUT + Figure 2. Continuous Operation The output voltage is regulated by pulse width modulating (PWM) the switch voltage to the average required output voltage. The switching can be broken up into two cycles; On and Off. Figure 4 demonstrates the off-time. During the off-time, the high-side internal P-channel MOSFET turns off. Since the current in the inductor has to discharge, the current flows through the free-wheeling Schottky diode to the output. In this case, the inductor discharge rate is During the on-time, the high side switch is turned on, September 2007 VIN 2 × 2.7MHz × L 9 M9999-090507-B Micrel, Inc. MIC4722 (where VD is the diode forward voltage); − (VOUT the inductor value. In Figure 5, it is drawn as a DC voltage, but to see actual operation (with ringing) refer to the functional characteristics. + VD ) L Discontinuous mode of operation has the advantage over full PWM in that at light loads, the MIC4722 will skip pulses as nessasary, reducing gate drive losses, drastically improving light load efficiency. The total off time can be calculated as; TOFF = 1− D 2.7MHz Efficiency Considerations Calculating the efficiency is as simple as measuring power out and dividing it by the power in; P Efficiency = OUT × 100 PIN Where input power (PIN) is; PIN = VIN × IIN and output power (POUT) is calculated as; POUT = VOUT × IOUT The Efficiency of the MIC4722 is determined by several factors. • • • • Figure 4. Off-Time Discontinuous Operation Discontinuous operation is when the inductor current discharges to zero during the off cycle. Figure 5 demonstrates the switch voltage and inductor currents during discontinuous operation. Rdson (Internal P-channel Resistance) Diode conduction losses Inductor Conduction losses Switching losses Rdson losses are caused by the current flowing through the high side P-channel MOSFET. The amount of power loss can be approximated by; PSW = R DSON × IOUT 2 × D Where D is the duty cycle. Since the MIC4722 uses an internal P-channel MOSFET, Rdson losses are inversely proportional to supply voltage. Higher supply voltage yields a higher gate to source voltage, reducing the Rdson, reducing the MOSFET conduction losses. A graph showing typical Rdson vs input supply voltage can be found in the typical characteristics section of this datasheet. Diode conduction losses occur due to the forward voltage drop (VF) and the output current. Diode power losses can be approximated as follows; PD = VF × IOUT × (1 − D) For this reason, the Schottky diode is the rectifier of choice. Using the lowest forward voltage drop will help reduce diode conduction losses, and improve efficiency. Figure 5. Discontinuous Operation Duty cycle, or the ratio of output voltage to input voltage, determines whether the dominant factor in conduction losses will be the internal MOSFET or the Schottky diode. Higher duty cycles place the power losses on the When the inductor current (IL) has completely discharged, the voltage on the switch node rings at the frequency determined by the parasitic capacitance and September 2007 10 M9999-090507-B Micrel, Inc. MIC4722 high side switch, and lower duty cycles place the power losses on the Schottky diode. Inductor conduction losses (PL) can be calculated by multiplying the DC resistance (DCR) times the square of the output current; PL = DCR × IOUT 2 Also, be aware that there are additional core losses associated with switching current in an inductor. Since most inductor manufacturers do not give data on the type of material used, approximating core losses becomes very difficult, so verify inductor temperature rise. Figure 6. Switching Transition Losses Normally, when the switch is on, the voltage across the switch is low (virtually zero) and the current through the switch is high. This equates to low power dissipation. When the switch is off, voltage across the switch is high and the current is zero, again with power dissipation being low. During the transitions, the voltage across the switch (VS-D) and the current through the switch (IS-D) are at middle, causing the transition to be the highest instantaneous power point. During continuous mode, these losses are the highest. Also, with higher load currents, these losses are higher. For discontinuous operation, the transition losses only occur during the “off” transition since the “on” transitions there is no current flow through the inductor. Switching losses occur twice each cycle, when the switch turns on and when the switch turns off. This is caused by a non-ideal world where switching transitions are not instantaneous, and neither are currents. Figure 6 demonstrates how switching losses due to the transitions dissipate power in the switch. September 2007 11 M9999-090507-B Micrel, Inc. MIC4722 Diode Selection Since the MIC4722 is non-synchronous, a free-wheeling diode is required for proper operation. A Schottky diode is recommended due to the low forward voltage drop and their fast reverse recovery time. The diode should be rated to be able to handle the average output current. Also, the reverse voltage rating of the diode should exceed the maximum input voltage. The lower the forward voltage drop of the diode the better the efficiency. Please refer to the layout recommendations to minimize switching noise. Component Selection Input Capacitor A 10µF ceramic is recommended on each VIN pin for bypassing. X5R or X7R dielectrics are recommended for the input capacitor. Y5V dielectrics lose most of their capacitance over temperature and are therefore not recommended. Also, tantalum and electrolytic capacitors alone are not recommended due their reduced RMS current handling, reliability, and ESR increases. An additional 0.1µF is recommended close to the VIN and PGND pins for high frequency filtering. Smaller case size capacitors are recommended due to their lower ESR and ESL. Please refer to layout recommendations for proper layout of the input capacitor. Feedback Resistors The feedback resistor set the output voltage by dividing down the output and sending it to the feedback pin. The feedback voltage is 1.0V. Calculating the set output voltage is as follows; Output Capacitor The MIC4722 is designed for a 4.7µF output capacitor. X5R or X7R dielectrics are recommended for the output capacitor. Y5V dielectrics lose most of their capacitance over temperature and are therefore not recommended. ⎛ R1 ⎞ VOUT = VFB ⎜ + 1⎟ ⎝ R2 ⎠ Where R1 is the resistor from VOUT to FB and R2 is the resistor from FB to GND. The recommended feedback resistor values for common output voltages are available in the bill of materials on page 19. Although the range of resistance for the FB resistors is very wide, R1 is recommended to be 10K. This minimizes the effect the parasitic capacitance of the FB node. In addition to a 4.7µF, a small 0.1µF is recommended close to the load for high frequency filtering. Smaller case size capacitors are recommended due to their lower equivalent series ESR and ESL. The MIC4722 utilizes type III voltage mode internal compensation and utilizes an internal zero to compensate for the double pole roll off of the LC filter. For this reason, larger output capacitors can create instabilities. In cases where a 4.7µF output capacitor is not sufficient, the MIC4722 offers the ability to externally control the compensation, allowing for a wide range of output capacitor types and values. Feedforward Capacitor (CFF) A capacitor across the resistor from the output to the feedback pin (R1) is recommended for most designs. This capacitor can give a boost to phase margin and increase the bandwidth for transient response. Also, large values of feedforward capacitance can slow down the turn-on characteristics, reducing inrush current. For maximum phase boost, CFF can be calculated as follows; Inductor Selection The MIC4722 is designed for use with a 0.47µH inductor. Proper selection should ensure the inductor can handle the maximum average and peak currents required by the load. Maximum current ratings of the inductor are generally given in two methods; permissible DC current and saturation current. Permissible DC current can be rated either for a 40°C temperature rise or a 10% to 20% loss in inductance. Ensure the inductor selected can handle the maximum operating current. When saturation current is specified, make sure that there is enough margin that the peak current will not saturate the inductor. September 2007 C FF = 1 2π × 200kHz × R1 Bias filter A small 10Ω resistor is recommended from the input supply to the bias pin along with a small 0.1µF ceramic capacitor from bias to ground. This will bypass the high frequency noise generated by the violent switching of high currents from reaching the internal reference and control circuitry. Tantalum and electrolytic capacitors are not recommended for the bias, these types of capacitors lose their ability to filter at high frequencies. 12 M9999-090507-B Micrel, Inc. MIC4722 Loop Stability and Bode Analysis Network Analyzer “R” Input Bode analysis is an excellent way to measure small signal stability and loop response in power supply designs. Bode analysis monitors gain and phase of a control loop. This is done by breaking the feedback loop and injecting a signal into the feedback node and comparing the injected signal to the output signal of the control loop. This will require a network analyzer to sweep the frequency and compare the injected signal to the output signal. The most common method of injection is the use of a transformer. Figure 7 demonstrates how a transformer is used to inject a signal into the feedback network. +8V MIC922BC5 Feedback R3 1k R4 1k 50 R1 1k Network Analyzer “A” Input Output Network Analyzer Source Figure 8. Op Amp Injection R1 and R2 reduce the DC voltage from the output to the non-inverting input by half. The network analyzer is generally a 50Ω source. R1 and R2 also divide the AC signal sourced by the network analyzer by half. These two signals are “summed” together at half of their original input. The output is then amplified by 2 by R3 and R4 (the 50Ω is to balance the network analyzer’s source impedance) and sent to the feedback signal. This essentially breaks the loop and injects the AC signal on top of the DC output voltage and sends it to the feedback. By monitoring the feedback “R” and output “A”, gain and phase are measured. This method has no minimum frequency. Ensure that the bandwidth of the op-amp being used is much greater than the expected bandwidth of the power supply’s control loop. An op-amp with >100MHz bandwidth is more than sufficient for most power supplies (which includes both linear and switching) and are more common and significantly cheaper than the injection transformers previously mentioned. The one disadvantage to using the op-amp injection method; is that the supply voltages need to be below the maximum operating voltage of the op-amp. Also, the maximum output voltage for driving 50Ω inputs using the MIC922 is 3V. For measuring higher output voltages, 1MΩ input impedance is required for the A and R channels. Remember to always measure the output voltage with an oscilloscope to ensure the measurement is working properly. You should see a single sweeping sinusoidal waveform without distortion on the output. If there is distortion of the sinusoid, reduce the amplitude of the source signal. You could be overdriving the feedback causing a large signal response. Figure 7. Transformer Injection A 50Ω resistor allows impedance matching from the network analyzer source. This method allows the DC loop to maintain regulation and allow the network analyzer to insert an AC signal on top of the DC voltage. The network analyzer will then sweep the source while monitoring A and R for an A/R measurement. While this is the most common method for measuring the gain and phase of a power supply, it does have significant limitations. First, to measure low frequency gain and phase, the transformer needs to be high in inductance. This makes frequencies <100Hz require an extremely large and expensive transformer. Conversely, it must be able to inject high frequencies. Transformers with these wide frequency ranges generally need to be custom made and are extremely expensive (usually to the tune of several hundred dollars!). By using an op-amp, cost and frequency limitations caused by an injection transformer are completely eliminated. Figure 8 demonstrates using an op-amp in a summing amplifier configuration for signal injection. September 2007 13 M9999-090507-B Micrel, Inc. MIC4722 Being that the MIC4722 is non-synchronous; the regulator only has the ability to source current. This means that the regulator has to rely on the load to be able to sink current. This causes a non-linear response at light loads. The following plot shows the effects of the pole created by the nonlinearity of the output drive during light load (discontinuous) conditions. The following Bode analysis show the small signal loop stability of the MIC4722, it utilizes type III compensation. This is a dominant low frequency pole, followed by 2 zeros and finally the double pole of the inductor capacitor filter, creating a final 20dB/decade roll off. Bode analysis gives us a few important data points; speed of response (Gain Bandwidth or GBW) and loop stability. Loop speed or GBW determines the response time to a load transient. Faster response times yield smaller voltage deviations to load steps. Bode Plot VIN=3.3V,V OUT=1.8V,IOUT=50mA 60 50 GAIN (dB) =3A 210 175 PHASE 40 GAIN (dB) OUT 140 30 105 20 70 10 L=1µH 0 COUT = 4.7µF 35 GAIN 0 -10 R1 = 10k R2 = 12.4k -20 C = 82pF FF -30 100 1k 10k 100k FREQUENCY (Hz) • • -35 -105 1M 50 GAIN (dB) 40 =3A PHASE 210 175 140 30 105 20 70 10 L=1µH 0 COUT = 4.7µF GAIN -10 R1 = 10k R2 = 12.4k -20 C = 82pF FF -30 100 1k 10k 100k FREQUENCY (Hz) GAIN (dB) OUT 0 35 -35 -70 -105 1M Gain and Phase vs. Frequency 35 0 0 L=1µH -1 C = 4.7µF OUT -2 R1 = 10k -3 R2 = 12.4k -4 CFF = 82pF -5 -6 -7 -8 -9 -10 100 PHASE (°) OUT 10 L=1µH 0 COUT = 4.7µF Phase Margin=90.5 Degrees GBW= 64.4KHz Gain will also increase with input voltage. The following graph shows the increase in GBW for an increase in supply voltage. 60 IN 70 Feed Forward Capacitor The feedback resistors are a gain reduction block in the overall system response of the regulator. By placing a capacitor from the output to the feedback pin, high frequency signal can bypass the resistor divider, causing a gain increase up to unity gain. -70 Phase Margin=47 Degrees GBW=156KHz Bode Plot =1.8V, I V =5V, V 105 20 3.3Vin, 1.8Vout Iout=50mA; Typically for 3.3Vin and 1.8Vout at 3A; • • 140 30 25 GAIN 20 PHASE BOOST (°) OUT PHASE -10 R1 = 10k GAIN R2 = 12.4k -20 C = 82pF FF -30 100 1k 10k 100k FREQUENCY (Hz) PHASE (°) 60 IN 175 40 PHASE (°) Instability in a control loop occurs when there is gain and positive feedback. Phase margin is the measure of how stable the given system is. It is measured by determining how far the phase is from crossing zero when the gain is equal to 1 (0dB). Bode Plot V =3.3V, V =1.8V, I 210 50 15 PHASE 1k 10k 100k FREQUENCY (Hz) 10 5 0 1M The graph above shows the effects on the gain and phase of the system caused by feedback resistors and a feedforward capacitor. The maximum amount of phase boost achievable with a feedforward capacitor is graphed below. -35 -70 -105 1M 5Vin, 1.8Vout at 3A load; • • Phase Margin=43.1 Degrees GBW= 218KHz September 2007 14 M9999-090507-B Micrel, Inc. MIC4722 50 Bode Plot V =3.3V, V =1.8V, I Voltage 60 IN OUT OUT =3A 210 45 40 50 35 30 30 105 20 70 GAIN (dB) PAHSE BOOST (°) 25 20 15 10 5 0 1 V REF = 1V 2 3 4 OUTPUT VOLTAGE (V) 5 10 L=1µH 0 COUT = 4.7µF GAIN -10 R1 = 10k R2 = 12.4k -20 C = 0pF FF -30 100 1k 10k 100k FREQUENCY (Hz) 175 140 35 0 -35 -70 -105 1M As you can see the typical phase margin, using the same resistor values as before without a feedforward capacitor results in 33.6 degrees of phase margin. Our prior measurement with a feedforward capacitor yielded a phase margin of 47 degrees. The feedforward capacitor has given us a phase boost of 13.4 degrees (47 degrees- 33.6 Degrees = 13.4 Degrees). By looking at the graph, phase margin can be affected to a greater degree with higher output voltages. The next bode plot shows the phase margin of a 1.8V output at 3A without a feedforward capacitor. September 2007 PHASE 40 PHASE (°) Max. Amount of Phase Boost Obtainable using CFF vs. Output 15 M9999-090507-B Micrel, Inc. MIC4722 Output Impedance and Transient Response ∆I = Output impedance, simply stated, is the amount of output voltage deviation vs. the load current deviation. The lower the output impedance, the better. Z OUT = × 1mW × 50Ω × 2 0.707 × R LOAD The following graph shows output impedance vs frequency at 2A load current sweeping the AC current from 10Hz to 10MHz, at 1A peak to peak amplitude. ∆VOUT ∆IOUT Output Impedance vs. Frequency 1 OUTPUT IMPEDANCE (Ohms) Output impedance for a buck regulator is the parallel impedance of the output capacitor and the MOSFET and inductor divided by the gain; Z TOTAL = dBm 10 10 R DSON + DCR + X L X COUT GAIN To measure output impedance vs. frequency, the load current must be swept across the frequencies measured, while the output voltage is monitored. Figure 9 shows a test set-up to measure output impedance from 10Hz to 1MHz using the MIC5190 high speed controller. VOUT=1.8V L=1µH =4.7µF + 0.1µ C OUT 0.1 3.3VIN 0.01 0.001 10 5VIN 100 1k 10k 100k 1M FREQUENCY (Hz) From this graph, you can see the effects of bandwidth and output capacitance. For frequencies <200KHz, the output impedance is dominated by the gain and inductance. For frequencies >200KHz, the output impedance is dominated by the capacitance. A good approximation for transient response can be calculated from determining the frequency of the load step in amps per second; f = Then, determine the output impedance by looking at the output impedance vs frequency graph. Next calculate the voltage deviation times the load step; Figure 9. Output Impedance Measurement ∆VOUT = ∆IOUT × Z OUT By setting up a network analyzer to sweep the feedback current, while monitoring the output of the voltage regulator and the voltage across the load resistance, output impedance is easily obtainable. To keep the current from being too high, a DC offset needs to be applied to the network analyzer’s source signal. This can be done with an external supply and 50Ω resistor. Make sure that the currents are verified with an oscilloscope first, to ensure the integrity of the signal measurement. It is always a good idea to monitor the A and R measurements with a scope while you are sweeping it. To convert the network analyzer data from dBm to something more useful (such as peak to peak voltage and current in our case); ∆V = 10 dBm 10 A/sec 2π The output impedance graph shows the relationship between supply voltage and output impedance. This is caused by the lower Rdson of the high side MOSFET and the increase in gain with increased supply voltages. This explains why higher supply voltages have better transient response. ↓Z TOTAL = ↓ R DSON + DCR + X L X COUT ↑ GAIN × 1mW × 50Ω × 2 0.707 and peak to peak current; September 2007 16 M9999-090507-B Micrel, Inc. MIC4722 Ripple measurements To properly measure ripple on either input or output of a switching regulator, a proper ring in tip measurement is required. Standard oscilloscope probes come with a grounding clip, or a long wire with an alligator clip. Unfortunately, for high frequency measurements, this ground clip can pick-up high frequency noise and erroneously inject it into the measured output ripple. The standard evaluation board accommodates a home made version by providing probe points for both the input and output supplies and their respective grounds. This requires the removing of the oscilloscope probe sheath and ground clip from a standard oscilloscope probe and wrapping a non-shielded bus wire around the oscilloscope probe. If there does not happen to be any non shielded bus wire immediately available, the leads from axial resistors will work. By maintaining the shortest possible ground lengths on the oscilloscope probe, true ripple measurements can be obtained. September 2007 17 M9999-090507-B Micrel, Inc. MIC4722 Recommended Layout: 3A Evaluation Board Recommended Top Layout Recommended Bottom Layout September 2007 18 M9999-090507-B Micrel, Inc. MIC4722 MIC4722 Schematic and BOM for 3A Output Item Part Number Description Manufacturer C1,C2 C2012JB0J106K 10µF Ceramic Capacitor X5R 0805 6.3V TDK GRM219R60J106KE19 10µF Ceramic Capacitor X5R 0805 6.3V Murata Qty 2 08056D106MAT 10µF Ceramic Capacitor X5R 0805 6.3V AVX C3 0402ZD104MAT 0.1µF Ceramic Capacitor X5R 0402 10V AVX C4 C2012JB0J475K 4.7µF Ceramic Capacitor X5R 0603 6.3V TDK GRM188R60J475KE19 4.7µF Ceramic Capacitor X5R 0603 6.3V Murata 06036D475MAT 4.7µF Ceramic Capacitor X5R 0603 6.3V AVX 1 C5 VJ0402A820KXAA 82pF Ceramic Capacitor 0402 Vishay VT 1 D1 SSA33L 3A Schottky 30V SMA Vishay Semi 1 L1 IHLP2525AHERR47M01 0.47µH Inductor 8.4mΩ 6.47mm(L)x6.86mm(W)x1.8mm(H) Vishay Dale 1 R1,R4 CRCW04021002F 10KΩ1% 0402 resistor Vishay Dale 1 R2 CRCW04026651F CRCW04021242F CRCW04022002F CRCW04024022F 6.65kΩ 1% 0402 For 2.5VOUT 12.4kΩ 1% 0402 For 1.8 VOUT 20kΩ 1% 0402 For 1.5 VOUT 40.2kΩ 1% 0402 For 1.2 VOUT Open For 1.0 VOUT Vishay Dale Vishay Dale Vishay Dale Vishay Dale Vishay Dale 1 1 R3 CRCW040210R0F 10Ω1% 0402 resistor Vishay Dale 1 U1 MIC4722YML 2.7MHz 3A Buck Regulator Micrel, Inc. 1 Notes: 1. TDK: www.tdk.com 2. Murata: www.murata.com 3. AVX: www.avx.com 4. Vishay: www.vishay.com 5. Wurth Elektronik: www.we-online.com 6. Micrel, Inc: www.micrel.com September 2007 19 M9999-090507-B Micrel, Inc. MIC4722 Package Information 12-Pin 3mm x 3mm MLF® (ML) MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http:/www.micrel.com The information furnished by Micrel in this data sheet is believed to be accurate and reliable. However, no responsibility is assumed by Micrel for its use. Micrel reserves the right to change circuitry and specifications at any time without notification to the customer. Micrel Products are not designed or authorized for use as components in life support appliances, devices or systems where malfunction of a product can reasonably be expected to result in personal injury. Life support devices or systems are devices or systems that (a) are intended for surgical implant into the body or (b) support or sustain life, and whose failure to perform can be reasonably expected to result in a significant injury to the user. A Purchaser’s use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser’s own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. © 2006 Micrel, Incorporated. September 2007 20 M9999-090507-B