MIC4742 2 MHz Dual 2A Integrated Switch Buck Regulator General Description Features The Micrel MIC4742 is a high efficiency dual PWM buck (step-down) regulator that provides dual 2A current output. The MIC4742 operates at 2MHz. A proprietary internal compensation technique allows a closed loop bandwidth of over 200kHz. • • • • • • • • The low on-resistance internal P-Channel MOSFET of the MIC4742 allows efficiencies over 92%, reduces external components count and eliminates the need for an expensive current sense resistor. The MIC4742 operates from 2.9V to 5.5V input and the output can be adjusted down to 0.6V. The device can operate with a maximum duty cycle of 100% for use in lowdropout conditions. The MIC4742 is available in the exposed pad 16-pin 3mm x 3mm MLF® and EPAD TSSOP-16 packages with a junction operating range from –40°C to +125°C. All support documentation can be found on Micrel’s web site at: www.micrel.com. • • • • • 2.9 to 5.5V supply voltage 2MHz PWM mode 2A Dual output Greater than 92% efficiency 100% maximum duty cycle Adjustable output voltage option down to 0.6V Ultra-fast transient response Ultra-small external components Stable with a 1µH inductor and a 4.7µF output capacitor Fully integrated 2A MOSFET switches Micro-power shutdown Thermal shutdown and current limit protection Available in a 3mm x 3mm 16-pin MLF® and 16-pin TSSOP –40°C to +125°C junction temperature range Applications • • • • • • Broadband: xDSL modems Automotive satellite radios HD STB, DVD/TV recorder Computer peripherals: printers and graphic cards FPGA/ASIC General point of load Typical Application 5.0Vin 3.3Vout Efficiency 100 98 EFFICIENCY (%) 96 94 92 90 88 86 84 82 80 0 0.4 0.8 1.2 1.6 2 OUTPUT CURRENT (A) 2A 2MHz Dual 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 March 2009 1 M9999-030209-D Micrel, Inc. MIC4742 Ordering Information Part Number Voltage Junction Temp. Range MIC4742YML Adj. –40° to +125°C 16-Pin 3mm x 3mm MLF® Package Lead Finish Pb-Free MIC4742YTSE Adj. –40° to +125°C 16-Pin EPAD TSSOP Pb-Free Note: ® MLF is a GREEN RoHS-compliant package. Lead finish is NiPdAu. Mold compound is Halogen Free. Pin Configuration 16-Pin 3mm x 3mm MLF® (YML) March 2009 16-Pin EPAD-TSSOP (YTSE) 2 M9999-030209-D Micrel, Inc. MIC4742 Pin Description Pin Number ® (MLF ) Pin Number (EPAD-TSSOP) Pin Name 1 7 FB2 Feedback for output 2 (Input). Input to the error amplifier, connect to the external resistor divider network to set the output voltage. 2(1) 8(1) EN2 Enable for output 2 (Input). Logic level low, will shutdown the device, reducing the current draw to less than 5µA (both EN1 and EN2 are low). 3,4 9,10 SW2 Switch for output 2 (Output): Internal power P-Channel MOSFET output switch 5 11 PGND2 Pin Function Power Ground for output 2. Provides the ground return path for the high-side drive current. PGND1 pin and PGND2 pin are internally connected by anti-parallel diodes. 6,15 5,12 VIN2 Supply Voltage for output 2 (Input): Supply voltage for the source of the internal P-channel MOSFET and driver. Requires bypass capacitor to GND. VIN1 pins and VIN2 pins are internally connected by anti-parallel diodes. 7,14 4,13 VIN1 Supply Voltage for output 1 (Input): Supply voltage for the source of the internal P-channel MOSFET and driver. Requires bypass capacitor to GND. VIN1 pins and VIN2 pins are internally connected by anti-parallel diodes. 8 14 PGND1 Power Ground for output 1. Provides the ground return path for the high-side drive current. PGND1 pin and PGND2 pin are internally connected by anti-parallel diodes. 9,10 15,16 SW1 Switch for output 1 (Output): Internal power P-Channel MOSFET output switch 11(1) 1(1) EN1 Enable for output 1 (Input). Logic level low, will shutdown the device, reducing the current draw to less than 5µA (both EN1 and EN2 are low). 12 2 FB1 Feedback for output 1 (Input). Input to the error amplifier, connect to the external resistor divider network to set the output voltage. 13 3 SGND Signal (Analog) Ground. Provides return path for control circuitry and internal reference. 16 6 BIAS Internal circuit bias supply. Must be bypassed with a 0.1µF ceramic capacitor to SGND. Biased through a 10Ω resistor to VIN. EPAD EPAD GND Connect to ground. Note: 1. Do not float Enable Input. March 2009 3 M9999-030209-D Micrel, Inc. MIC4742 Absolute Maximum Ratings(1) Operating Ratings(2) Supply Voltage (VIN) ......................................... -0.3V to +6V Output Switch Voltage (VSW1, VSW2).................. -0.3V to +6V Output Switch Current (ISW1, ISW2)………...Internally Limited Input Voltage (VEN), (VFB), (VBIAS) .......................-0.3V to VIN Storage Temperature (Ts) .........................–60°C to +150°C Junction Temperature ................................................ 150°C Lead Temperature (soldering, 10sec.)....................... 260°C ESD Rating(3) ................................................................. 2KV Supply Voltage (VIN)..................................... +2.9V to +5.5V Logic Input Voltage (VEN) ....................................... 0V to VIN Junction Temperature (TJ) ........................ –40°C to +125°C Junction Thermal Resistance 3mm x 3mm MLF® (θJA).....................................60°C/W EPAD TSSOP (θJA)............................................35°C/W Electrical Characteristics(4) VIN = VEN = 3.6V; L = 1.0µH; COUT = 4.7µF; TA = 25°C, unless noted. Bold values indicate –40°C< TJ < +125°C. Parameter Condition Min 2.9 Supply Voltage Range Under-Voltage Lockout Threshold 2.45 Turn-on UVLO Hysteresis Quiescent Current Typ 2.6 Max Units 5.5 V 2.7 100 VFB = 0.9 * VNOM (not switching); VIN = 5.5V 1.4 V mV 3.0 mA VFB = 0.9 * VNOM (not switching); VIN = 3.6V 1.0 Shutdown Current VEN = 0V 1.6 10 µA [Adjustable] Feedback Voltage ± 2% (over temperature) ILOAD = 100µA 0.6 0.612 V 0.588 FB pin input current mA 1 2.5 nA Current Limit VFB = 0.9 * VNOM 4.3 A Output Voltage Line Regulation VIN = 2.9V to 5.5V; ILOAD= 100 µA 0.07 % Output Voltage Load Regulation 20mA < ILOAD < 2A 0.2 % Maximum Duty Cycle VFB ≤ 0.9 * VNOM Switch ON-Resistance ISW = 50mA VFB =GND 100 % 155 1.5 Oscillator Frequency Switching Phase 2 mΩ 2.3 180 0.5 Enable Threshold Enable Hysteresis 0.9 MHz Deg 1.3 55 V mV 2 Enable Input Current 0.1 Over-Temperature Shutdown 153 µA °C Over-Temperature Hysteresis 18 °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. March 2009 4 M9999-030209-D Micrel, Inc. MIC4742 Typical Characteristics 78 78 82 76 76 80 74 74 72 70 68 66 72 70 68 66 64 3.0Vin 64 62 3.3Vin 3.6Vin 62 0 0.4 0.8 1.2 1.6 EFFICIENCY (%) 84 4.5Vin 74 72 70 66 0.4 0.8 1.2 1.6 2 0 3.3Vin 1.5Vout Efficiency 86 86 80 84 84 EFFICIENCY (%) 76 74 72 70 82 80 78 76 74 68 4.5Vin 72 66 5.0Vin 5.5Vin 70 64 0.8 1.2 1.6 EFFICIENCY (%) 82 78 80 78 76 0 0.4 0.8 1.2 1.6 2 0 94 88 92 86 86 90 3.0Vin 76 84 82 80 78 4.5Vin 5.0Vin 5.5Vin 76 3.3Vin 74 EFFICIENCY (%) 90 88 EFFICIENCY (%) 90 78 74 3.6Vin 1.6 0.4 OUTPUT CURRENT (A) 5.0Vin 2.5Vout Efficiency 98 92 96 90 94 EFFICIENCY (%) 94 88 86 84 82 4.5Vin 5.0Vin 5.5Vin 78 0.8 1.2 1.6 0.8 1.2 1.6 OUTPUT CURRENT (A) March 2009 82 3.0Vin 3.3Vin 3.6Vin 0 2 0.4 0.8 1.6 2 Load Regulation 92 90 88 86 4.5Vin 1.000 0.995 0.990 5.0Vin 5.5Vin 82 2 1.2 OUTPUT CURRENT (A) Vin=3.3V 80 0.4 84 1.005 84 76 0 86 5.0Vin 3.3Vout Efficiency 100 80 88 OUTPUT CURRENT (A) 96 2 76 0 2 1.6 78 OUTPUT VOLTAGE (V) 1.2 1.2 80 72 72 0.8 3.3Vin 2.5Vout Efficiency 5.0Vin 1.8Vout Efficiency 96 0.8 0.4 OUTPUT CURRENT (A) 92 0.4 5.0Vin 5.5Vin 68 3.3Vin 1.8Vout Efficiency 0 4.5Vin 70 92 80 2 82 OUTPUT CURRENT (A) 82 1.6 72 3.3Vin 3.6Vin OUTPUT CURRENT (A) 84 1.2 74 3.0Vin 68 2 0.8 5.0Vin 1.5Vout Efficiency 88 0.4 0.4 OUTPUT CURRENT (A) 88 0 3.0Vin 3.3Vin 3.6Vin OUTPUT CURRENT (A) 5.0Vin 1.2Vout Efficiency 84 EFFICIENCY (%) 76 64 0 2 OUTPUT CURRENT (A) EFFICIENCY (%) 78 68 5.0Vin 5.5Vin 60 60 EFFICIENCY (%) 3.3Vin 1.2Vout Efficiency 5.0Vin 1.0Vout Efficiency 80 EFFICIENCY (%) EFFICIENCY (%) 3.3Vin 1.0Vout Efficiency 80 0.985 0 0.4 0.8 1.2 1.6 OUTPUT CURRENT (A) 5 2 0 0.4 0.8 1.2 1.6 2 OUTPUT CURRENT (A) M9999-030209-D Micrel, Inc. MIC4742 Typical Characteristics (continue) Feedback Voltage vs. Temperature 0.605 0.9948 0.604 2.4 0.9946 0.603 2.3 0.9944 0.9942 0.9940 0.9938 0.9936 0.9934 0.602 0.601 0.600 0.599 0.598 2.0 1.9 1.8 1.7 0.596 1.6 0.9930 0.595 3.3 3.7 4.1 4.5 4.9 5.3 1.5 -40 -20 0.4 0.2 0.0 1 2 3 4 20 40 60 80 100 5 170 140 130 120 110 20 40 60 March 2009 80 100 120 60 80 100 120 160 800 400 150 140 130 120 110 100 1.00 2.00 3.00 4.00 5.00 2.9 3.3 3.7 4.1 4.5 4.9 SUPPLY VOLTAGE (V) SUPPLY VOLTAGE (V) Enable Threshold vs. Supply Voltage Enable Threshold vs. Temperature 1.2 0.80 0.60 0.40 0.20 0.00 100 40 180 ENABLE THRESHOLD (V) ENABLE THRESHOLD (V) 150 20 Rdson vs. Supply Voltage 1.00 160 TEMPERATURE (°C) 0 170 1.20 180 0 -20 TEMPERATURE (°C) 1200 0 0.00 Rdson vs. Temperature -20 -40 Quiescent Current vs. Supply Voltage SUPPLY VOLTAGE (V) -40 120 Rdson (mΩ) QUIESCENT CURRENT (μA) 1600 0.6 0 0 TEMPERATURE (°C) Feedback Voltage vs. Supply Voltage 0.8 FEEDBACK VOLTAGE (V) 2.1 0.9932 SUPPLY VOLTAGE (V) Rdson (mΩ) 2.2 0.597 2.9 Frequency vs. Temperature 2.5 FREQUENCY (MHz) 0.9950 FEEDBACK VOLTAGE (V) OUTPUT VOLTAGE (V) Line Regulation 5.3 1.0 0.8 0.6 0.4 0.2 0.0 2.9 3.3 3.7 4.1 4.5 4.9 SUPPLY VOLTAGE (V) 6 5.3 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (°C) M9999-030209-D Micrel, Inc. MIC4742 Functional Characteristics March 2009 7 M9999-030209-D Micrel, Inc. MIC4742 Functional Diagram MIC4742 Block Diagram March 2009 8 M9999-030209-D Micrel, Inc. MIC4742 Pin Description SW1/SW2 The switch pins (SW1 and SW2) connect directly to the inductor and provide the switching current necessary to operate in PWM mode. Due to the high speed switching on these pins, the switch nodes should be routed away from sensitive nodes. These pins also connect to the cathodes of the free-wheeling diodes. VIN1/VIN2 VIN pins (two pins for VIN1 and two pins for VIN2) provide power to the source of the internal P-channel MOSFET along with the current limiting sensing. VIN1 pins and VIN2 pins are internally connected by antiparallel diodes. The VIN operating voltage range is from 2.9V 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 for more details. PGND1/PGND2 Power ground pins (PGND1 and PGND2) are the ground paths for the MOSFET drive current. PGND1 pin and PGND2 pin are internally connected by anti-parallel diodes. 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 recommendation for more details. BIAS The bias (BIAS) provides power to the internal reference and control sections of the MIC4742. A 10Ω resistor from VIN to BIAS and a 0.1µF from BIAS to SGND are required for clean operation. 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 recommendation for more details. EN1/EN2 The enable pins (EN1 and EN2) provides a logic level control of the outputs 1 and 2. In the off state, supply current of the device is greatly reduced (typically <2µA). Do not drive the enable pin above the supply voltage. FB1/FB2 The feedback pins (FB1 and FB2) provides the control path to control the outputs 1 and 2. A resistor divider connecting the feedback to the output is used to adjust the desired output voltage. The output voltage is calculated as follows: EPAD The exposed pad on the bottom of the part must be connected to ground. ⎛ R1 ⎞ VOUT = VREF × ⎜ + 1⎟ ⎝ R2 ⎠ where VREF is equal to 0.6V. A feed-forward capacitor is recommended for most designs. To reduce current draw, 10K feedback resistors are recommended from the outputs to the FB pins (R1 in the equation). Also, feed-forward capacitors should be connected between the outputs and feedback pins (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 feed-forward capacitor, these effects can be significantly reduced. Feed-forward capacitance (CFF) can be calculated as follows: C FF = March 2009 1 2π × R1 × 200kHz 9 M9999-030209-D Micrel, Inc. MIC4742 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. During the On-Time, Figure 3 illustrates the high side switch is turned on, current flows from the input supply through the inductor and to the output. The inductor current is charged at the rate; Application Information The MIC4742 is a dual 2A 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. (VIN − VOUT ) L 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; Figure 3. On-Time 2⎤ ⎡ V ⎢VOUT − OUT ⎥ VIN ⎥⎦ ⎢ I OUT = ⎣ fsw × 2 × 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; D= Continuous or discontinuous operation determines how we calculate peak inductor current. VOUT VIN and the On time is; Continuous Operation Figure 2 illustrates the switch voltage and inductor current during continuous operation. TON = D fsw Therefore, peak to peak ripple current is; I pk − pk = fsw × L VIN 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. Figure 2. Continuous Operation March 2009 (V IN −VOUT )× VOUT 10 M9999-030209-D Micrel, Inc. MIC4742 I pk = IOUT + (VIN − VOUT )× VOUT 2 × fsw × L VIN 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 (where VD is the diode forward voltage); − (VOUT + VD ) L The total off time can be calculated as; TOFF = 1− D fsw Figure 5. Discontinuous Operation When the inductor current (IL) has completely discharged, the voltage on the switch node rings at the frequency determined by the parasitic capacitance and 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. Discontinuous mode of operation has the advantage over full PWM in that at light loads, the MIC4742 will skip pulses as necessary, reducing gate drive losses, drastically improving light load efficiency. 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; Figure 4. Off-Time PIN = VIN × IIN and output power (POUT) is calculated as; 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. POUT = VOUT × IOUT The Efficiency of the MIC4742 is determined by several factors. • 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 March 2009 11 M9999-030209-D Micrel, Inc. MIC4742 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. Where D is the duty cycle. Since the MIC4742 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. 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 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; 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. 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. March 2009 12 M9999-030209-D Micrel, Inc. MIC4742 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 to 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 recommendation for proper layout of the input capacitor. Diode Selection Since the MIC4742 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 recommendation to minimize switching noise. 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 0.6V. Calculating the set output voltage is as follows; Output Capacitor The MIC4742 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. 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 there lower equivalent series ESR and ESL. The MIC4742 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, other values of capacitance can be used but the original LC filter pole frequency determined by COUT = 4.7µF and L = 1µH (which is approximately 73.4KHz) must remain fixed. Increasing COUT forces L to decrease and vice versa. ⎛ 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 17. 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. 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 MIC4742 is designed for use with a 1µH 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. March 2009 C FF = 1 2π × 200kHz × R1 Large values of feedforward capacitance may introduce negative FB pin voltage during load shorting, which will cause latch-off. In that case, a Schottky diode from FB pin to the ground is recommended. 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. 13 M9999-030209-D Micrel, Inc. MIC4742 Loop Stability and Bode Analysis GAIN (dB) 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 transformer. Figure 7 demonstrates how a transformer is used to inject a signal into the feedback network. 60 210 50 175 40 140 30 105 20 70 10 35 0 0 -10 -35 -20 -70 -30 1.E+02 1.E+03 1.E+04 1.E+05 PHASE (°) Bode Plot Vin=3.6V Vout=1.8V Iout=2A -105 1.E+06 FREQUENCY (Hz) Typically for 3.6Vin and 1.8Vout at 2A; • • Phase Margin=77.8 Degrees GBW=229KHz Being that the MIC4742 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. Figure 7. Transformer Injection March 2009 GAIN (dB) 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. The following Bode analysis show the small signal loop stability of the MIC4742, 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. 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). 60 210 50 175 40 140 30 105 20 70 10 35 0 0 -10 -35 -20 -70 -30 1.E+02 1.E+03 1.E+04 1.E+05 PHASE (°) Bode Plot Vin=3.6V Vout=1.8V Iout=0.1A -105 1.E+06 FREQUENCY (Hz) 3.6Vin, 1.8Vout Iout=0.1A; • • Phase Margin=89.9 Degrees GBW= 43.7kHz 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. 14 M9999-030209-D Micrel, Inc. MIC4742 Gain and Phase vs. Frequency 35 0 60 Max. Amount of Phase Boost Obtainable Using Cff vs. Output Voltage 30 20 -10 15 10 -15 PHASE BOOST (°) GAIN (dB) 25 PHASE BOOST (°) 50 -5 1000 10000 100000 0 1000000 20 0 0 FREQUENCY (Hz) 1 2 3 4 5 OUTPUT VOLTAGE (V) By looking at the graph, phase margin can be affected to a greater degree with higher output voltages. 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. March 2009 30 10 5 -20 100 40 15 M9999-030209-D Micrel, Inc. MIC4742 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. March 2009 16 M9999-030209-D Micrel, Inc. MIC4742 Bill of Materials MIC4742YML Schematic for 2A Output Item Part Number C11,C12 GRM188R60J106M C21,C22 C1608X5R0J106M 06036D106KMAT2A C13*,C23* C14,C24 C15,C25 C16,C26,C7 C18,C28 D1,D2 L1,L2 Manufacturer TDK(2) VJ0603A820KXXCW Vishay Murata C1608 X5R0J475M TDK(2) 0603D475MAT AVX(3) VJ0603Y104KXXAT Vishay(4) SS2P3L Vishay(4) SSA23L Vishay (4) Diodes IHLP2525AH-01 1R0 Vishay(4) CRCW06033K16FKXX TDK (2) 82pF Ceramic Capacitor NPO 0603 10V 2 4.7µF Ceramic Capacitor X5R 0603 6.3V 2 0.1µF Ceramic Capacitor X7R 0603 25V 5 2A Schottky 30V 2 COOPER 1µH Inductor 17.5mΩ 6.86mm(L) x 6.47mm(W) x 1.8mm(H) 1µH Inductor 8.8mΩ 7.3mm(L) x 6.8mm(W) x 3.2mm(H) (6) Vishay(4) 2 1µH Inductor 10mΩ 7.3mm(L) x 7.0mm(W) x 3.0mm(H) 10KΩ1% 0603 resistor 2 3.16kΩ 1% 0603 For 2.5VOUT CRCW06034K99FKXX CRCW06036K65FKXX 2 (5) B230A R12,R22 680pF Ceramic Capacitor NPO 0603 6.3V (1) GRM188R60J475K HCP0703-1R0 4 AVX (4) CRCW060310K0FKXX 10µF Ceramic Capacitor X5R 0603 6.3V (3) Vishay(4) R11,R12 Qty Murata VJ0603A681KXXCW RLF7030-1R0 N Description (1) 4.99kΩ 1% 0603 For 1.8 VOUT (4) Vishay 6.65kΩ 1% 0603 For 1.5 VOUT CRCW060310K0FKXX 10kΩ 1% 0603 For 1.2 VOUT CRCW060315K0FKXX 15kΩ 1% 0603 For 1.0 VOUT 2 * only for ultra-low noise applications. March 2009 17 M9999-030209-D Micrel, Inc. Item R13*, R23* R14, R24 R5 U1 MIC4742 Part Number CRCW06032R70FKXX CRCW060349K9FKXX CRCW060310R0FKXX MIC4742YML Manufacturer Description Qty (4) 2.7Ω1% 0603 resistor 2 (4) 49.9kΩ1% 0603 resistor 2 10Ω1% 0603 resistor 1 Dual 2A 2MHz Integrated Switch Buck Regulator 1 Vishay Vishay (4) Vishay (7) Micrel, Inc. Notes: 1. Murata: www.murata.com 2. TDK: www.tdk.com 3. AVX: www.avx.com 4. Vishay: www.vishay.com 5. Cooper: www.cooperbussmann.com 6. Diode: www.diodes.com 7. Micrel, Inc: www.micrel.com * only for ultra-low noise applications. March 2009 18 M9999-030209-D Micrel, Inc. MIC4742 MIC4742YML Layout Recommendation: 2A Evaluation Board Recommended TOP Layout Recommended Bottom Layout March 2009 19 M9999-030209-D Micrel, Inc. MIC4742 Recommended Mid-Layer 1 Layout Recommended Mid-Layer 2 Layout March 2009 20 M9999-030209-D Micrel, Inc. MIC4742 Package Information 16-Pin 3mm x 3mm MLF® (ML) March 2009 21 M9999-030209-D Micrel, Inc. MIC4742 16-Pin ETSSOP March 2009 22 M9999-030209-D Micrel, Inc. MIC4742 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. © 2008 Micrel, Incorporated. March 2009 23 M9999-030209-D