MIC2177 2.5A Synchronous Buck Regulator General Description Features The Micrel MIC2177 is a 200kHz synchronous buck (stepdown) switching regulator designed for high-efficiency, battery-powered applications. The MIC2177 operates from a 4.5V to 16.5V input and features internal power MOSFETs that can supply up to 2.5A output current. It can operate with a maximum duty cycle of 100% for use in low-dropout conditions. It also features a shutdown mode that reduces quiescent current to less than 5µA. The MIC2177 achieves high efficiency over a wide output current range by switching between PWM and skip mode. Operating mode is automatically selected according to output conditions. Switching frequency is preset to 200kHz and can be synchronized to an external clock signal of up to 300kHz. The MIC2177 uses current-mode control with internal current sensing. Current-mode control provides superior line regulation and makes the regulator control loop easy to compensate. The output is protected with pulse-by-pulse current limiting and thermal shutdown. Undervoltage lockout turns the output off when the input voltage is less than 4.5V. The MIC2177 is packaged in a 20-pin wide power SO package with an operating temperature range of –40°C to +85°C. See the MIC2178 for externally selected PWM or skip-mode operation. Data sheets and support documentation can be found on Micrel’s web site at: www.micrel.com. • 4.5V to 16.5V input voltage range • Dual-mode operation for high efficiency (up to 96%) – PWM mode for > 200mA load current – Skip mode for < 200mA load current • 100mΩ internal power MOSFETs at 12V input • 200kHz preset switching frequency • Low quiescent current – 1.0mA in PWM mode – 500µA in skip mode – < 5µA in shutdown mode • 100% duty cycle for low dropout operation • Current-mode control – Simplified loop compensation – Superior line regulation • Current limit • Thermal shutdown • Undervoltage lockout Applications • High-efficiency, battery-powered supplies • Buck (step-down) dc-to-dc converters • Cellular telephones • Laptop computers • Hand-held instruments • Battery Charger _________________________________________________________________________________________________________ Typical Application VIN 5.4V to 18V 100 U1 1,2,9 95 VIN ENABLE 20 SHUTDOWN 18 11 2.2 nF OUT EN SW MIC SYNC 2177-5.0 PGND AUTO FB COMP SGND BIAS R1 10k CC 6.8nF 13 14–17 10 3,8 4–7 L1, 50µH D1 MBRS130L 12 19 C3 0.01µF VOUT 5V/1A C2 100µF 10V EFFICIENCY (%) C1 22µF 35V 5V Output Efficiency 90 85 80 75 R1 10k VIN = 6V 70 10 SKIP PWM 100 1000 2500 OUTPUT CURRENT (mA) Micrel Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel +1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com April 2008 M9999-042108 Micrel, Inc. MIC2177 Ordering Information Part Number Output Voltage Switching Frequency Temperature Range Package Lead Finish MIC2177-3.3BWM 3.3V 200kHz –40°C to +85°C 20-Pin Wide SOIC Standard MIC2177-5.0BWM 5.0V 200kHz –40°C to +85°C 20-Pin Wide SOIC Standard MIC2177BWM Adj. 200kHz –40°C to +85°C 20-Pin Wide SOIC Standard MIC2177-3.3YWM 3.3V 200kHz –40°C to +85°C 20-Pin Wide SOIC Pb-Free Pin Configuration VIN 1 20 E N VIN 2 19 B I A S SW 3 18 S Y NC P GND 4 17 S G N D P GND 5 16 S G N D P GND 6 15 S G N D P GND 7 14 S G N D SW 8 13 C O M P VIN 9 12 F B 11 AUTO OUT 10 20-Pin Wide SOIC (WM) April 2008 2 M9999-042108 Micrel, Inc. MIC2177 Pin Description Pin Number Pin Name 1, 2, 9 VIN Supply Input: Controller and switch supply. Unregulated supply input to internal regulator, output switches, and control circuitry. Requires bypass capacitor to PGND. All three pins must be connected to VIN. 3, 8 SW Switch (Output): Internal power MOSFET switch output. Both pins must be externally connected together. 4, 5, 6, 7 PGND 10 OUT 11 AUTO 12 FB 13 COMP Compensation: Internal error amplifier output. Connect to capacitor or series RC network to compensate the regulator control loop. 14, 15, 16, 17 SGND Signal Ground: Ground connection of control section. Connect all pins to common ground plane. 18 SYNC Frequency Synchronization (Input): Optional clock input. Connect to external clock signal to synchronize oscillator. Leading edge of signal above 1.7V terminates switching cycle. Connect to SGND if not used. 19 BIAS Bias Supply: Internal 3.3V bias supply output. Decouple with 0.01µF bypass capacitor and 10kΩ to SGND. Do not apply any external load. 20 EN April 2008 Pin Function Power Ground: Output stage ground connections. Connect all pins to a common ground plane. Output Voltage Sense (Input): Senses output voltage to determine minimum switch current for PWM operation. Connect directly to VOUT. Automatic Mode: Connect 2.2nF timing capacitor for automatic PWM-/skip-mode switching. Regulator operates exclusively in PWM mode when pin is pulled low. Feedback (Input): Error amplifier inverting input. For adjustable output version, connect FB to external resistive divider to set output voltage. For 3.3V and 5V fixed output versions, connect FB directly to output. Enable (Input): Logic high enables operation. Logic low shuts down regulator. Do not allow pin to float. 3 M9999-042108 Micrel, Inc. MIC2177 Absolute Maximum Ratings Operating Ratings Supply Voltage [100ms transient] (VIN) ..........................18V Output Switch Voltage (VSW). .........................................18V Output Switch Current (ISW)...........................................6.0A Enable, Output-Sense Voltage (VEN, VOUT). ...................18V Sync Voltage (VSYNC)........................................................6V Supply Voltage (VIN)....................................... 4.5V to 16.5V Junction Temperature (TJ) ........................ –40°C to +125°C Electrical Characteristics VIN = 7.0V; TA = 25°C, bold values indicate –40°C< TA < +85°C, unless noted. Symbol Parameter Condition ISS Input Supply Current Min Typ Max Units PWM mode, output not switching, 4.5V ≤ VIN ≤ 16.5V 1.0 1.5 mA skip mode, output not switching, 4.5V ≤ VIN ≤ 16.5V 500 650 µA 1 25 µA VEN = 0V, 4.5V ≤ VIN ≤ 16.5V VBIAS Bias Regulator Output Voltage VFB VOUT VTH VIN = 16.5V 3.10 3.30 3.40 V Feedback Voltage MIC2177 [adj.]: VOUT = 3.3V, ILOAD = 0 1.22 1.245 1.27 V Output Voltage MIC2177 [adj.]: VOUT = 3.3V, 5V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 2A 3.20 3.14 3.3 3.40 3.46 V V MIC2177-5.0: ILOAD = 0 4.85 5.0 5.15 V MIC2177-5.0: 6V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 2A 4.85 4.75 5.0 5.15 5.25 V V MIC2177-3.3: ILOAD = 0 3.20 3.3 3.40 V MIC2177-3.3: 5V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 2A 3.20 3.14 3.3 3.40 3.46 V V 4.25 4.35 V Undervoltage Lockout VTL IFB AVOL upper threshold lower threshold Feedback Bias Current 3.9 4.15 60 150 nA MIC2177-5.0, MIC2177-3.3 20 40 µA 20 V Error Amplifier Gain 0.6V ≤ VCOMP ≤ 0.8V 15 18 Error Amplifier Output Swing upper limit 0.9 1.5 Lower limit Error Amplifier Output Current V MIC2177 [adj.] 0.05 source and sink fO Oscillator Frequency DMAX Maximum Duty Cycle VFB = 1.0V tON min Minimum On-Time VFB = 1.5V V 0.1 V 15 25 35 µA 160 200 240 kHz % 100 300 SYNC Frequency Range 220 SYNC Threshold 0.8 SYNC Minimum Pulse Width 500 1.6 400 ns 300 kHz 2.2 V ns ISYNC SYNC Leakage VSYNC = 0V to 5.5V –1 0.01 1 µA ILIM Current Limit PWM mode, VIN = 12V 3.8 4.7 5.7 A RON April 2008 Switch On-Resistance skip mode 600 high-side switch, VIN = 12V 90 250 mΩ low-side switch, VIN = 12V 110 250 mΩ 4 mA M9999-042108 Micrel, Inc. MIC2177 Symbol Parameter Condition ISW Output Switch Leakage VSW = 16.5V Min Enable Threshold IEN Enable Leakage VEN = 0V to 5.5V AUTO Threshold Typ Max Units 1 10 µA 0.8 1.6 2.2 V 1 µA –1 0.01 0.8 1.6 7 11 V AUTO Source Current VFB = 1.5V, VAUTO < 0.8V Minimum Switch Current for PWM Operation VIN – VOUT = 0V 220 mA VIN – VOUT = 3V 420 mA 15 µA General Note: Devices are ESD sensitive. Handling precautions recommended. April 2008 5 M9999-042108 Micrel, Inc. MIC2177 Typical Characteristics 190 185 180 175 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 1.248 1.246 1.244 1.242 1.240 1.238 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) Reference Voltage vs. Temperature 5.030 MIC21775.0 5.020 5.010 5.000 4.990 4.980 3.315 18.0 17.5 17.0 16.5 3.305 3.300 3.295 3.290 3.285 3.280 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) Feedback Input Bias Current vs. Temperature 120 16.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) Current Limit vs. Temperature 4.9 4.8 200 4.7 4.6 150 100 4.2 4.1 50 0 PWM-Mode Supply Current 12 125°C 85°C 25°C 0°C 40 20 Low-Side Switch On-Resistance 125°C 85°C 25°C 0°C 300 250 200 150 10 8 6 4 2 50 2 100 OUTPUT SWITCHING 4 3.3V Output Efficiency 90 April 2008 4 6 8 10 12 14 16 18 INPUT VOLTAGE (V) 85 8V 80 12V 75 70 60 10 2 100 95 VIN = 5V SKIP PWM 65 2 0 6 8 10 12 14 16 18 INPUT VOLTAGE (V) 95 EFFICIENCY (%) SUPPLY CURRENT (mA) 60 100 4.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 0 80 350 4.5 4.4 4.3 100 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) High-Side Switch On-Resistance 250 MIC2177-3.3 3.310 100 1000 2500 OUTPUT CURRENT (mA) 6 EFFICIENCY (%) CURRENT LIMIT (A) 18.5 Reference Voltage vs. Temperature 3.320 Error-Amplifier Gain vs. Temperature 19.0 4.970 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 5.0 MIC2177 [adj.] BIAS CURRENT (nA) 195 1.250 AMPLIFIER VOLTAGE GAIN FREQUENCY (kHz) 200 REFERENCE VOLTAGE (V) 1.252 REFERENCE VOLTAGE (V) 205 Reference Voltage vs. Temperature REFERENCE VOLTAGE (V) Oscillator Frequency vs. Temperature 4 6 8 10 12 14 16 18 INPUT VOLTAGE (V) 5V Output Efficiency VIN = 6V 90 8V 85 80 75 70 10 12V SKIP PWM 100 1000 2500 OUTPUT CURRENT (mA) M9999-042108 Micrel, Inc. MIC2177 Functional Diagram VIN 4.5V to 16.5V CIN VIN UVLO, Thermal Shutdown 1 2 9 R1 VOUT = 1.245 ( + 1) R2 100m P-channel SW ISENSE Amp. Output Control Logic L1 3 D EN Enable Shutdown 20 VOUT 8 3.3V Regulator 100m N-channel COUT PGND 4 BIAS 10k 0.01µF 5 ILIMIT Comp. 19 6 7 internal supply voltage Bold lines indicate high current traces PWM/ Skip-Mode Select Logic IMIN Comp. IMIN Thrshld. OUT 10 SYNC 18 CORRECTIVE RAMP 200kHz Oscillator R1 3.3V Low Output Comp. FB 12 10µA AUTO Auto-Mode PWM 2.2nF R2 11 40mV Skip-Mode Comp. RESET PULSE Q R S PWM Comp. Error Amp. COMP RC CC VREF 1.245V 13 MIC2177 [Adjustable] SGND April 2008 7 14 15 16 17 M9999-042108 Micrel, Inc. MIC2177 Shutdown Mode The MIC2177 has a low-current shutdown mode that is controlled by the enable input (EN). When a logic 0 is applied to EN, the MIC2177 is in shutdown mode and its quiescent current drops to less than 5µA. Functional Description Micrel’s MIC2177 is a synchronous buck regulator that operates from an input voltage of 4.5V to 16.5V and provides a regulated output voltage of 1.25V to 16.5V. It has internal power MOSFETs that supply up to 2.5A of load current and operates with up to 100% duty cycle to allow low-dropout operation. To optimize efficiency, the MIC2177 operates in PWM and skip mode. Skip mode provides the best efficiency when load current is less than 200mA, while PWM mode is more efficient at higher current. A patented technique allows the MIC2177 to automatically select the correct operating mode as the load current changes. During PWM operation, the MIC2177 uses current-mode control which provides superior line regulation and makes the control loop easier to compensate. The PWM switching frequency is set internally to 200kHz and can be synchronized to an external clock frequency up to 300kHz. Other features include a low-current shutdown mode, current limit, undervoltage lockout, and thermal shutdown. See the following sections for details. Internal Bias Regulator An internal 3.3V regulator provides power to the MIC2177 control circuits. This internal supply is brought out to the BIAS pin for bypassing by an external 0.01µF capacitor. Do not connect any external load to the BIAS pin. It is not designed to provide an external supply voltage. Frequency Synchronization The MIC2177 operates at a preset switching frequency of 200kHz. It can be synchronized to a higher frequency by connecting an external clock to the SYNC pin. The SYNC pin is a logic level input that synchronizes the oscillator to the rising edge of an external clock signal. It has a frequency range of 220kHz–300kHz, and can operate with a minimum pulse-width of 500ns. If synchronization is not required, connect SYNC to ground. Switch Output The switch output (SW) is a half H-bridge consisting of a high-side P-channel and low-side N-channel power MOSFET. These MOSFETs have a typical on-resistance of 100mΩ when the MIC2177 operates from a 12V supply. Anti-shoot-through circuitry prevents the Pchannel and N-channel from turning on at the same time. Low-Dropout Operation Output regulation is maintained in PWM or skip mode even when the difference between VIN and VOUT decreases below 1V. As VIN – VOUT decreases, the duty cycle increases until it reaches 100%. At this point, the P-channel is kept on for several cycles at a time, and the output stays in regulation until VIN – VOUT falls below the dropout voltage (dropout voltage = P-channel on resistance × load current). Current Limit The MIC2177 uses pulse-by-pulse current limiting to protect the output. During each switching period, a current limit comparator detects if the P-channel current exceeds 4.7A. When it does, the P-channel is turned off until the next switching period begins. PWM-Mode Operation Refer to “PWM-Mode Functional Diagram” which is a simplified block diagram of the MIC2177 operating in PWM mode with its associated waveforms. When operating in PWM mode, the output P-channel and N-channel MOSFETs are alternately switched on at a constant frequency and variable duty cycle. A switching period begins when the oscillator generates a reset pulse. This pulse resets the RS latch which turns on the P-channel and turns off the N-channel. During this time, inductor current (IL1) increases and energy is stored in the inductor. The current sense amplifier (ISENSE Amp) measures the P-channel drain-to-source voltage and outputs a voltage proportional to IL1. The output of ISENSE Amp is added to a saw tooth waveform (corrective ramp) generated by the oscillator, creating a composite waveform labeled ISENSE on the timing diagram. When ISENSE is greater than the error amplifier output, the PWM comparator will set the RS latch which turns off the Pchannel and turns on the N-channel. Energy is then Undervoltage Lockout Undervoltage lockout (UVLO) turns off the output when the input voltage (VIN) is too low to provide sufficient gate drive for the output MOSFETs. It prevents the output from turning on until VIN exceeds 4.3V. Once operating, the output will not shut off until VIN drops below 4.2V. Thermal Shutdown Thermal shutdown turns off the output when the MIC2177 junction temperature exceeds the maximum value for safe operation. After thermal shutdown occurs, the output will not turn on until the junction temperature drops approximately 10°C. April 2008 8 M9999-042108 Micrel, Inc. MIC2177 switching cycles that turn on the P-channel. To begin analyzing MIC2177 skip-mode operation, assume the skip-mode comparator output is high and the latch output has been reset to a logic 1. This turns on the P-channel and causes IL1 to increase linearly until it reaches a current limit of 600mA. When I L1 reaches this value, the current limit comparator sets the RS latch output to logic 0, turning off the P-channel. The output switch voltage (VSW) then swings from VIN to 0.4V below ground, and IL1 flows through the Schottky diode. L1 discharges its energy to the output and IL1 de-creases to zero. When IL1 = 0, VSW swings from –0.4V to VOUT, and this triggers a one-shot that resets the RS latch. Resetting the RS latch turns on the P-channel, which begins another switching cycle. The skip-mode comparator regulates VOUT by controlling when the MIC2177 skips cycles. It compares VFB to VREF and has 10mV of hysteresis to prevent oscillations in the control loop. When VFB is less than VREF – 5mV, the comparator output is logic 1, allowing the P-channel to turn on. Conversely, when VFB is greater than VREF + 5mV, the P-channel is turned off. Note that this is a self-oscillating topology which explains why the switching frequency and duty cycle are a function of VIN, VOUT, and the value of L1. It has the unique feature (for a pulse-skipping regulator) of supplying the same value of maximum load current for any value of VIN, VOUT, or L1. This allows the MIC2177 to always supply up to 300mA of load current (ILOAD) when operating in skip mode. discharged from the inductor and IL1 decreases until the next switching cycle begins. By varying the P-channel on-time (duty cycle), the average inductor current is adjusted to whatever value is required to regulate the output voltage. The MIC2177 uses current-mode control to adjust the duty cycle and regulate the output voltage. Currentmode control has two signal loops that determine the duty cycle. One is an outer loop that senses the output voltage, and the other is a faster inner loop that senses the inductor current. Signals from these two loops control the duty cycle in the following way: VOUT is fed back to the error amplifier which compares the feedback voltage (VFB) to an internal reference voltage (VREF). When VOUT is lower than its nominal value, the error amplifier output voltage increases. This voltage then intersects the current-sense waveform later in switching period which increases the duty cycle and average inductor current. If VOUT is higher than nominal, the error amplifier output voltage decreases, reducing the duty cycle. The PWM control loop is stabilized in two ways. First, the inner signal loop is compensated by adding a corrective ramp to the output of the current sense amplifier. This allows the regulator to remain stable when operating at greater than 50% duty cycle. Second, a series resistor-capacitor load is connected to the error amplifier output (COMP pin). This places a pole-zero pair in the regulator control loop. One more important item is synchronous rectification. As mentioned earlier, the N-channel output MOSFET is turned on after the P-channel turns off. When the Nchannel turns on, its on-resistance is low enough to create a short across the output diode. As a result, inductor current flows through the N-channel and the voltage drop across; it is significantly lower than a diode forward voltage. This reduces power dissipation and improves efficiency to greater than 95% under certain operating conditions. To prevent shoot through current, the output stage employs break-before-make circuitry that provides approximately 50ns of delay from the time one MOSFET turns off and the other turns on. As a result, inductor current briefly flows through the output diode during this transition. Changing from PWM to Skip Mode Refer to “Block Diagram” for circuits described in the following sections. The MIC2177 automatically changes from PWM to skip mode operation when ILOAD drops below a minimum value. IMIN is determined indirectly by detecting when the peak inductor current (IL(peak)) is less than 420mA. This is done by the minimum current comparator which detects if the output P-Channel current equals 420mA during each switching cycle. If it does not, the PWM/skip-mode select logic places the MIC2177 into skip-mode operation. The value of IMIN that corresponds to IL1(peak) = 420mA is given by the following equation: Skip-Mode Operation Refer to “Skip-Mode Functional Diagram” which is a simplified block diagram of the MIC2177 operating in skip mode and its associated waveforms. Skip-mode operation turns on the output P-channel at a frequency and duty cycle that is a function of VIN, VOUT, and the output inductor value. While in skip mode, the Nchannel is kept off to optimize efficiency by reducing gate charge dissipation. VOUT is regulated by skipping April 2008 IMIN = 420mA − ∆IL1 2 Where: ∆IL1 = inductor ripple current This equation shows IMIN varies as a function of ∆IL. Therefore, the user must select an inductor value that results in IMIN = 200mA when IL(peak) = 420mA. The formulas for calculating the correct inductor value are 9 M9999-042108 Micrel, Inc. MIC2177 given in the “Applications Information” section. Note that ∆IL varies as a function of input voltage, and this also causes IMIN to vary. In applications where the input voltage changes by a factor of two, IMIN will typically vary from 130mA to 250mA. During low-dropout operation, the minimum current thresh-old circuit reduces the minimum value of IL1(peak) for PWM operation. This compensates for ∆IL1 decreasing to almost zero when the difference between VIN and VOUT is very low. The low output comparator determines when VOUT is low enough for the regulator to change operating modes. It detects when the feedback voltage is 3% below nominal, and pulls the AUTO pin to ground. When AUTO is less than 1.6V, the PWM/skip-mode select logic places the MIC2177 into PWM operation. The external 2.2nF capacitor connected to AUTO is charged by a 10µA current source after the regulator begins operating in PWM mode. As a result, AUTO stays below 1.6V for several switching cycles after PWM operation begins, forcing the MIC2177 to remain in PWM mode during this transition. Changing from Skip to PWM Mode The MIC2177 will automatically change from skip to PWM mode when ILOAD exceeds 300mA. During skipmode operation, it can supply up to 300mA, and when ILOAD exceeds this limit, VOUT will fall below its nominal value. At this point, the MIC2177 begins operating in PWM mode. Note that the maximum value of ILOAD for skip mode is greater than the minimum value required for PWM mode. This current hysteresis prevents the MIC2177 from toggling between modes when ILOAD is in the range of 100mA to 300mA. April 2008 External PWM-Mode Selection The MIC2177 can be forced to operate in only PWM mode by connecting AUTO to ground. This prevents skip-mode operation in applications that are sensitive to switching noise. 10 M9999-042108 Micrel, Inc. MIC2177 PWM-Mode Functional Diagram VIN 4.5V to 16.5V CIN VIN 1 2 9 R1 VOUT = 1.245 ( + 1) R2 100m P-channel SW IS E N S E Amp. L1 3 8 IL1 D 100m N-channel VO U T COU T P GND 4 5 6 7 Stop S Y NC 18 Corrective Ramp 200kHz Oscillator R1 Reset Pulse FB 12 R2 Q R S PWM Comp. Error Amp. COMP CC RC 13 VR E F1.245V MIC2177 [Adjustable] PWM-Mode Signal Path S G N D 14 15 16 17 VS W Reset Pulse IL1 I LOAD IL1 Error Amp. Output ISE NS E April 2008 11 M9999-042108 Micrel, Inc. MIC2177 Skip-Mode Functional Diagram VIN 4.5V to 16.5V CIN VIN 1 2 9 Output Control Logic S R Q R1 VOUT = 1.245 ( + 1) R2 100m P-channel One Shot IS E N S E Amp. SW L1 3 8 VO U T IL1 D CO U T P GND 4 5 ILIMIT Comp. 6 7 ILIMIT Thresh. Voltage R1 Skip-Mode Comp. FB 12 R2 VR E F1.245V MIC2177 [Adjustable] Skip-Mode Signal Pat S G N D 14 VS W 15 16 17 VIN VO U T 0 One-Shot Pulse I LIM IL1 0 VR E F + 5mV VF B VR E F – 5mV April 2008 12 M9999-042108 Micrel, Inc. MIC2177 increase significantly. Both of these effects will degrade efficiency. The formula for peak inducto rcurrent is: IL(peak) = ILOAD(max) + 300mA To maximize efficiency, the inductor’s resistance must be less than the output switch on-resistance (preferably 50mΩor less). Application Information Feedback Resistor Selection (Adjustable Version) The output voltage is configured by connecting an external resistive divider to the FB pin as shown in “MIC2177 Block Diagram.” The ratio of R1 to R2 determines the output voltage. To optimize efficiency during low output current operation, R2 should not be less than 20kΩ. However, to prevent feedback error due to input bias current at the FB pin, R2 should not be greater than 100kΩ. After selecting R2, calculate R1 using the following formula: Output Capacitor Selection Select an output capacitor that has a low value of ESR. This parameter determines a regulator’s output ripple voltage (VRIPPLE) which is generated by ∆IL × ESR. As mentioned in “Inductor Selection,” the maximum value for ∆IL is 600mA. Therefore, the maximum value of ESR is: ⎡⎛ V ⎞ ⎤ R1 = R2⎢⎜⎜ OUT ⎟⎟ − 1⎥ ⎣⎝ 1.245V ⎠ ⎦ ESR MAX = Input Capacitor Selection The input capacitor is selected for its RMS current and voltage rating and should be a low ESR (equivalent series resistance) electrolytic or tantalum capacitor. As a rule-of-thumb, the voltage rating for a tantalum capacitor should be twice the value of VIN, and the voltage rating for an electrolytic should be 40% higher than VIN. The RMS current rating must be equal or greater than the maximum RMS input ripple current. A simple, worst-case formula for calculating this RMS current is: IRMS(max) = Where: VRIPPLE < 1% of VOUT Typically, capacitors in the range of 100µF to 220µF have ESR less than this maximum value. The output capacitor can be either a low ESR electrolytic or tantalum capacitor, but tantalum is a better choice for compact layout and operation at temperatures below 0°C. The voltage rating of a tantalum capacitor must be 2 × VOUT, and the voltage rating of an electrolytic must be 1.4 × VOUT. ILOAD(max) 2 Tantalum capacitors are a better choice for applications that require the most compact layout or operation below 0°C. The input capacitor must be located very close to the VIN pin (within 0.2 inches, 5mm). Also place a 0.1µF ceramic bypass capacitor as close as possible to VIN. Output Diode Selection In PWM operation, inductor current flows through the output diode approximately 50ns during the dead time when one output MOSFET turns off and the other turns on. In skip-mode, the inductor current flows through the diode during the entire P-channel off time. The correct diode for both of these conditions is a 1A diode with a reverse voltage rating greater than VIN. It must be a Schottky or ultra fast-recovery diode (tR<100ns) to minimize power dissipation from the diode’s reverserecovery charge. Inductor Selection The inductor must be at least a minimum value in order for the MIC2177 to change from PWM to skip mode at the correct value of output current. This minimum value ensures the inductor ripple current never exceeds 600mA, and is calculated using the following formula: ⎛ VOUT L MIN = VOUT ⎜1 − ⎜ VIN(max) ⎝ Compensation Compensation is provided by connecting a series RC load to the COMP pin. This creates a pole-zero pair in the regulator control loop, allowing the regulator to remain stable with enough low frequency loop-gain for good load and line regulation. At higher frequencies pole-zero reduces loop-gain to a level referred to as the mid-band gain. The mid-band gain is low enough so that the loop gain crosses 0dB with sufficient phase margin. Typical values for the RC load are 4.7nF – 10nF for the capacitor and 5kΩ – 20kΩ for the resistor. ⎞ ⎟ × 8.3µ.3µ ⎟ ⎠ Where: VIN(max) = maximum input voltage In general, a value at least 20% greater than LMIN should be selected because inductor values have a tolerance of ±20%. Two other parameters to consider in selecting an inductor are winding resistance and peak current rating. The inductor must have a peak current rating equal or greater than the peak inductor current. Otherwise, the inductor may saturate, causing excessive current in the output switch. Also, the inductor’s core loss may April 2008 600mA VRIPPLE Printed Circuit Board Layout A well designed PC board will prevent switching noise and ground bounce from interfering with the operation of 13 M9999-042108 Micrel, Inc. MIC2177 the MIC2177. A good design takes into consideration component placement and routing of power traces. The first thing to consider is the locations of the input capacitor, inductor, output diode, and output capacitor. The input capacitor must be placed very close to the VIN pin, the inductor and output diode very close to the SW pin, and the output capacitor near the inductor. These components pass large high-frequency current pulses, so they must use short, wide power traces. In addition, their ground pins and PGND are connected to a ground plane that is nearest the power supply ground bus. The feedback resistors, RC compensation network, and BIAS pin bypass capacitor should be located near their respective pins. To prevent ground bounce, their ground traces and SGND should not be in the path of switching currents returning to the power supply ground bus. SGND and PGND should be tied together by a ground plane that extends under the MIC2177. VIN 4.5V to 16.5V C1 22µF 35V U1 1,2,9 VIN 20 18 11 EN SW SYNC MIC2177 PGND AUTO COMP SGND C5 0.01 µF OUT R4 10k 13 C4 6.8nF FB BIAS 14–17 C3 0.01µF 10 L1, 50µH 3,8 D1 MBRS130L 4–7 VOUT 3.3V/1A C2 100µF 10V 12 U1 C1 C2 C3 C4 D1 L1 L1 L1 19 R4 10k Micrel MIC2177-3.3BWM AVX AVX Z5UorX7R Ceramic Dielectric Material X7RorNP0 Ceramic Dielectric Material Motorola MBRS130LT3 Coiltronics CTX50-4P, DCR = 0.097 Coilcraft Bi HM77-11003, DCR = 0.073 Bill of Materials Inductors Capacitors Diodes Transistors Coilcraft AVX General Instruments (GI) Siliconix 1102 Silver Lake Rd. Cary, IL 60013 Tel: (708) 639-2361 Fax: (708) 639-1469 801 17 Ave. Myrtle Beach, SC 29577 Tel: (803) 448-9411 Fax: (803) 448-1973 10 Melville Park Rd. Melville, NY 11747 Tel: (516) 847-3222 Fax: (516) 847-3150 2201 Laurelwood Rd. Santa Clara, CA 96056 Tel: (800) 554-5565 Coiltronics Sanyo Video Components Corp. International Rectifier Corp. 6000 Park of Commerce Blvd. Boca Raton, FL 33487 Tel: (407) 241-7876 Fax: (407) 241-9339 2001 Sanyo Ave. San Diego, CA 92173 Tel: (619) 661-6835 Fax: (619) 661-1055 233 Kansas St. El Segundo, CA 90245 Tel: (310) 322-3331 Fax: (310) 322-3332 Bi Technologies Sprague Electric Motorola, Inc. 4200 Bonita Place Fullerton, CA 92835 Tel: (714) 447-2345 Fax: (714) 447-2500 60005 Lower Main St. Sanford, ME 04073 Tel: (207) 324-4140 MS 56-126 th 3102 North 56 St. Phoenix, AZ 85018 Tel: (602) 244-3576 Fax: (602) 244-4015 April 2008 th 14 M9999-042108 Micrel, Inc. MIC2177 Package Information 20-Pin Wide SOIC (WM) 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. © 1999 Micrel, Incorporated. April 2008 15 M9999-042108