Micrel, Inc. MIC2179 MIC2179 1.5A Synchronous Buck Regulator General Description Features The Micrel MIC2179 is a 200kHz synchronous buck (stepdown) switching regulator designed for high-efficiency, battery-powered applications. The MIC2179 operates from a 4.5V to 16.5V input and features internal power MOSFETs that can supply up to 1.5A 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 MIC2179 achieves high efficiency over a wide output current range by operating in either PWM or skip mode. The operating mode is externally selected, typically by an intelligent system, which chooses the appropriate mode according to operating conditions, efficiency, and noise requirements. The switching frequency is preset to 200kHz and can be synchronized to an external clock signal of up to 300kHz. The MIC2179 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 MIC2179 and is packaged in a 20-lead SSOP package with an operating temperature range of –40°C to +85°C. • 4.5V to 16.5V input voltage range • Dual-mode operation for high efficiency (up to 96%) PWM mode for > 150mA load current Skip mode for <150mA load current • 150mΩ internal power MOSFETs at 12V input • 200kHz preset switching frequency • Low quiescent current 1.0mA in PWM mode 600µA in skip mode < 5µA in shutdown mode • Current-mode control Simplified loop compensation Superior line regulation • 100% duty cycle for low dropout operation • Current limit • Thermal shutdown • Undervoltage lockout Typical Application Applications • • • • • • High-efficiency, battery-powered supplies Buck (step-down) dc-to-dc converters Cellular telephones Laptop computers Hand-held instruments Battery Charger VIN 5.4V to 16.5V C1 10µF 20V Output Good Output Low Skip Mode PWM Mode R1 20k U1 15 6 5 13 16,17 VIN EN SW PWRGD PWM MIC 2179-3.3 PGND SYNC FB COMP SGND 8 9–12 3,4 1,2, 19,20 D1 MBRM120 VOUT 3.3V/600mA C2 100µF 6.3V 7 BIAS 14 C3 0.01µF C4 6.8nF L1 22µH R5 4.02k Pins 4 and 18 are not connected. Pins 3 and 4 can be connected together for a low-impedance connection. Micrel, Inc. • 2180 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 474-1000 • http://www.micrel.com June 2009 1 M9999-063009 Micrel, Inc. MIC2179 Ordering Information Part Number Voltage Temperature Range Package Adj. -40°C to +85°C 20-Lead SSOP MIC2179-3.3BSM MIC2179-3.3YSM 3.3V -40°C to +85°C 20-Lead SSOP MIC2179-5.0BSM MIC2179-5.0YSM 5.0V -40°C to +85°C 20-Lead SSOP Standard* Pb-Free MIC2179BSM MIC2179YSM * Standard product will be supported as Pb-Free IAW PCCN #040004 effective 1-1-2005 pending residual depletion. Pin Configuration PGND 1 20 PGND PGND 2 19 PGND SW 3 18 NC NC 4 17 VIN PWM 5 16 VIN 15 EN PWRGD 6 FB 7 14 BIAS COMP 8 13 SYNC SGND 9 12 SGND SGND 10 11 SGND 20-Lead Wide SSOP Pin Description Pin Number Pin Name 1, 2, 19, 20 PGND 3 SW 5 PWM PWM/Skip-Mode Control (Input): Logic-level input. Controls regulator operating mode. Logic low enables PWM mode. Logic high enables skip mode. Do not allow pin to float. 6 PWRGD 7 FB Error Flag (Output): Open-drain output. Active low when FB input is 10% below the reference voltage (VREF). 8 COMP Compensation: Output of internal error amplifier. Connect capacitor or series RC network to compensate the regulator control loop. 9–12 SGND Signal Ground: Connect all pins to ground, PGND. 13 SYNC Frequency Synchronization (Input): Optional. Connect an external clock signal to synchronize the oscillator. Leading edge of signal above 1.7V terminates switching cycle. Connect to SGND if not used. 14 BIAS Internal 3.3V Bias Supply: Decouple with 0.01µF bypass capacitor to SGND. Do not apply any external load. 15 EN Enable (Input): Logic high enables operation. Logic low shuts down regulator. Do not allow pin to float. 16, 17 VIN 4, 18 NC Supply Voltage (Input): Requires bypass capacitor to PGND. Both pins must be connected to VIN. June 2009 Pin Function Power Ground: Connect all pins to central ground point. Switch (Output): Internal power MOSFET output switches. Feedback (Input): Connect to output voltage divider resistors. not internally connected. 2 M9999-063009 Micrel, Inc. MIC2179 Absolute Maximum Ratings(1) Operating Ratings(2) Supply Voltage [100ms transient] (VIN).......................... 18V Output Switch Voltage (VSW).................................................... 18V Output Switch Current (ISW).......................................... 6.0A Enable, PWM Control Voltage (VEN, VPWM)................... 18V Sync Voltage (VSYNC)....................................................... 6V Supply Voltage (VIN)........................................4.5V to 16.5V Junction Temperature Range (TJ)............. –40°C to +125°C Electrical Characteristics(3) VIN = 7.0V; TA = 25°C, bold indicates –40°C ≤ TA ≤ 85°C; unless noted. Symbol Parameter Input Supply Current ISS VBIAS VFB Bias Regulator Output Voltage Feedback Voltage Output Voltage VOUT VTH Undervoltage Lockout IFB Feedback Bias Current Error Amplifier Gain VTL Condition Min PWM mode, output not switching, 4.5V ≤ VIN ≤ 16.5V skip mode, output not switching, 4.5V ≤ VIN ≤ 16.5V Typ Max Units 1.0 1.5 mA 600 750 µA 1 25 µA VIN = 16.5V 3.10 3.30 3.4 V 1.22 1.245 1.27 V MIC2179 [adj.]: VOUT = 3.3V, 5V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 1A 3.20 3.3 3.14 3.40 3.46 V V 4.85 5.0 5.15 V MIC2179-5.0: 6V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 1A 4.85 5.0 4.75 5.15 5.25 V 3.20 3.3 3.40 V MIC2179-3.3: 5V ≤ VIN ≤ 16V, 10mA ≤ ILOAD ≤ 1A 3.20 3.3 3.14 3.40 3.46 V V 4.35 V lower threshold 3.90 VEN = 0V, 4.5V ≤ VIN ≤ 16.5V MIC2179 [adj.]: VOUT = 3.3V, ILOAD = 0 MIC2179-5.0: ILOAD = 0 MIC2179-3.3: ILOAD = 0 upper threshold 4.25 4.15 V MIC2179 [adj.] 60 150 nA MIC2179-5.0, MIC2179-3.3 20 40 µA AVOL 0.6V ≤ VCOMP ≤ 0.8V 18 20 Error Amplifier Output Swing upper limit 15 0.9 1.5 V lower limit Error Amplifier Output Current source and sink 15 25 35 µA fO Oscillator Frequency 160 200 240 kHz DMAX Maximum Duty Cycle 100 VFB = 1.0V 0.05 0.1 V % 400 ns 220 300 kHz 0.8 2.2 V tON min Minimum On-Time SYNC Frequency Range SYNC Threshold SYNC Minimum Pulse Width 500 ns ISYNC SYNC Leakage –1 µA ILIM Current Limit RON Switch On-Resistance ISW Output Switch Leakage June 2009 VFB = 1.5V VSYNC = 0V to 5.5V PWM mode, VIN = 12V 3.4 skip mode high-side switch, VIN = 12V low-side switch, VIN = 12V VSW = 16.5V 3 300 1.6 0.01 1 4.3 5.5 600 A mA 160 350 mΩ 140 350 mΩ 1 10 µA M9999-063009 Micrel, Inc. MIC2179 Symbol Parameter Condition Enable Threshold IEN Enable Leakage PWM Threshold IPWM PWM Leakage Min VEN = 0V to 5.5V VPWM = 0V to 5.5V Typ Max Units 0.8 1.6 2.2 V –1 0.01 1 µA 0.6 1.1 1.4 V –1 0.01 1 µA MIC2179 [adj.]: measured at FB pin 1.09 1.13 1.17 V MIC2179-5.0: measured at FB pin 4.33 4.54 4.75 V MIC2179-3.3: measured at FB pin 2.87 PWRGD Output Low PWRGD Off Leakage ISINK = 1.0mA PWRGD Threshold VPWRGD = 5.5V 3.00 3.13 V 0.25 0.4 V 0.01 1 µA Notes: 1. Exceeding the absolute maximum rating may damage the device. 2. The device is not guaranteed to function outside its operating rating. 3. Specification for packaged product only. General. Devices are ESD sensitive. Handling precautions recommended. June 2009 4 M9999-063009 Micrel, Inc. MIC2179 Typical Characteristics 1.252 REFERENCE VOLTAGE (V) 195 190 185 180 Reference Voltage vs. Temperature 5.010 5.000 4.990 4.980 Current Limit vs. Temperature 4.9 4.7 4.5 4.3 4.1 3.9 3.7 3.5 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 6 4 2 0 1.240 18.0 17.5 17.0 16.5 125°C 85°C 25°C 0°C 250 200 150 100 50 2 90 4 June 2009 6 8 10 12 14 16 18 INPUT VOLTAGE (V) 6 8 10 12 14 16 18 INPUT VOLTAGE (V) 3.310 3.305 3.300 3.295 3.290 3.285 Feedback Input Bias Current vs. Temperature 100 80 60 40 20 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) Low-Side Switch On-Resistance 400 125°C 85°C 25°C 0°C 350 300 250 200 150 100 50 0 2 4 6 8 10 12 14 16 18 INPUT VOLTAGE (V) Skip- and PWM-Mode Efficiency 5.4V PWM 85 8.4V Skip 80 75 8.4V PWM 70 65 2 4 MIC2179-3.3 120 High-Side Switch On-Resistance 300 3.315 Reference Voltage vs. Temperature 3.280 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) Error-Amplifier Gain vs. Temperature 18.5 95 EFFICIENCY (%) SUPPLY CURRENT (mA) 8 1.242 0 OUTPUT SWITCHING 10 1.244 350 PWM-Mode Supply-Current 12 1.246 16.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) ON-RESISTANCE (mΩ) CURRENT LIMIT (A) 4.970 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 5.5 5.3 5.1 1.248 19.0 AMPLIFIER VOLTAGE GAIN REFERENCE VOLTAGE (V) MIC2179-5.0 5.020 3.320 1.238 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 175 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 5.030 MIC2179 [adj.] BIAS CURRENT (nA) 200 1.250 ON-RESISTANCE (mΩ) FREQUENCY (kHz) 205 Reference Voltage vs. Temperature REFERENCE VOLTAGE (V) Oscillator Frequency vs. Temperature 60 10 5.4V Skip 100 600 OUTPUT CURRENT (mA) 5 M9999-063009 Micrel, Inc. MIC2179 Block Diagram VIN 4.5V to 16.5V 100µF VIN UVLO, Thermal Shutdown EN Enable Shutdown 15 ISENSE Amp. Output Control Logic 3.3V Regulator 110mΩ N-channel PWM 5 SYNC 13 COUT PGND 2 ILIMIT Comp. * 19 * Connect SGND to PGND 20 PWM/ Skip-Mode Select ILIMIT Thresh. Voltage Bold lines indicate high current traces Corrective Ramp 200kHz Oscillator Reset Pulse R1 Skip-Mode Comp. R S FB 7 Q VIN Power Good Comp. PWM Comp. CC VOUT 1 internal supply Voltage RC L SW D 0.01µF Stop R1 R1 VOUT = 1.245 VOUT 1.245( R2 + 1) 1 R2 3 14 Skip Mode PWM Mode 17 110mΩ P-channel BIAS R3 4.02k 16 PWRGD R2 20k Output Good 6 COMP 8 1.13V VREF 1.245V MIC2179 [Adjustable] SGND June 2009 9 6 10 11 12 M9999-063009 Micrel, Inc. MIC2179 Functional Description connect an external load to the BIAS pin. It is not designed to provide an external supply voltage. Frequency Synchronization The MIC2179 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 to 300kHz, and can operate with a minimum pulse width of 500ns. If synchronization is not required, connect SYNC to ground. Power Good Flag The power good flag (PWRGD) is an error flag that alerts a system when the output is not in regulation. When the output voltage is 10% below its nominal value, PWRGD is logic low, signaling that VOUT is to low. PWRGD is an open-drain output that can sink 1mA from a pull-up resistor connected to VIN. 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). PWM-Mode Operation Refer to “PWM Mode Functional Diagram” which is a simplified block diagram of the MIC2179 operating in PWM mode and its associated waveforms. When operating in PWM mode, the output P-channel and Nchannel 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 sawtooth 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 P-channel and turns on the N-channel. Energy is then 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 MIC2179 uses current-mode control to adjust the duty cycle and regulate the output voltage. Current-mode 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 Micrel’s MIC2179 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. Its has internal power MOSFETs that supply up to 1.5A load current and operates with up to 100% duty cycle to allow low-dropout operation. To optimize efficiency, the MIC2179 operates in PWM and skip mode. Skip mode provides the best efficiency when load current is less than 150mA, while PWM mode is more efficient at higher current. PWM or skip-mode operation is selected externally, allowing an intelligent system (i.e. microprocessor controlled) to select the correct operating mode for efficiency and noise requirements. During PWM operation, the MIC2179 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 more detail. 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 150mΩ when the MIC2179 operates from a 12V supply. Antishoot-through circuitry prevents the P-channel and N-channel from turning on at the same time. Current Limit The MIC2179 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 4.3A. When it does, the P-channel is turned off until the next switching period begins. Undervoltage Lockout Undervoltage lockout (UVLO) turns off the output when the input voltage (VIN) is to 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 MIC2179 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. Shutdown Mode The MIC2179 has a low-current shutdown mode that is controlled by the enable input (EN). When a logic 0 is applied to EN, the MIC2179 is in shutdown mode, and its quiescent current drops to less than 5µA. Internal Bias Regulator An internal 3.3V regulator provides power to the MIC2179 control circuits. This internal supply is brought out to the BIAS pin for bypassing by an external 0.01µF capacitor. Do not June 2009 7 M9999-063009 Micrel, Inc. MIC2179 (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 the 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 N-channel 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. Skip-Mode Operation Refer to “Skip Mode Functional Diagram” which is a simplified block diagram of the MIC2179 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 N-channel is kept off to optimize efficiency by reducing gate charge dissipation. VOUT is regulated by skipping switching cycles that turn on the P-channel. To begin analyzing MIC2179 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 400mA. When IL1 reaches this value, the current limit comparator sets the RS latch output to logic 0, turning off June 2009 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 decreases 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, and this begins another switching cycle. The skip-mode comparator regulates VOUT by controlling when the MIC2179 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 Pchannel 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 MIC2179 to always supply up to 300mA of load current when operating in skip mode. Selecting PWM- or Skip-Mode Operation PWM or skip mode operation is selected by an external logic signal applied to the PWM pin. A logic low places the MIC2179 into PWM mode, and logic high places it into skip mode. Skip mode operation provides the best efficiency when load current is less than 150mA, and PWM operation is more efficient at higher currents. The MIC2179 was designed to be used in intelligent systems that determine when it should operate in PWM or skip mode. This makes the MIC2179 ideal for applications where a regulator must guarantee low noise operation when supplying light load currents, such as cellular telephone, audio, and multimedia circuits. There are two important items to be aware of when selecting PWM or skip mode. First, the MIC2179 can start-up only in PWM mode, and therefore requires a logic low at PWM during start-up. Second, in skip mode, the MIC2179 will supply a maximum load current of approximately 300mA, so the output will drop out of regulation when load current exceeds this limit. To prevent this from occurring, the MIC2179 should change from skip to PWM mode when load current exceeds 200mA. 8 M9999-063009 Micrel, Inc. MIC2179 PWM-Mode Functional Diagram VIN 4.5V to 16.5V CIN VIN 16 17 R1 VOUT = 1.245 ( R2 + 1) 110mΩ P-channel IS E N S E Amp. L1 SW 3 IL1 D 110mΩ N-channel VOU T COU T P GND 1 2 19 20 Stop S Y NC 13 Corrective Ramp 200kHz Oscillator R1 Reset Pulse FB 7 R2 Q R S PWM Comp. Error Amp. COMP CC RC 8 VR E F1.245V MIC2179 [Adjustable] PWM-Mode Signal Path SGND 9 10 11 12 VS W Reset Pulse IL 1 ILOAD ∆IL1 Error Amp. Output IS E N S E June 2009 9 M9999-063009 Micrel, Inc. MIC2179 Skip-Mode Functional Diagram VIN 4.5V to 16.5V CIN VIN 16 17 Output Control Logic S R Q VOUT = 1.245 ( 110mΩ P-channel One Shot IS E N S E Amp. R1 + 1) R2 L1 SW 3 VOU T IL1 D COU T P GND 1 2 ILIMIT Comp. 19 20 ILIMIT Thresh. Voltage R1 Skip-Mode Comp. FB 7 R2 VR E F1.245V MIC2179 [Adjustable] Skip-Mode Signal Path SGND VS W 9 10 11 12 VIN VOU T 0 One-Shot Pulse ILIM IL 1 0 VR E F + 5mV VF B VR E F – 5mV June 2009 10 M9999-063009 Micrel, Inc. MIC2179 Application Information To maximize efficiency, the inductor’s resistance must be less than the output switch on-resistance (preferably, 50mΩ or less). 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. Therefore, ESR must be equal or less than a maximum value calculated for a specified VRIPPLE (typically less than 1% of the output voltage) and ∆IL(max): VRIPPLE ESR MAX = ∆IL(max) Typically, capacitors in the range of 100 to 220µF have ESR less than this maximum value. The output capacitor can be 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. 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 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 ultrafast-recovery diode (tR < 100ns) to minimize power dissipation from the diode’s reverse-recovery charge. 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, the 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 to 10nF for the capacitor and 5kΩ to 20kΩ for the resistor. Printed Circuit Board Layout A well designed PC board will prevent switching noise and ground bounce from interfering with the operation of the MIC2179. 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. Feedback Resistor Selection (Adjustable Version) The output voltage is programmed by connecting an external resistive divider to the FB pin as shown in “MIC2179 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 with the following formula: VOUT R1 = R2 (( ) -1) 1.245V 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: ILOAD(max) IRMS(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 0.2in, 5mm). Also, place a 0.1µF ceramic bypass capacitor as close as possible to VIN. Inductor Selection The MIC2179 is a current-mode controller with internal slope compensation. As a result, the inductor must be at least a minimum value to prevent subharmonic oscillations. This minimum value is calculated by the following formula: LMIN = VOUT x 3.0 µH/V 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 increase significantly. Both of these effects will degrade efficiency. The formula for peak inductor current is: ∆IL(max) IL(peak) = ILOAD(max) + 2 Where: ∆IL(max) = VOUT (1 June 2009 VOUT VIN(max) )x 1 L•f 11 M9999-063009 Micrel, Inc. MIC2179 The feedback resistors, RC compensation network, and BIAS pin bypass capacitor should be located close to their respective pins. To prevent ground bounce, their ground traces and SGND should not be in the path of switching Suggested Manufacturers List Inductors Capacitors Coilcraft 1102 Silver Lake Rd. Cary, IL 60013 tel: (708) 639-2361 fax: (708) 639-1469 AVX Corp. 801 17th Ave. South Myrtle Beach, SC 29577 tel: (803) 448-9411 fax: (803) 448-1943 Coiltronics 6000 Park of Commerce Blvd. Boca Raton, FL 33487 tel: (407) 241-7876 fax: (407) 241-9339 Bi Technologies 4200 Bonita Place Fullerton, CA tel: (714) 447-2345 fax: (714) 447-2500 June 2009 currents returning to the power supply ground bus. SGND and PGND should be tied together by a ground plane that extends under the MIC2179. Sanyo Video Components Corp. 2001 Sanyo Ave. San Diego, CA 92173 tel: (619) 661-6835 fax: (619) 661-1055 Sprague Electric Lower Main St. 60005 Sanford, ME 04073 tel: (207) 324-4140 12 Diodes Transistors General Instruments (GI) 10 Melville Park Rd. Melville, NY 11747 tel: (516) 847-3222 fax: (516) 847-3150 Siliconix 2201 Laurelwood Rd. Santa Clara, CA 96056 tel: (800) 554-5565 International Rectifier Corp. 233 Kansas St. El Segundo, CA 90245 tel: (310) 322-3331 fax: (310) 322-3332 Motorola Inc. MS 56-126 3102 North 56th St. Phoenix, AZ 85018 tel: (602) 244-3576 fax: (602) 244-4015 M9999-063009 Micrel, Inc. MIC2179 Package Information 20-Pin SSOP (SM) 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 This 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. © 2001 Micrel Incorporated June 2009 13 M9999-063009