MIC2177 Micrel 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. • 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 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. Applications • • • • • • The MIC2177 is packaged in a 20-lead 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. High-efficiency, battery-powered supplies Buck (step-down) dc-to-dc converters Cellular telephones Laptop computers Hand-held instruments Battery Charger Typical Application VIN 6V to 16.5V C2 22µF 35V U1 ON ENABLE 20 OFF 1,2,9 100 VIN EN OUT MIC2177-5.0 SW AUTO MODE Mode PWM MODE C6 2.2nF April 1999 11 18 AUTO PGND SYNC COMP SGND R7 15k C5 10nF 13 14–17 FB BIAS 5V Output Efficiency 10 3,8 4–7 L1, 33µH VOUT 5V/2.5A D1 MBRS140 C7 220µF 10V 12 19 C3 220µF 10V 95 EFFICIENCY (%) C1 22µF 35V 90 85 80 75 C4 0.01µF 70 10 1 VIN = 6V SKIP PWM 100 1000 2500 OUTPUT CURRENT (mA) MIC2177 MIC2177 Micrel Ordering Information Part Number Output Voltage Switching Frequency Temperature Range Package MIC2177-3.3BWM 3.3V 200kHz –40°C to +85°C 20-lead wide SOP MIC2177-5.0BWM 5.0V 200kHz –40°C to +85°C 20-lead wide SOP MIC2177BWM adj. 200kHz –40°C to +85°C 20-lead wide SOP Pin Configuration VIN 1 20 EN VIN 2 19 BIAS SW 3 18 SYNC PGND 4 17 SGND PGND 5 16 SGND PGND 6 15 SGND PGND 7 14 SGND SW 8 13 COMP VIN 9 12 FB 11 AUTO OUT 10 20-Lead Wide SOP 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 Output Voltage Sense (Input): Senses output voltage to determine minimum switch current for PWM operation. Connect directly to VOUT. 11 AUTO Automatic Mode: Connect 2.2nF timing capacitor for automatic PWM-/skipmode switching. Regulator operates exclusively in PWM mode when pin is pulled low. 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 to SGND. Do not apply any external load. 20 EN Enable (Input): Logic high enables operation. Logic low shuts down regulator. Do not allow pin to float. MIC2177 Pin Function Power Ground: Output stage ground connections. Connect all pins to a common ground plane. 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. 2 April 1999 MIC2177 Micrel 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 Range (TJ) ........... –40°C to +125°C Electrical Characteristics VIN = 7.0V; TA = 25°C, bold indicates –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 VIN = 16.5V 3.10 3.30 3.4 V VFB Feedback Voltage MIC2177 [adj.]: VOUT = 3.3V, ILOAD = 0 1.22 1.245 1.27 V VOUT 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 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 VTH Undervoltage Lockout VTL IFB AVOL upper threshold lower threshold Feedback Bias Current 3.9 4.15 MIC2177 [adj.] 60 150 nA MIC2177-5.0, MIC2177-3.3 20 40 µA 20 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 source and sink fO Oscillator Frequency DMAX Maximum Duty Cycle VFB = 1.0V tON min Minimum On-Time VFB = 1.5V V 0.05 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 ISW April 1999 Switch On-Resistance Output Switch Leakage skip mode 600 high-side switch, VIN = 12V 90 250 mΩ low-side switch, VIN = 12V 110 250 mΩ 1 10 µA VSW = 16.5V 3 mA MIC2177 MIC2177 Symbol Micrel Parameter Condition Enable Threshold IEN Enable Leakage VEN = 0V to 5.5V AUTO Threshold Min Typ Max Units 0.8 1.6 2.2 V –1 0.01 1 µA 0.8 1.6 7 11 V 15 µA 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 General Note: Devices are ESD sensitive. Handling precautions recommended. MIC2177 4 April 1999 MIC2177 Micrel Typical Characteristics 180 175 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) REFERENCE VOLTAGE (V) 1.242 1.240 1.238 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) Reference Voltage vs. Temperature 5.030 MIC2177-5.0 5.020 5.010 5.000 4.990 4.980 4.970 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 18.5 18.0 17.5 17.0 16.5 4.7 4.6 4.5 4.4 4.3 4.2 4 2 0 April 1999 150 100 50 2 4 4 6 8 10 12 14 16 18 INPUT VOLTAGE (V) 3.280 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) Feedback Input Bias Current vs. Temperature 120 60 40 20 Low-Side Switch On-Resistance 125°C 85°C 25°C 0°C 300 250 200 150 100 50 0 2 100 95 VIN = 5V 85 8V 80 12V 75 70 60 10 80 350 95 90 100 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 6 8 10 12 14 16 18 INPUT VOLTAGE (V) SKIP PWM 65 2 3.285 3.3V Output Efficiency EFFICIENCY (%) SUPPLY CURRENT (mA) 6 3.290 100 OUTPUT SWITCHING 8 125°C 85°C 25°C 0°C 200 0 PWM-Mode Supply Current 10 3.295 High-Side Switch On-Resistance 250 4.1 4.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 12 3.300 16.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) ON-RESISTANCE (mΩ) 4.9 4.8 3.305 Error-Amplifier Gain vs. Temperature 19.0 Current Limit vs. Temperature 5.0 CURRENT LIMIT (A) 1.244 3.310 BIAS CURRENT (nA) 185 1.246 MIC2177-3.3 3.315 ON-RESISTANCE (mΩ) 190 1.248 Reference Voltage vs. Temperature 3.320 100 1000 2500 OUTPUT CURRENT (mA) 5 EFFICIENCY (%) 195 MIC2177 [adj.] 1.250 AMPLIFIER VOLTAGE GAIN FREQUENCY (kHz) 200 Reference Voltage vs. Temperature 1.252 REFERENCE VOLTAGE (V) 205 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 12V 80 75 70 10 SKIP PWM 100 1000 2500 OUTPUT CURRENT (mA) MIC2177 MIC2177 Micrel Block Diagram VIN 4.5V to 16.5V CIN VIN UVLO, Thermal Shutdown 1 2 9 100mΩ P-channel 20 R1 R2 1 L1 3 VOUT 8 D EN Enable Shutdown 1.245 SW ISENSE Amp. Output Control Logic VOUT 3.3V Regulator 100mΩ N-channel COUT PGND 4 BIAS 5 ILIMIT Comp. 19 0.01µF 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 R2 11 2.2nF 40mV Skip-Mode Comp. RESET PULSE R Q S PWM Comp. Error Amp. COMP RC CC VREF 1.245V 13 MIC2177 [Adjustable] SGND MIC2177 6 14 15 16 17 April 1999 MIC2177 Micrel 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. 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. 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 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. Switch Output The switch output (SW) is a half H-bridge consisting of a highside 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. Antishootthrough circuitry prevents the P-channel and N-channel from turning on at the same time. 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 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-tosource 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 Pchannel 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. 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. 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 The MIC2177 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 (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 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. 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. 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. April 1999 7 MIC2177 MIC2177 Micrel Changing from PWM to Skip Mode Refer to “Block Diagram” for circuits described in the following sections. 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. 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 PChannel 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: 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 MIC2177 operating in skip mode and its associated waveforms. IMIN = 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 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 threshold 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. Changing from Skip to PWM Mode 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 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 IL1 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 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, which begins another switching cycle. The MIC2177 will automatically change from skip to PWM mode when ILOAD exceeds 300mA. During skip-mode 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. 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. 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. MIC2177 420mA − ∆IL1 2 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. 8 April 1999 MIC2177 Micrel PWM-Mode Functional Diagram VIN 4.5V to 16.5V CIN VIN 1 2 9 100mΩ P-channel VOUT 1.245 R1 R2 1 SW ISENSE Amp. L1 3 VOUT 8 IL1 D 100mΩ N-channel COUT PGND 4 5 6 7 Corrective Ramp Stop SYNC 18 200kHz Oscillator R1 Reset Pulse FB 12 R2 R Q S PWM Comp. Error Amp. COMP CC RC 13 VREF 1.245V MIC2177 [Adjustable] PWM-Mode Signal Path SGND 14 15 16 17 VSW Reset Pulse IL1 ILOAD ∆IL1 Error Amp. Output ISENSE April 1999 9 MIC2177 MIC2177 Micrel Skip-Mode Functional Diagram VIN 4.5V to 16.5V CIN VIN 1 2 9 Output Control Logic S Q 100mΩ P-channel R One Shot ISENSE Amp. VOUT 1.245 R1 R2 1 SW L1 3 VOUT 8 IL1 D COUT PGND 4 5 ILIMIT Comp. 6 7 ILIMIT Thresh. Voltage R1 Skip-Mode Comp. FB 12 R2 VREF 1.245V MIC2177 [Adjustable] Skip-Mode Signal Path SGND VSW 14 15 16 17 VIN VOUT 0 One-Shot Pulse ILIM IL1 0 VREF + 5mV VFB VREF – 5mV MIC2177 10 April 1999 MIC2177 Micrel To maximize efficiency, the inductor’s resistance must be less than the output switch on-resistance (preferably 50mΩ or less). Output Capacitor Selection 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: 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: ESRMAX = V R1 = R2 OUT − 1 1.245V 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. 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-ofthumb, 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) = 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 ultrafast-recovery diode (tR < 100ns) to minimize power dissipation from the diode’s reverse-recovery charge. Compensation 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. 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: 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 loopgain 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. Printed Circuit Board Layout VOUT LMIN = VOUT 1 – × 8.3µH/V VIN(max) 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 increase significantly. Both of these effects will degrade efficiency. The formula for peak inductor current is: A well designed PC board will prevent switching noise and ground bounce from interfering with the operation of 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. IL(peak) = ILOAD(max) + 300mA April 1999 600mA VRIPPLE 11 MIC2177 MIC2177 Micrel 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 OUT EN SW 18 SYNC MIC2177 11 AUTO PGND FB COMP SGND BIAS 13 C5 0.01 µF 19 14–17 R4 10k C4 6.8nF 10 L1, 50µH 3,8 4–7 VOUT 3.3V/1A D1 MBRS130L C2 100µF 10V 12 U1 C1 C2 C3 C3 C4 0.01µF D1 L1 L1 L1 Micrel AVX AVX Z5UorX7R X7RorNP0 Motorola Coiltronics Coilcraft Bi MIC2177-3.3BWM TPSE226M035R0300, ESR = 0.3Ω TPSD107M010R0100, ESR = 0.1Ω Ceramic Dielectric Material Ceramic Dielectric Material MBRS130LT3 CTX50-4P, DCR = 0.097Ω DO3316P-473, DCR = 0.12Ω HM77-11003, DCR = 0.073Ω Figure 1. MIC2177 4.5V–16.5V to 3.3/1A Regulator Suggested Manufacturers List Inductors Capacitors Diodes Transistors 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 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 Coiltronics 6000 Park of Commerce Blvd. Boca Raton, FL 33487 tel: (407) 241-7876 fax: (407) 241-9339 Sanyo Video Components Corp. 2001 Sanyo Ave. San Diego, CA 92173 tel: (619) 661-6835 fax: (619) 661-1055 International Rectifier Corp. 233 Kansas St. El Segundo, CA 90245 tel: (310) 322-3331 fax: (310) 322-3332 Bi Technologies 4200 Bonita Place Fullerton, CA tel: (714) 447-2345 fax: (714) 447-2500 Sprague Electric Lower Main St. 60005 Sanford, ME 04073 tel: (207) 324-4140 Motorola Inc. MS 56-126 3102 North 56th St. Phoenix, AZ 85018 tel: (602) 244-3576 fax: (602) 244-4015 MIC2177 12 April 1999 MIC2177 Micrel Package Information PIN 1 DIMENSIONS: INCHES (MM) 0.301 (7.645) 0.297 (7.544) 0.027 (0.686) 0.031 (0.787) 0.094 (2.388) 0.090 (2.286) 0.050 (1.270) TYP 0.016 (0.046) TYP 0.509 (12.929) 0.505 (12.827) 0.103 (2.616) 0.099 (2.515) 7° TYP 0.015 R (0.381) 0.015 (0.381) SEATING MIN PLANE 0.297 (7.544) 0.293 (7.442) 0.330 (8.382) 0.326 (8.280) 0.022 (0.559) 0.018 (0.457) 5° TYP 10° TYP 0.032 (0.813) TYP 0.408 (10.363) 0.404 (10.262) 20-Lead Wide SOP (WM) April 1999 13 MIC2177 MIC2177 MIC2177 Micrel 14 April 1999 MIC2177 April 1999 Micrel 15 MIC2177 MIC2177 Micrel MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131 TEL + 1 (408) 944-0800 FAX + 1 (408) 944-0970 WEB USA http://www.micrel.com This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel Inc. © 1999 Micrel Incorporated MIC2177 16 April 1999