MIC2179YSM

MIC2179 1.5A Synchronous Buck Regulator Features General Description • Input Voltage Range: +4.5V to +16.5V • Dual-Mode Operation for High Efficiency (up to 96%): - PWM Mode for > 150 mA Load Current - Skip Mode for < 150 mA Load Current • 150 mΩ Internal Power MOSFETs at 12V Input • 200 kHz Preset Switching Frequency • Low Quiescent Current - 1.0 mA in PWM Mode - 600 μA in Skip Mode - < 5 μA 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 The MIC2179 is a 200 kHz synchronous buck (step-down) switching regulator designed for high-efficiency, battery-powered applications. Applications • • • • • • High-Efficiency, Battery-Powered Supplies Buck (Step-Down) DC/DC Converters Laptop Computers Cellular Telephones Handheld Instruments Battery Chargers 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 5 μA. 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 200 kHz and can be synchronized to an external clock signal of up to 300 kHz. 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 is packaged in a 20-lead SSOP package with an operating temperature range of -40°C to +85°C. Package Type MIC2179 20-Lead SSOP (SM) (Top View) PGND 1 20 PGND PGND 2 19 PGND SW 3 18 NC NC 4 17 VIN PWM 5 16 VIN PWRGD 6 FB 7  2021 Microchip Technology Inc. 15 EN 14 BIAS COMP 8 13 SYNC SGND 9 12 SGND SGND 10 11 SGND DS20006284B-page 1 MIC2179 Typical Application Circuit VIN 5.4V to 16.5V C1 10μF 20V U1 R1 20k 15 Output Good Output Low Skip Mode PWM Mode 6 5 13 16,17 L1 22μH VIN EN 3,4 SW PWRGD PWM SYNC COMP 8 1,2, 19,20 MIC 2179-3.3 PGND C2 100μF 6.3V 7 FB SGND VOUT 3.3V/600mA D1 MBRM120 BIAS 9–12 14 C3 0.01μF C4 6.8nF R5 4.02k Pins 4 and 18 are not connected. Pins 3 and 4 can be connected together for a low-impedance connection. Functional Block Diagram V IN 4.5V to 16.5V 100μF VIN UVLO, Thermal Shutdown Enable Shutdown EN 15 16 17 V OUT 150m Ω P-channel I SENSE Amp. Output Control Logic 3.3V Regulator 1.245 D 150m Ω N-channel 0.01μF C OUT PGND 1 Internal Supply Voltage PWM 5 2 I LIMIT Comp. SYNC 13 * 19 * Connect S GND to P GND 20 PWM/ Skip-Mode Select I LIMIT Thresh. Voltage Bold lines indicate high current traces Corrective Ramp Stop V OUT 3 14 Skip Mode PWM Mode 200kHz Oscillator R1 Skip-Mode Comp. Reset Pulse FB R 7 Q S R2 V IN Power Good Comp. PWM Comp. RC 1 L SW BIAS R3 4.02kΩ R1 R2 20kΩ PWRGD Output Good 6 COMP 8 CC 1.13V V REF 1.245V MIC2179 [Adjustable] SGND DS20006284B-page 2 9 10 11 12  2021 Microchip Technology Inc. MIC2179 1.0 ELECTRICAL CHARACTERISTICS Absolute Maximum Ratings † Supply Voltage, 100 ms 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 Operating Ratings †† Supply Voltage (VIN) ............................................................................................................................... +4.5V to +16.5V † Notice: Stresses above those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at those or any other conditions above those indicated in the operational sections of this specification is not intended. Exposure to maximum rating conditions for extended periods may affect device reliability. †† Notice: The device is not guaranteed to function outside its operating ratings. ELECTRICAL CHARACTERISTICS Electrical Characteristics: VIN = 7.0V; TA = +25°C, bold indicates –40°C ≤ TA ≤ +85°C; unless noted. Devices are ESD sensitive. Handling precautions recommended. Parameter Input Supply Current Bias Regulator Output Voltage Feedback Voltage Output Voltage Undervoltage Lockout Feedback Bias Current Error Amplifier Gain Error Amplifier Output Swing  2021 Microchip Technology Inc. Sym. Min. Typ. Max. Units Conditions — 1.0 1.5 mA PWM mode, output not switching, 4.5V ≤ VIN ≤ 16.5V — 600 750 µA Skip mode, output not switching, 4.5V ≤ VIN ≤ 16.5V — 1 25 µA VEN = 0V, 4.5V ≤ VIN ≤ 16.5V VBIAS 3.10 3.30 3.4 V VIN = 16.5V VFB 1.22 1.245 1.27 V MIC2179 [adj.]: VOUT = 3.3V, ILOAD = 0A 3.20 3.3 3.40 V 3.14 — 3.46 V 4.85 5.0 5.15 V MIC2179-5.0: ILOAD = 0A 4.85 5.0 5.15 V 4.75 — 5.25 V MIC2179-5.0: 6V ≤ VIN ≤ 16V, 10 mA ≤ ILOAD ≤ 1A 3.20 3.3 3.40 V MIC2179-3.3: ILOAD = 0A 3.20 3.3 3.40 V 3.14 — 3.46 V MIC2179-3.3: 5V ≤ VIN ≤ 16V, 10 mA ≤ ILOAD ≤ 1A VTH — 4.25 4.35 V Upper threshold VTL 3.90 4.15 — V Lower threshold — 60 150 nA MIC2179 [adj.] — 20 40 µA MIC2179-5.0, MIC2179-3.3 15 18 20 — 0.6V ≤ VCOMP ≤ 0.8V 0.9 15 — — 0.05 0.1 ISS VOUT IFB AVOL — V MIC2179 [adj.]: VOUT = 3.3V, 5V ≤ VIN ≤ 16V, 10 mA ≤ ILOAD ≤ 1A Upper Limit Lower Limit DS20006284B-page 3 MIC2179 ELECTRICAL CHARACTERISTICS (CONTINUED) Electrical Characteristics: VIN = 7.0V; TA = +25°C, bold indicates –40°C ≤ TA ≤ +85°C; unless noted. Devices are ESD sensitive. Handling precautions recommended. Parameter Sym. Min. Typ. Max. Units Error Amplifier Output Current — 15 25 35 µA Source and sink Oscillator Frequency fO 160 200 240 kHz — Maximum Duty Cycle DMAX 100 — — % VFB = 1.0V tON(MIN) — 300 400 ns VFB = 1.5V SYNC Frequency Range — 220 — 300 kHz — SYNC Threshold — 0.8 1.6 2.2 V — SYNC Minimum Pulse Width — 500 — — ns — ISYNC –1 0.01 1 µA VSYNC = 0V to 5.5V 3.4 4.3 5.5 A PWM mode, VIN = 12V — 600 — mA — 160 350 — 140 350 Minimum On-Time SYNC Leakage Conditions Current Limit ILIM Switch On-Resistance RON Output Switch Leakage ISW — 1 10 µA VSW = 16.5V Enable Threshold — 0.8 1.6 2.2 V — Enable Leakage IEN –1 0.01 1 µA VEN = 0V to 5.5V PWM Threshold — 0.6 1.1 1.4 V — PWM Leakage IPWM –1 0.01 1 µA VPWM = 0V to 5.5V 1.09 1.13 1.17 4.33 4.54 4.75 mΩ Skip mode High-side switch, VIN = 12V Low-side switch, VIN = 12V MIC2179 [adj.]: measured at FB pin PWRGD Threshold — V 2.87 3.00 3.13 PWRGD Output Low — — 0.25 0.4 V ISINK = 0.5 mA PWRGD Off Leakage — — 0.01 1 µA VPWRGD = 5.5V MIC2179-5.0: measured at FB pin MIC2179-3.3: measured at FB pin TEMPERATURE SPECIFICATIONS Parameters Sym. Min. Typ. Max. Units TJ –40 — +125 °C Conditions Temperature Ranges Operating Junction Temperature Range Note 1: Note 1 The maximum allowable power dissipation is a function of ambient temperature, the maximum allowable junction temperature and the thermal resistance from junction to air (i.e., TA, TJ, JA). Exceeding the maximum allowable power dissipation will cause the device operating junction temperature to exceed the maximum +125°C rating. Sustained junction temperatures above +125°C can impact the device reliability. DS20006284B-page 4  2021 Microchip Technology Inc. MIC2179 2.0 TYPICAL PERFORMANCE CURVES Note: The graphs and tables provided following this note are a statistical summary based on a limited number of samples and are provided for informational purposes only. The performance characteristics listed herein are not tested or guaranteed. In some graphs or tables, the data presented may be outside the specified operating range (e.g., outside specified power supply range) and therefore outside the warranted range. 205 REFERENCE VOLTAGE (V) 5.030 FREQUENCY (kHz) 200 195 190 185 180 175 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) FIGURE 2-1: Temperature. Oscillator Frequency vs. MIC2179-5.0 5.020 5.010 5.000 4.990 4.980 4.970 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) FIGURE 2-4: Temperature. 19.0 MIC2179 [adj.] 1.248 1.246 1.244 1.242 1.240 AMPLIFIER VOLTAGE GAIN REFERENCE VOLTAGE (V) 1.252 1.250 Reference Voltage vs. 17.5 17.0 16.5 Error-Amplifier Gain vs. 120 MIC2179-3.3 BIAS CURRENT (nA) REFERENCE VOLTAGE (V) 18.0 FIGURE 2-5: Temperature. 3.320 3.315 18.5 16.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) 1.238 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) FIGURE 2-2: Temperature. Reference Voltage vs. 3.310 3.305 3.300 3.295 3.290 100 80 60 40 20 3.285 3.280 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) FIGURE 2-3: Temperature. Reference Voltage vs.  2021 Microchip Technology Inc. 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (°C) FIGURE 2-6: Feedback Input Bias Current vs. Temperature. DS20006284B-page 5 MIC2179 12 5.3 5.1 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) FIGURE 2-7: Temperature. Current Limit vs. SUPPLY CURRENT (mA) CURRENT LIMIT (A) 5.5 6 4 2 2 4 6 8 10 12 14 16 18 INPUT VOLTAGE (V) FIGURE 2-10: PWM-Mode Supply Current. 95 125°C 85°C 25°C 0°C 300 250 90 200 150 100 2 4 FIGURE 2-8: On-Resistance. 6 8 10 12 14 16 18 ,138792/7$*(9 High-Side Switch 8.4V S kip 80 75 8.4V P WM 70 65 50 5.4V P WM 85 EFFICIENCY (%) 215(6,67$1&(PŸ 8 0 350 0 OUTPUT SWITCHING 10 60 10 FIGURE 2-11: Efficiency. 5.4V S kip 100 OUTPUT CURRENT (mA) 600 Skip-Mode and PWM-Mode 215(6,67$1&(PŸ 400 125°C 85°C 25°C 0°C 350 300 250 200 150 100 50 0 2 4 FIGURE 2-9: On-Resistance. DS20006284B-page 6 6 8 10 12 14 16 18 ,138792/7$*(9 Low-Side Switch  2021 Microchip Technology Inc. MIC2179 3.0 PIN DESCRIPTIONS The descriptions of the pins are listed in Table 3-1. TABLE 3-1: PIN FUNCTION TABLE Pin Number Pin Name 1, 2, 19, 20 PGND Description Power ground: Connect all pins to central ground point. 3 SW Switch (output): Internal power MOSFET output switches. 4, 18 NC Not internally connected. 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 Error flag (output): Open-drain output. Active-low when FB input is 10% below the reference voltage (VREF). 7 FB Feedback (input): Connect to output voltage divider resistors. 8 COMP Compensation: Output of internal error amplifier. Connect capacitor or series RC network to compensate the regulator control loop. 9, 10, 11, 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 0.01 µF 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 Supply voltage (input): Requires bypass capacitor to PGND. Both pins must be connected to VIN.  2021 Microchip Technology Inc. DS20006284B-page 7 MIC2179 4.0 FUNCTIONAL DESCRIPTION The 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. It 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 150 mA, 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 that provides superior line regulation and makes the control loop easier to compensate. The PWM switching frequency is set internally to 200 kHz and can be synchronized to an external clock frequency up to 300 kHz. Other features include a low-current shutdown mode, current limit, undervoltage lockout, and thermal shutdown. See the following sections for more detail. 4.1 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 150 mΩ when the MIC2179 operates from a 12V supply. Anti-shoot-through circuitry prevents the P-channel and N-channel from turning on at the same time. 4.2 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. 4.3 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. 4.4 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. DS20006284B-page 8 4.5 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 5 μA. 4.6 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 0.01 μF capacitor. Do not connect an external load to the BIAS pin. It is not designed to provide an external supply voltage. 4.7 Frequency Synchronization The MIC2179 operates at a preset switching frequency of 200 kHz. 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 220 kHz to 300 kHz, and can operate with a minimum pulse width of 500 ns. If synchronization is not required, connect SYNC to ground. 4.8 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 too low. PWRGD is an open-drain output that can sink 1 mA from a pull-up resistor connected to VIN. 4.9 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). 4.10 PWM-Mode Operation Refer to the 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 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  2021 Microchip Technology Inc. MIC2179 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 (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 50 ns 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.  2021 Microchip Technology Inc. 4.11 Skip Mode Operation Refer to the 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 600 mA. 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, 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 10 mV of hysteresis to prevent oscillations in the control loop. When VFB is less than VREF – 5 mV, the comparator output is logic 1, allowing the P-channel to turn on. Conversely, when VFB is greater than VREF + 5 mV, the P-channel is turned off. Note that this is a self-oscillating topology that 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 300 mA of load current when operating in skip mode. 4.12 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 150 mA, 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. DS20006284B-page 9 MIC2179 current of approximately 300 mA, 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 200 mA. 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 PWM Mode Functional Diagram VIN 4.5V to 16.5V CIN VIN 16 17 VOUT = 1.245 150mŸ P-channel IS E N S E Amp. ( R1 + 1) R2 L1 SW VOUT 3 IL1 D 150mŸ N-channel COUT PGND 1 2 19 20 Corrective Ramp Stop S Y NC 13 200kHz Oscillator R1 Reset Pulse FB 7 R2 R Q 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 IL1 ILOAD ¨IL1 Error Amp. Output IS E N S E DS20006284B-page 10  2021 Microchip Technology Inc. MIC2179 Skip Mode Functional Diagram VIN 4.5V to 16.5V CIN VIN 16 17 Output Control Logic S Q VOUT = 1.245 150mŸ P-channel R One Shot IS E N S E Amp. ( R1 + 1) R2 L1 SW VOUT 3 IL1 D COUT PGND 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 VOUT 0 One-Shot Pulse ILIM IL1 0 VR E F + 5mV VF B VR E F – 5mV  2021 Microchip Technology Inc. DS20006284B-page 11 MIC2179 5.0 APPLICATION INFORMATION 5.1 Feedback Resistor Selection (Adjustable Version) The output voltage is programmed by connecting an external resistive divider to the FB pin as shown in the Functional 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 20 kΩ. However, to prevent feedback error due to input bias current at the FB pin, R2 should not be greater than 100 kΩ. After selecting R2, calculate R1 with the following formula: EQUATION 5-1: V OUT R1 = R2    ----------------- – 1   1.245V  EQUATION 5-3: L MIN = V OUT  3.0H/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: EQUATION 5-4: 5.2 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: EQUATION 5-2: I L  MAX  I L  PEAK  = I LOAD  MAX  + ---------------------2 Where: V OUT  1 I L  MAX  = V OUT   1 – ----------------------  ----------V IN  MAX  L  f To maximize efficiency, the inductor’s resistance must be less than the output switch on-resistance (preferably, 50 mΩ or less). 5.4 I RMS  MAX  I LOAD  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 in, 5 mm). Also, place a 0.1 μF ceramic bypass capacitor as close as possible to VIN. 5.3 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: DS20006284B-page 12 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 ΔIL x 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 ΔIL(MAX): EQUATION 5-5: V RIPPLE ESR MAX = ---------------------I L  MAX  Typically, capacitors in the range of 100 µF to 220 μF 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  2021 Microchip Technology Inc. MIC2179 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. 5.5 Output Diode Selection In PWM operation, inductor current flows through the output diode approximately 50 ns 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