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MIC2177
Micrel
MIC2177
2.5A Synchronous Buck Regulator
General Description
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 5A. 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-lead wide power SO package with an operating temperature range of -40C to +85C.
Features
* 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 * 100m internal power MOSFETs at 12V input * 200kHz preset switching frequency * Low quiescent current 1.0mA in PWM mode 500A in skip mode < 5A 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
See the MIC2178 for externally selected PWM or skip-mode operation.
Typical Application
VIN 6V to 16.5V C1 22F 35V C2 22F 35V
ON OFF ENABLE 20
U1 EN
1,2,9
VIN OUT MIC2177-5.0 SW
10 3,8 4-7 12
100
5V Output Efficiency
L1, 33H D1 MBRS140 C7 220F 10V
EFFICIENCY (%)
AUTO MODE
Mode
PWM MODE
VOUT 5V/2.5A C3 220F 10V
95 90 85 80 75 70 10 SKIP PWM 100 1000 2500 OUTPUT CURRENT (mA) VIN = 6V
11 18
AUTO SYNC COMP SGND
13 14-17
PGND FB BIAS
19
C6 2.2nF
R7 15k C5 10nF
C4 0.01F
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MIC2177
MIC2177
Micrel
Ordering Information
Part Number MIC2177-3.3BWM MIC2177-5.0BWM MIC2177BWM Output Voltage 3.3V 5.0V adj. Switching Frequency 200kHz 200kHz 200kHz Temperature Range -40C to +85C -40C to +85C -40C to +85C Package 20-lead wide SOP 20-lead wide SOP 20-lead wide SOP
Pin Configuration
VIN 1 VIN 2 SW 3 PGND 4 PGND 5 PGND 6 PGND 7 SW 8 VIN 9 OUT 10 20 EN 19 BIAS 18 SYNC 17 SGND 16 SGND 15 SGND 14 SGND 13 COMP 12 FB 11 AUTO
20-Lead Wide SOP
Pin Description
Pin Number 1, 2, 9 Pin Name VIN Pin Function 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. Switch (Output): Internal power MOSFET switch output. Both pins must be externally connected together. 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-/skipmode 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. Compensation: Internal error amplifier output. Connect to capacitor or series RC network to compensate the regulator control loop. Signal Ground: Ground connection of control section. Connect all pins to common ground plane. 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. Bias Supply: Internal 3.3V bias supply output. Decouple with 0.01F bypass capacitor to SGND. Do not apply any external load. Enable (Input): Logic high enables operation. Logic low shuts down regulator. Do not allow pin to float.
3,8 4,5,6,7 10 11
SW PGND OUT AUTO
12
FB
13 14,15,16,17 18
COMP SGND SYNC
19 20
BIAS EN
MIC2177
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MIC2177
Micrel
Absolute Maximum 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
Operating Ratings
Supply Voltage (VIN) ..................................... 4.5V to 16.5V Junction Temperature Range (TJ) ........... -40C to +125C
Electrical Characteristics
VIN = 7.0V; TA = 25C, bold indicates -40C TA 85C; unless noted. Symbol ISS Parameter Input Supply Current Condition PWM mode, output not switching, 4.5V VIN 16.5V skip mode, output not switching, 4.5V VIN 16.5V VEN = 0V, 4.5V VIN 16.5V VBIAS VFB VOUT Bias Regulator Output Voltage Feedback Voltage Output Voltage VIN = 16.5V MIC2177 [adj.]: VOUT = 3.3V, ILOAD = 0 MIC2177 [adj.]: VOUT = 3.3V, 5V VIN 16V, 10mA ILOAD 2A MIC2177-5.0: ILOAD = 0 MIC2177-5.0: 6V VIN 16V, 10mA ILOAD 2A MIC2177-3.3: ILOAD = 0 MIC2177-3.3: 5V VIN 16V, 10mA ILOAD 2A VTH VTL IFB AVOL Feedback Bias Current Undervoltage Lockout upper threshold lower threshold MIC2177 [adj.] MIC2177-5.0, MIC2177-3.3 Error Amplifier Gain Error Amplifier Output Swing 0.6V VCOMP 0.8V upper limit lower limit Error Amplifier Output Current fO DMAX tON min Oscillator Frequency Maximum Duty Cycle Minimum On-Time SYNC Frequency Range SYNC Threshold SYNC Minimum Pulse Width ISYNC ILIM RON ISW SYNC Leakage Current Limit VSYNC = 0V to 5.5V PWM mode, VIN = 12V skip mode Switch On-Resistance high-side switch, VIN = 12V low-side switch, VIN = 12V Output Switch Leakage VSW = 16.5V VFB = 1.0V VFB = 1.5V 220 0.8 500 -1 3.8 0.01 4.7 600 90 110 1 250 250 10 1 5.7 1.6 source and sink 15 160 100 300 400 300 2.2 15 0.9 3.9 3.10 1.22 3.20 3.14 4.85 4.85 4.75 3.20 3.20 3.14 Min Typ 1.0 500 1 3.30 1.245 3.3 5.0 5.0 3.3 3.3 4.25 4.15 60 20 18 1.5 0.05 25 200 0.1 35 240 150 40 20 V V A kHz % ns kHz V ns A A mA m m A Max 1.5 650 25 3.4 1.27 3.40 3.46 5.15 5.15 5.25 3.40 3.40 3.46 4.35 Units mA A A V V V V V V V V V V V nA A
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MIC2177
MIC2177
Symbol Parameter Enable Threshold IEN Enable Leakage AUTO Threshold AUTO Source Current Minimum Switch Current for PWM Operation VFB = 1.5V, VAUTO < 0.8V VIN - VOUT = 0V VIN - VOUT = 3V VEN = 0V to 5.5V Condition Min 0.8 -1 0.8 7 Typ 1.6 0.01 1.6 11 220 420 15 Max 2.2 1
Micrel
Units V A V A mA mA
General Note: Devices are ESD sensitive. Handling precautions recommended.
MIC2177
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MIC2177
Micrel
Typical Characteristics
Oscillator Frequency vs. Temperature
REFERENCE VOLTAGE (V)
205 200 FREQUENCY (kHz) 195 190 185 180
1.252 1.250 1.248 1.246 1.244 1.242 1.240
Reference Voltage vs. Temperature
REFERENCE VOLTAGE (V) MIC2177 [adj.]
3.320 3.315 3.310 3.305 3.300 3.295 3.290 3.285
Reference Voltage vs. Temperature
MIC2177-3.3
175 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
1.238 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
3.280 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
5.030 REFERENCE VOLTAGE (V) 5.020 5.010 5.000 4.990 4.980
Reference Voltage vs. Temperature
AMPLIFIER VOLTAGE GAIN MIC2177-5.0
19.0 18.5 18.0 17.5 17.0 16.5
Error-Amplifier Gain vs. Temperature
120 BIAS CURRENT (nA) 100
Feedback Input Bias Current vs. Temperature
80 60 40 20 0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
4.970 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
16.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
5.0 CURRENT LIMIT (A) 4.9 4.8 4.7 4.6 4.5 4.4 4.3 4.2
Current Limit vs. Temperature
ON-RESISTANCE (m)
250 200 150 100 50 0
High-Side Switch On-Resistance
ON-RESISTANCE (m) 125C 85C 25C 0C
350 300 250 200 150 100 50 0 2 4
Low-Side Switch On-Resistance
125C 85C 25C 0C
4.1 4.0 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
2
4
6 8 10 12 14 16 18 INPUT VOLTAGE (V)
6 8 10 12 14 16 18 INPUT VOLTAGE (V)
12 SUPPLY CURRENT (mA) 10 8 6 4 2 0 2 4
PWM-Mode Supply Current
OUTPUT SWITCHING
EFFICIENCY (%)
3.3V Output Efficiency
100 95
EFFICIENCY (%)
100 95
5V Output Efficiency
VIN = 6V
90 85 80 75 70 65
VIN = 5V 8V 12V
90 8V 85 12V 80 75 70 10 SKIP PWM 100 1000 2500 OUTPUT CURRENT (mA)
SKIP PWM 100 1000 2500 OUTPUT CURRENT (mA)
6 8 10 12 14 16 18 INPUT VOLTAGE (V)
60 10
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MIC2177
MIC2177
Micrel
Block Diagram
VIN 4.5V to 16.5V
CIN VIN
1 2 9
UVLO, Thermal Shutdown Output Control Logic ISENSE Amp.
100m P-channel SW
3 8
VOUT
1.245
L1
R1 R2
1
VOUT
D
Enable Shutdown
EN
20
3.3V Regulator ILIMIT Comp.
COUT
100m N-channel
PGND
4 5 6 7
BIAS
19
0.01F
internal supply voltage
PWM/ Skip-Mode Select Logic IMIN Comp.
Bold lines indicate high current traces
IMIN Thrshld.
OUT
10
SYNC
18
CORRECTIVE RAMP
200kHz Oscillator
3.3V Low Output Comp. 10A FB
12
R1
Auto-Mode PWM
AUTO
11
R2 40mV Skip-Mode Comp.
RESET PULSE
2.2nF
R Q S COMP RC CC
13
PWM Comp.
Error Amp. VREF 1.245V
MIC2177 [Adjustable] SGND
14 15 16 17
MIC2177
6
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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. 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 x load current). 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 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. 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 7 MIC2177
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. 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 100m when the MIC2177 operates from a 12V supply. Antishootthrough circuitry prevents the P-channel and N-channel from turning on at the same time. 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 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 10C. 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 5A. 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 0.01F capacitor. Do not connect any external load to the BIAS pin. It is not designed to provide an external supply voltage. April 1999
MIC2177
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 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 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 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.
Micrel
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 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:
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 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 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 10A 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. 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.
MIC2177
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MIC2177
Micrel
PWM-Mode Functional Diagram
VIN 4.5V to 16.5V
CIN VIN
1 2 9
100m P-channel ISENSE Amp. SW
3 8
VOUT
1.245
R1 R2
1
L1 IL1 D COUT
VOUT
100m N-channel
PGND
4 5 6 7
Corrective Ramp
Stop
SYNC
18
200kHz Oscillator Reset Pulse FB
12
R1
R2 R Q S COMP CC RC
13
PWM Comp.
Error Amp.
VREF 1.245V MIC2177 [Adjustable] PWM-Mode Signal Path SGND
14 15 16 17
VSW
Reset Pulse
IL1
ILOAD IL1
Error Amp. Output ISENSE
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MIC2177
MIC2177
Micrel
Skip-Mode Functional Diagram
VIN 4.5V to 16.5V
CIN VIN Output Control Logic S Q R One Shot ISENSE Amp. 100m P-channel SW
3 8 1 2 9
VOUT
1.245
R1 R2
1
L1 IL1 D COUT
VOUT
PGND
4 5
ILIMIT Comp. ILIMIT Thresh. Voltage
6 7
Skip-Mode Comp.
R1 FB
12
R2
VREF 1.245V MIC2177 [Adjustable] Skip-Mode Signal Path SGND
14 15 16 17
VSW
VIN VOUT 0
One-Shot Pulse
ILIM IL1 0 VREF + 5mV VFB VREF - 5mV
MIC2177
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MIC2177
Micrel
To maximize efficiency, the inductor's resistance must be less than the output switch on-resistance (preferably 50m 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 x ESR. As mentioned in "Inductor Selection," the maximum value for IL is 600mA. Therefore, the maximum value of ESR is: ESRMAX = Where: VRIPPLE < 1% of VOUT Typically, capacitors in the range of 100F to 220F 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 0C. The voltage rating of a tantalum capacitor must be 2 x VOUT, and the voltage rating of an electrolytic must be 1.4 x 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 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 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 5k - 20k 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 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. 600mA VRIPPLE
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 20k. However, to prevent feedback error due to input bias current at the FB pin, R2 should not be greater than 100k. After selecting R2, calculate R1 using the following formula:
V R1 = R2 OUT - 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-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) = ILOAD(max) 2
Tantalum capacitors are a better choice for applications that require the most compact layout or operation below 0C. The input capacitor must be located very close to the VIN pin (within 0.2 inches, 5mm). Also place a 0.1F 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:
VOUT LMIN = VOUT 1 - x 8.3H/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:
IL(peak) = ILOAD(max) + 300mA
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MIC2177
MIC2177
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
Micrel
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 22F 35V
20 18 11
U1
1,2,9
VIN OUT EN SYNC MIC2177 AUTO PGND FB COMP SGND BIAS
13 14-17 19
10 3,8
L1, 50H D1 MBRS130L
SW
4-7 12
C2 100F 10V
VOUT 3.3V/1A
C5 0.01 F
R4 10k C4 6.8nF
U1 C1 C2 C3 C3 C4 0.01F D1 L1 L1 L1
Micrel AVX AVX Z5UorX7R X7RorNP0 Motorola Coiltronics Coilcraft Bi
MIC2177-3.3BWM TPSE226M035R0300, ESR = 0.3 TPSD107M010R0100, ESR = 0.1 Ceramic Dielectric Material Ceramic Dielectric Material MBRS130LT3 CTX50-4P, DCR = 0.097 DO3316P-473, DCR = 0.12 HM77-11003, DCR = 0.073
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 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
AVX Corp. 801 17th Ave. South Myrtle Beach, SC 29577 tel: (803) 448-9411 fax: (803) 448-1943 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
General Instruments (GI) 10 Melville Park Rd. Melville, NY 11747 tel: (516) 847-3222 fax: (516) 847-3150 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
Siliconix 2201 Laurelwood Rd. Santa Clara, CA 96056 tel: (800) 554-5565
MIC2177
12
April 1999
MIC2177
Micrel
Package Information
PIN 1
0.301 (7.645) 0.297 (7.544)
DIMENSIONS: INCHES (MM)
0.027 (0.686) 0.031 (0.787)
0.050 (1.270) TYP
0.016 (0.046) TYP
0.094 (2.388) 0.090 (2.286)
0.509 (12.929) 0.505 (12.827)
7 TYP 0.015 R (0.381) 0.015 (0.381) SEATING MIN PLANE
0.103 (2.616) 0.099 (2.515)
0.297 (7.544) 0.293 (7.442)
0.022 (0.559) 0.018 (0.457) 5 TYP 10 TYP
0.330 (8.382) 0.326 (8.280) 0.032 (0.813) TYP 0.408 (10.363) 0.404 (10.262)
20-Lead Wide SOP (WM)
April 1999
13
MIC2177
MIC2177
Micrel
MIC2177
14
April 1999
MIC2177
Micrel
April 1999
15
MIC2177
MIC2177
Micrel
MICREL INC. 1849 FORTUNE DRIVE SAN JOSE, CA 95131
TEL
USA
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
WEB
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. (c) 1999 Micrel Incorporated
MIC2177
16
April 1999

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