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Micrel, Inc.
MIC2179
MIC2179
1.5A Synchronous Buck Regulator
General Description
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 5A. 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 -40C to +85C.
Features
* 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 * 150m internal power MOSFETs at 12V input * 200kHz preset switching frequency * Low quiescent current 1.0mA in PWM mode 600A in skip mode < 5A 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
Applications
* * * * * * High-efficiency, battery-powered supplies Buck (step-down) dc-to-dc converters Cellular telephones Laptop computers Hand-held instruments Battery Charger
Typical Application
VIN 5.4V to 16.5V C1 10F 20V Output Good Output Low Skip Mode PWM Mode U1
15 6 5 13 16,17
R1 20k
EN PWRGD PWM SYNC
VIN SW
3,4 1,2, 19,20 7
L1 22H D1 MBRM120
MIC 2179-3.3 PGND
FB BIAS
14 9-12
C2 100F 6.3V
VOUT 3.3V/600mA
COMP SGND
8
C4 6.8nF
C3 0.01F
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
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MIC2179 Part Number Voltage Adj. 3.3V 5.0V Temperature Range -40C to +85C -40C to +85C -40C to +85C Package 20-Lead SSOP 20-Lead SSOP 20-Lead SSOP
Ordering Information
Standard* MIC2179BSM Pb-Free MIC2179YSM
MIC2179-3.3BSM MIC2179-3.3YSM MIC2179-5.0BSM MIC2179-5.0YSM
* Standard product will be supported as Pb-Free IAW PCCN #040004 effective 1-1-2005 pending residual depletion.
Pin Configuration
PGND 1 PGND 2 SW 3 NC 4 PWM 5 PWRGD 6 FB 7 COMP 8 SGND 9 SGND 10 20 PGND 19 PGND 18 NC 17 VIN 16 VIN 15 EN 14 BIAS 13 SYNC 12 SGND 11 SGND
20-Lead Wide SSOP
Pin Description
Pin Number 1, 2, 19, 20 3 5 Pin Name PGND SW PWM Pin Function Power Ground: Connect all pins to central ground point. Switch (Output): Internal power MOSFET output switches. 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. Error Flag (Output): Open-drain output. Active low when FB input is 10% below the reference voltage (VREF).
6 7 8 9-12 13
PWRGD FB COMP SGND SYNC
Feedback (Input): Connect to output voltage divider resistors.
Compensation: Output of internal error amplifier. Connect capacitor or series RC network to compensate the regulator control loop. Signal Ground: Connect all pins to ground, PGND. 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. Internal 3.3V Bias Supply: 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. Supply Voltage (Input): Requires bypass capacitor to PGND. Both pins must be connected to VIN.
14 15 16, 17 4, 18
BIAS EN VIN NC
not internally connected.
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Absolute Maximum Ratings(1)
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
Operating Ratings(2)
Supply Voltage (VIN) .......................................4.5V to 16.5V Junction Temperature Range (TJ) ............ -40C to +125C
Electrical Characteristics(3)
Symbol ISS VIN = 7.0V; TA = 25C, bold indicates -40C TA 85C; unless noted. Parameter Condition Input Supply Current PWM mode, output not switching, 4.5V VIN 16.5V Min Typ 1.0 600 1 3.10 1.22 3.20 3.14 4.85 4.85 4.75 3.20 3.20 3.14 3.90 3.30 1.245 3.3 5.0 5.0 3.3 3.3 4.25 4.15 60 20 15 0.9 15 160 VFB = 1.0V VFB = 1.5V 100 300 220 0.8 500 PWM mode, VIN = 12V VSYNC = 0V to 5.5V -1 3.4 0.01 4.3 600 160 140 1 350 350 10 1 5.5 1.6 400 300 2.2 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 1.5 750 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
skip mode, output not switching, 4.5V VIN 16.5V VBIAS VFB VOUT Bias Regulator Output Voltage Feedback Voltage Output Voltage VEN = 0V, 4.5V VIN 16.5V VIN = 16.5V
MIC2179-5.0: ILOAD = 0
MIC2179 [adj.]: VOUT = 3.3V, 5V VIN 16V, 10mA ILOAD 1A
MIC2179 [adj.]: VOUT = 3.3V, ILOAD = 0
MIC2179-3.3: ILOAD = 0
MIC2179-5.0: 6V VIN 16V, 10mA ILOAD 1A MIC2179-3.3: 5V VIN 16V, 10mA ILOAD 1A upper threshold lower threshold
VTH IFB VTL
Undervoltage Lockout Feedback Bias Current Error Amplifier Gain Error Amplifier Output Swing Error Amplifier Output Current
MIC2179 [adj.] MIC2179-5.0, MIC2179-3.3 0.6V VCOMP 0.8V upper limit lower limit source and sink
AVOL
DMAX
fO
Oscillator Frequency Maximum Duty Cycle Minimum On-Time SYNC Frequency Range SYNC Threshold SYNC Minimum Pulse Width
tON min
ILIM
ISYNC
SYNC Leakage Current Limit Switch On-Resistance Output Switch Leakage
skip mode
RON ISW
VSW = 16.5V
low-side switch, VIN = 12V
high-side switch, VIN = 12V
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Symbol IEN IPWM Parameter Enable Threshold Enable Leakage PWM Threshold PWM Leakage PWRGD Threshold VEN = 0V to 5.5V VPWM = 0V to 5.5V Condition Min 0.8 -1 0.6 -1 1.09 4.33 2.87 Typ 1.6 0.01 1.1 0.01 1.13 4.54 3.00 0.25 0.01 Max 2.2 1 1.4 1 1.17 4.75 3.13 0.4 1
MIC2179
Units V A V A V V V V A
MIC2179 [adj.]: measured at FB pin MIC2179-5.0: measured at FB pin MIC2179-3.3: measured at FB pin
PWRGD Output Low PWRGD Off Leakage
Notes:
VPWRGD = 5.5V
ISINK = 1.0mA
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.
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MIC2179
Typical Characteristics
Oscillator Frequency vs. Temperature
REFERENCE VOLTAGE (V)
205 FREQUENCY (kHz) 200 195 190 185 180
1.252 1.250 1.248 1.246 1.244 1.242 1.240
Reference Voltage vs. Temperature
REFERENCE VOLTAGE (V) MIC2179 [adj.]
3.320 3.315 3.310 3.305 3.300 3.295 3.290 3.285
Reference Voltage vs. Temperature
MIC2179-3.3
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 MIC2179-5.0
19.0 18.5 18.0 17.5 17.0 16.5
Error-Amplifier Gain vs. Temperature
BIAS CURRENT (nA)
120 100
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)
ON-RESISTANCE (m)
ON-RESISTANCE (m)
CURRENT LIMIT (A)
5.5 5.3 5.1 4.9 4.7 4.5 4.3 4.1
Current Limit vs. Temperature
350 300 250 200 150 100 50 0 2 4
High-Side Switch On-Resistance
125C 85C 25C 0C
400 350 300 250 200 150 100 50 0 2 4
Low-Side Switch On-Resistance
125C 85C 25C 0C
3.9 3.7 3.5 -60 -30 0 30 60 90 120 150 TEMPERATURE (C)
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 (%)
95 90 85 80 75 70 65
Skip- and PWM-Mode Efficiency
5.4V PWM 8.4V Skip 8.4V PWM 5.4V Skip 100 600 OUTPUT CURRENT (mA)
6 8 10 12 14 16 18 INPUT VOLTAGE (V)
60 10
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MIC2179
VIN 4.5V to 16.5V
Block Diagram
100F VIN
16 17
UVLO, Thermal Shutdown Output Control Logic ISENSE Amp.
110m P-channel
R1 R1 VOUT 1.245 VOUT = 1.245 ( R2 + 1) 1 R2
Enable Shutdown
EN
15
3.3V Regulator
SW
3
L
VOUT
BIAS R3 4.02k
14
D 110m N-channel internal supply Voltage ILIMIT Comp. ILIMIT Thresh. Voltage Corrective Ramp 200kHz Oscillator Reset Pulse R S Skip-Mode Comp. Q Power Good Comp. PGND
1 2 19 20
COUT
0.01F
* * Connect SGND to PGND Bold lines indicate high current traces
Skip Mode PWM Mode
PWM
5
PWM/ Skip-Mode Select
Stop
SYNC
13
R1 FB
7
VIN PWM Comp. 20k
R2
PWRGD
6
Output Good
RC CC
COMP
8
VREF 1.245V MIC2179 [Adjustable] SGND
9 10 11 12
1.13V
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MIC2179 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 x 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 7
Functional Description
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 150m 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 10C. 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 5A. 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 0.01F capacitor. Do not
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Micrel, Inc. (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
MIC2179 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.
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MIC2179
PWM-Mode Functional Diagram
VIN 4.5V to 16.5V
CIN VIN
16 17
110m P-channel IS E N S E Amp. SW
3
R1 VOUT = 1.245 ( R2 + 1)
L1 IL1 D COU T
VOU T
110m N-channel
P GND
1 2 19 20
Stop
S Y NC
13
Corrective Ramp 200kHz Oscillator Reset Pulse FB
7
R1
R2 Q R S PWM Comp. Error Amp. VR E F1.245V MIC2179 [Adjustable] PWM-Mode Signal Path SGND
9 10 11 12
COMP CC RC
8
VS W
Reset Pulse
IL 1
ILOAD IL1
Error Amp. Output IS E N S E
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Skip-Mode Functional Diagram
VIN 4.5V to 16.5V
CIN VIN Output Control Logic S R One Shot Q 110m P-channel IS E N S E Amp. SW
3 16 17
VOUT = 1.245 (
R1 + 1) R2
L1 IL1 D COU T
VOU T
P GND
1 2
ILIMIT Comp. ILIMIT Thresh . Voltage
19 20
Skip-Mode Comp.
R1 FB
7
R2
VR E F1.245V MIC2179 [Adjustable] Skip-Mode Signal Path SGND
9 10 11 12
VS W
VIN VOU T 0
One-Shot Pulse ILIM IL 1 0 VR E F + 5mV VF B VR E F - 5mV
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MIC2179 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. 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 220F 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 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 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 5k to 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 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.
Application Information
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 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 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 0C. The input capacitor must be located very close to the VIN pin (within 0.2in, 5mm). Also, place a 0.1F 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 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: IL(max) IL(peak) = ILOAD(max) + 2 Where: IL(max) = VOUT (1 VOUT VIN(max)
)x
1 L*f
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Micrel, Inc. 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
MIC2179 currents returning to the power supply ground bus. SGND and PGND should be tied together by a ground plane that extends under the MIC2179.
Suggested Manufacturers List
Inductors 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
Capacitors
Diodes 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
Transistors Siliconix 2201 Laurelwood Rd. Santa Clara, CA 96056 tel: (800) 554-5565
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
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MIC2179
Package Information
20-Pin SSOP (SM)
MICREL INC.
tel + 1 (408) 944-0800 fax + 1 (408) 474-1000 web http://www.micrel.com
2180 FORTUNE DRIVE SAN JOSE, CA 95131
USA
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. (c) 2001 Micrel Incorporated
June 2009
13
M9999-063009

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