MIC2171 100kHz 2.5A Switching Regulator General Description Features The MIC2171 is a complete 100kHz SMPS current-mode controller with an internal 65V 2.5A power switch. Although primarily intended for voltage step-up applications, the floating switch architecture of the MIC2171 makes it practical for step-down, inverting, and Cuk configurations as well as isolated topologies. Operating from 3V to 40V, the MIC2171 draws only 7mA of quiescent current, making it attractive for battery operated supplies. The MIC2171 is available in a 5-pin TO-220 or TO-263 for -40C to +85C operation. Data sheets and support documentation can be found on Micrel's web site at: www.micrel.com. * * * * * * * * * * 2.5A, 65V internal switch rating 3V to 40V input voltage range Current-mode operation, 2.5A peak Internal cycle-by-cycle current limit Thermal shutdown Twice the frequency of the LM2577 Low external parts count Operates in most switching topologies 7mA quiescent current (operating) Fits LT1171/LM2577 TO-220 and TO-263 sockets Applications * * * * Laptop/palmtop computers Battery operated equipment Hand-held instruments Off-line converter up to 50W(requires external power switch) * Pre-driver for higher power capability ___________________________________________________________________________________________________________ Typical Application +5V (4.75V min.) C1* 47F L1 15H D1 IN SW 1N5822 MIC2171 R3 1k COMP FB GND C3 1F VOU T +12V, 0.25A VIN 4V to 6V C1 47F R4* SW MIC2171 R3 1k * Locate near MIC2171 when supply leads > 2" COMP GND C3* D1* IN R1 10.7k 1% R2 C2 1.24k 470F 1% T1 D2 1N5818 VOU T 5V, 0.5A R1 C4 3.74k 470F 1% 1:1.25 LP R I = 12H FB R2 1.24k 1% C2 1F * Optional voltage clipper (may be req'd if T1 leakage inductance too high) Figure 1. MIC2171 5V to 12V Boost Converter Figure 2. MIC2171 5VFlyback Converter Micrel Inc. * 2180 Fortune Drive * San Jose, CA 95131 * USA * tel +1 (408) 944-0800 * fax + 1 (408) 474-1000 * http://www.micrel.com May 2007 1 M9999-051107 Micrel, Inc. MIC2171 Ordering Information Part Number Standard RoHS Compliant* Temperature Range Package MIC2171BT MIC2171WT -40 to +85C 5-Pin TO-220 MIC2171BU MIC2171WU -40 to +85C 5-Pin TO-263 *RoHS compliant with "high-melting solder" exemption. Pin Configuration 5 4 3 2 1 5 4 3 2 1 IN SW GND FB COMP Tab GND IN SW GND FB COMP Tab GND 5-Pin TO-220 (T) 5-Pin TO-263 (U) Pin Description Pin Number Pin Name 1 COMP 2 FB 3 GND Ground: Connect directly to the input filter capacitor for proper operation (see applications info). 4 SW Power Switch Collector: Collector of NPN switch. Connect to external inductor or input voltage depending on circuit topology. 5 IN May 2007 Pin Function Frequency Compensation: Output of transconductance-type error amplifier. Primary function is for loop stabilization. Can also be used for output voltage soft-start and current limit tailoring. Feedback: Inverting input of error amplifier. Connect to external resistive divider to set power supply output voltage. Supply Voltage: 3.0V to 40V 2 M9999-051107 Micrel, Inc. MIC2171 Absolute Maximum Ratings Operating Ratings Supply Voltage (VIN) .......................................................40V Switch Voltage (VSW)......................................................65V Feedback Voltage (transient, 1ms) (VFB) .....................15V Lead Temperature (soldering, 10 sec.)...................... 300C Storage Temperature (Ts) .........................-65C to +150C ESD Rating(1) Operating Temperature Range ................... -40C to +85C Junction Temperature (TJ) ........................ -55C to +150C Thermal Resistance TO-220-5 (JA) (2) ...............................................45C/W TO-263-5 (JA) (3) ................................................45C/W Electrical Characteristics VIN = 5V; TA = 25C, bold values indicate -40C< TA < +85C, unless noted. Parameter Condition Min Typ Max Units 1.220 1.214 1.240 1.264 1.274 V V %/V 310 750 1100 nA nA 3.9 6.0 7.0 2000 A/mV A/mV V/V 350 400 2.3 0.52 1.08 1.25 A A V V V V 0.50 0.55 5.0 5.5 5.0 A A A V 112 115 95 kHz kHz % 2.7 7 3.0 9 V mA 9 20 mA Reference Section Feedback Voltage (VFB) VCOMP = 1.24V Feedback Voltage Line Regulation Feedback Bias Current (IFB) 3V VIN 40V VCOMP = 1.24V VFB = 1.24V 0.6 Error Amplifier Section Transconductance (gm) ICOMP = 25A Voltage Gain (AV) Output Current 0.9V VCOMP 1.4V VCOMP = 1.5V Output Swing High Clamp, VFB = 1V Low Clamp, VFB = 1.5V Duty Cycle = 0 Compensation Pin Threshold 3.0 2.4 400 125 100 1.8 0.25 0.8 0.6 800 175 2.1 0.35 0.9 Output Switch Section ON Resistance ISW = 2A, VFB = 0.8V Current Limit Duty Cycle = 50%, TJ 25C Duty Cycle = 50%, TJ < 25C Duty Cycle = 80%, Note 4 3V VIN 40V ISW = 5mA Breakdown Voltage (BV) 0.37 2.5 2.5 2.5 65 3.6 4.0 3.0 75 88 85 80 100 Oscillator Section Frequency (fO) Duty Cycle [(max)] Input Supply Voltage Section Minimum Operating Voltage Quiescent Current (IQ) 3V VIN 40V, VCOMP = 0.6V, ISW = 0 Supply Current Increase (IIN) ISW = 2A, VCOMP = 1.5V, during switch on-time 90 Notes: 1. Devices are ESD sensitive. Handling precautions recommended. 2. Mounted vertically, no external heat sink, 1/4 inch leads soldered to PC board containing approximately 4 inch squared copper area surrounding leads. 3. All ground leads soldered to approximately 2 inches squared of horizontal PC board copper area. 4. For duty cycles () between 50% and 95%, minimum guaranteed switch current is ICL = 1.66 (2-) Amp (Pk). May 2007 3 M9999-051107 Micrel, Inc. MIC2171 Typical Characteristics 2.8 2.7 2.6 Switch Current = 2A 2.5 2.4 -50 0 50 100 Temperature (C) 400 300 200 100 0 -100 D.C . = 50% 8 7 D.C . = 0% 0 10 20 30 VIN Operating Voltage (V) Switch On-Voltage Frequency (kHz) T = -40C J 0.8 0.6 T = 125C J 0.4 5.0 4.5 4.0 1 2 Switch Current (A) 1 2 3 Switch Current (A) 4 T = 125C J 1 0 -1 TJ = 25C -2 -3 T = -40C J -4 -5 0 10 20 30 VIN Operating (V) 40 Supply Current 10 9 8 VC OMP = 0.6V 7 6 5 4 3 2 1 0 -100 Oscillator Frequency -50 0 50 100 Temperature(C) 150 Current Limit 8 2.0 1.5 -50 0 50 100 Temperature(C) 90 80 150 0 50 100 Temperature(C) 6 -40C 4 25C 125C 2 0 150 Error Amplifier Gain 7000 3.5 3.0 2.5 May 2007 0 60 -50 3 Error Amplifier Gain 1.0 0.5 0.0 -100 10 4 3 2 70 Transconductance (S) Transconductance (A/mV) 0 = 50% 100 0.2 0 20 5 110 T = 25C J 1.0 = 90% 30 120 1.4 1.2 40 0 40 150 Supply Current 50 D.C . = 90% 11 10 9 -50 0 50 100 Temperature (C) Feedback Voltage Line Regulation Switch Current (A) Supply Current (mA) 500 150 IS W = 0 13 12 1.6 Switch ON Voltage (V) 600 Supply Current 15 14 6 5 700 20 40 60 80 Duty Cycle (%) 100 0 6000 30 5000 4000 3000 2000 1000 0 0 Error Amplifier Phase -30 Phase Shift () 2.3 -100 Feedback Voltage Change (mV) Feedback Bias Current (nA) 800 Average Supply Current (mA) Minimum Operating Voltage (V) 2.9 Feedback Bias Current Supply Current (mA) Minimum Operating Voltage 60 90 120 150 180 1 10 100 1000 Frequency (kHz) 4 10000 210 1 10 100 1000 Frequency (kHz) 10000 M9999-051107 Micrel, Inc. MIC2171 Functional Diagram IN Reg. D1 2.3V 100kHz Osc. Logic Q1 Driver Comparator FB Current Amp. Error 1.24V Amp. Ref. COMP GND reliability and provides automatic output current limiting. Finally, current-mode operation provides automatic input voltage feed forward which prevents instantaneous input voltage changes from disturbing the output voltage setting. Functional Description Refer to "Block Diagram MIC2171". Internal Power The MIC2171 operates when VIN is 2.6V. An internal 2.3V regulator supplies biasing to all internal circuitry including a precision 1.24V band gap reference. Anti-Saturation The anti-saturation diode (D1) increases the usable duty cycle range of the MIC2171 by eliminating the base to collector stored charge which would delay Q1's turnoff. PWM Operation The 100kHz oscillator generates a signal with a duty cycle of approximately 90%. The current-mode comparator output is used to reduce the duty cycle when the current amplifier output voltage exceeds the error amplifier output voltage. The resulting PWM signal controls a driver which supplies base current to output transistor Q1. Compensation Loop stability compensation of the MIC2171 can be accomplished by connecting an appropriate network from either COMP to circuit ground (see "Typical Applications") or COMP to FB. The error amplifier output (COMP) is also useful for soft start and current limiting. Because the error amplifier output is a transconductance type, the output impedance is relatively high which means the output voltage can be easily clamped or adjusted externally. Current-Mode Advantages The MIC2171 operates in current mode rather than voltage mode. There are three distinct advantages to this technique. Feedback loop compensation is greatly simplified because inductor current sensing removes a pole from the closed loop response. Inherent cycle-bycycle current limiting greatly improves the power switch May 2007 SW Anti-Sat. 5 M9999-051107 Micrel, Inc. MIC2171 the losses of the power switch driver circuitry. The dc losses are calculated from the supply voltage (VIN) and device supply current (IQ).The MIC2171 supply current is almost constant regardless of the supply voltage (see "Electrical Characteristics"). The driver section losses (not including the switch) are a function of supply voltage, power switch current, and duty cycle. P(bias+driver) = (VIN IQ) + (VIN(min) x ISW x IIN) where: P(bias+driver) = device operating losses VIN(min) = supply voltage = VIN - VSW IQ = typical quiescent supply current ICL = power switch current limit IIN = typical supply current increase As a practical example refer to Figure 1. VIN = 5.0V IQ = 0.007A ICL = 2.21A Application Information Soft Start A diode-coupled capacitor from COMP to circuit ground slows the output voltage rise at turn on (Figure 3). VIN IN MIC2171 COMP D1 D2 R1 C1 C2 Figure 3. Soft Start The additional time it takes for the error amplifier to charge the capacitor corresponds to the time it takes the output to reach regulation. Diode D1 discharges C1 when VIN is removed. = 66.2% (0.662) then: Current Limit VIN VIN(min) = 5.0V - (2.21 x 0.37) = 4.18V P(bias+driver) = (5 x 0.007) + (4.18 x 2.21 x 0.009) P(bias+driver) = 0.1W Power switch dissipation calculations are greatly simplified by making two assumptions which are usually fairly accurate. First, the majority of losses in the power switch are due to on-losses. To find these losses, assign a resistance value to the collector/emitter terminals of the device using the saturation voltage versus collector current curves (see Typical Performance Characteristics). Power switch losses are calculated by modeling the switch as a resistor with the switch duty cycle modifying the average power dissipation. IN SW MIC2171 GND Q1 R1 C1 R2 VOU T FB COMP R3 IC L 0.6V/R2 and output C2 Note: Input returns not common Figure 4. Current Limit PSW = (ISW)2 RSW The maximum current limit of the MIC2171 can be reduced by adding a voltage clamp to the COMP output (Figure 4). This feature can be useful in applications requiring either a complete shutdown of Q1's switching action or a form of current fold-back limiting. This use of the COMP output does not disable the oscillator, amplifiers or other circuitry, therefore, the supply current is never less than approximately 5mA. where: = duty cycle = VOUT + VF VSW = ICL (RSW) VOUT = output voltage VF = D1 forward voltage drop at IOUT From the Typical performance Characteristics: RSW = 0.37 then: PSW = (2.21)2 x 0.37 x 0.662 PSW = 1.2W P(total) = 1.2 + 0.1 P(total) = 1.3W Thermal Management Although the MIC2171 family contains thermal protection circuitry, for best reliability, avoid prolonged operation with junction temperatures near the rated maximum. The junction temperature is determined by first calculating the power dissipation of the device. For the MIC2171, the total power dissipation is the sum of the device operating losses and power switch losses. The device operating losses are the dc losses associated with biasing all of the internal functions plus May 2007 VOUT + VF - VIN(min) 6 M9999-051107 Micrel, Inc. MIC2171 Discontinuous mode is preferred because the feedback control of the converter is simpler. When L1 discharges its current completely during the MIC2171 off-time, it is operating in discontinuous mode. L1 is operating in continuous mode if it does not discharge completely before the MIC2171 power switch is turned on again. The junction temperature for any semiconductor is calculated using the following: TJ = TA + P(total) JA where: TJ = junction temperature TA = ambient temperature (maximum) P(total) = total power dissipation JA = junction to ambient thermal resistance For the practical example: TA = 70C JA = 45C/W (TO-220) then: TJ = 70 + (1.24 x 45) TJ = 126C This junction temperature is below the rated maximum of 150C. Discontinuous Mode Design Given the maximum output current, solve equation (1) to determine whether the device can operate in discontinuous mode without initiating the internal device current limit. Grounding Refer to Figure 5. Heavy lines indicate high current paths. (1) IOUT (1a) = VIN(min) VOUT VOUT + VF - VIN(min) VOUT + VF where: ICL = internal switch current limit ICL = 2.5A when < 50% VIN ICL = 1.67 (2 - ) when 50% (Refer to Electrical Characteristics.) IOUT = maximum output current VIN(min) = minimum input voltage = VIN - VSW IN SW MIC2171 GND FB VC = duty cycle VOUT = required output voltage VF = D1 forward voltage drop For the example in Figure 1. IOUT = 0.25A ICL = 1.67 (2-0.662) = 2.24A VIN(min) = 4.18V Single point ground Figure 5. Single Point Ground A single point ground is strongly recommended for proper operation. The signal ground, compensation network ground, and feed-back network connections are sensitive to minor voltage variations. The input and output capacitor grounds and power ground conductors will exhibit voltage drop when carrying large currents. Keep the sensitive circuit ground traces separate from the power ground traces. Small voltage variations applied to the sensitive circuits can prevent the MIC2171 or any switching regulator from functioning properly. = 0.662 VOUT = 12.0V VF = 0.36V (@ .26A, 70C) then: 2.235 x 4.178 x 0.662 2 IOUT 12 IOUT 0.258A This value is greater than the 0.25A output current requirement, so we can proceed to find the minimum inductance value of L1 for discontinuous operation at POUT. Boost Conversion Refer to Figure 1 for a typical boost conversion application where a +5V logic supply is available but +12V at 0.25A is required. The first step in designing a boost converter is determining whether inductor L1 will cause the converter to operate in either continuous or discontinuous mode. May 2007 ICL 2 (2) 7 L1 (VIN ) 2 2POUT f SW M9999-051107 Micrel, Inc. MIC2171 solutions to be connected to circuit ground, although a more conventional technique of connecting the components from the error amplifier output to its inverting input is also possible. where: POUT = 12 x 0.25 = 3W fSW = 1x105Hz (100kHz) For our practical example: (4.178 x 0.662)2 L1 Voltage Clipper Care must be taken to minimize T1's leakage inductance, otherwise it may be necessary to incorporate the voltage clipper consisting of D1, R4, and C3 to avoid second breakdown (failure) of the MIC2171's internal power switch. 2 x 3.0 x 1 x 10 5 L1 12.4H (use 15H) Equation (3) solves for L1's maximum current value. (3) IL1(peak) = VIN TON L1 Discontinuous Mode Design When designing a discontinuous flyback converter, first determine whether the device can safely handle the peak primary current demand placed on it by the output power. Equation (8) finds the maximum duty cycle required for a given input voltage and output power. If the duty cycle is greater than 0.8, discontinuous operation cannot be used. where: TON = / fSW = 6.62x10-6 sec IL1(peak) = 4.178 x 6.62 x 10 -6 15 x 10 - 6 IL1(peak) = 1.84A Use a 15H inductor with a peak current rating of at least 2A. (8) Flyback Conversion Flyback converter topology may be used in low power applications where voltage isolation is required or whenever the input voltage can be less than or greater than the output voltage. As with the step-up converter the inductor (transformer primary) current can be continuous or discontinuous. Discontinuous operation is recommended. Figure 2 shows a practical flyback converter design using the MIC2171. ICL 2POUT VIN(min) - VSW ( ) For a practical example let: (see Figure 2) POUT = 5.0V x 0.5A = 2.5W VIN = 4.0V to 6.0V ICL = 2.5A when < 50% 1.67 (2 - ) when 50% then: VIN(min) = VIN - (ICL x RSW V IN(min) = 4 - 0.78V V IN(min) = 3.22V Switch Operation During Q1's on time (Q1 is the internal NPN transistor-- see block diagrams), energy is stored in T1's primary inductance. During Q1's off time, stored energy is partially discharged into C4 (output filter capacitor). Careful selection of a low ESR capacitor for C4 may provide satisfactory output ripple voltage making additional filter stages unnecessary. C1 (input capacitor) may be reduced or eliminated if the MIC2171 is located near a low impedance voltage source. 0.74 (74%), less than 0.8 so discontinuous is permitted. A few iterations of equation (8) may be required if the duty cycle is found to be greater than 50%. Calculate the maximum transformer turns ratio a, or NPRI/NSEC, that will guarantee safe operation of the MIC2171 power switch. (9) a VCE FCE - VIN(max) VSEC where: Output Diode The output diode allows T1 to store energy in its primary inductance (D2 non-conducting) and release energy into C4 (D2 conducting). The low forward voltage drop of a Schottky diode minimizes power loss in D2. a = transformer maximum turns ratio VCE = power switch collector to emitter maximum voltage FCE = safety derating factor (0.8 for most commercial and industrial applications) VIN(max) = maximum input voltage VSEC = transformer secondary voltage (VOUT + VF) Frequency Compensation A simple frequency compensation network consisting of R3 and C2 prevents output oscillations. High impedance output stages (transconductance type) in the MIC2171 often permit simplified loop-stability May 2007 8 M9999-051107 Micrel, Inc. MIC2171 For the practical example: VCE = 65V max. for the MIC2171 FCE = 0.8 VSEC = 5.6V then: a then: a This ratio is less than the ratio calculated in equation (9). When specifying the transformer it is necessary to know the primary peak current which must be withstood without saturating the transformer core. 65 x 0.8 - 6.0 5.6 a 8.2 (NPRI/NSEC) Next, calculate the maximum primary inductance required to store the needed output energy with a power switch duty cycle of 55%. 2 (10) L PRI 0.5f SW VIN(min) TON (13) IPEAK(pri) = 2 POUT (14) then: 2 0.5f SW VSEC TOFF POUT VBR 2 FBR a 6.0 + (5.0 x 1.2) 0.8 x 1.2 Forward Converters Micrel's MIC2171 can be used in several circuit configurations to generate an output voltage which is less than the input voltage (buck or step-down topology). Figure 6 shows the MIC2171 in a voltage step-down application. Because of the internal architecture of these devices, more external components are required to implement a step-down regulator than with other devices offered by Micrel (refer to the LM257x or MIC457x family of buck switchers). However, for step-down conversion requiring a transformer (forward), the MIC2171 is a good choice. A 12V to 5V step-down converter using transformer isolation (forward) is shown in Figure 6. Unlike the isolated flyback converter which stores energy in the primary inductance during the controller's on-time and releases it to the load during the off-time, the forward converter transfers energy to the output during the on- 0.5 x 1 x 10 5 x (5.41) 2 x (2.6 x 10 -6 ) 2 2.5 LSEC 7.9H Finally, recalculate the transformer turns ratio to insure that it is less than the value earlier found in equation (9). May 2007 VIN(max) + (VOUT a) VBR 12.5V A 1N5817 will safely handle voltage and current requirements in this example. then: a VBR then: LSEC = maximum secondary inductance TOFF = power switch off time (12) 3.22 x 7.6 x 10 -6 L PRI VBR = output rectifier maximum peak reverse voltage rating a = transformer turns ratio (1.2) FBR = reverse voltage safety derating factor (0.8) where: L SEC L PRI where: 0.5 x 1 x 10 5 x (3.22) 2 x (7.4 x 10 -6 )2 2.5 LPRI 11.4H Use a 12H primary inductance to overcome circuit inefficiencies. To complete the design the inductance value of the secondary is found which will guarantee that the energy stored in the transformer during the power switch on time will be completed discharged into the output during the off-time. This is necessary when operating in discontinuous-mode. L SEC VIN(min) TON IPEAK(pri) = 2.1A Now find the minimum reverse voltage requirement for the output rectifier. This rectifier must have an average current rating greater than the maximum output current of 0.5A. LPRI = maximum primary inductance fSW = device switching frequency (100kHz) VIN(min) = minimum input voltage TON = power switch on time (11) IPEAK(pri) = so: where: L PRI 11.4 = 1.20 7.9 L PRI L SEC 9 M9999-051107 Micrel, Inc. MIC2171 off-time would require the voltage across the power switch to be ten times the input voltage. This would limit the input voltage to 6V or less for forward converter applications. To prevent core saturation, the application given here uses a duty cycle limiter consisting of Q1, C4 and R3. Whenever the MIC2171 exceeds a duty cycle of 50%, T1's reset winding current turns Q1 on. This action reduces the duty cycle of the MIC2171 until T1 is able to reset during each cycle. time, using the off-time to reset the transformer core. In the application shown, the transformer core is reset by the tertiary winding discharging T1's peak magnetizing current through D2. For most forward converters the duty cycle is limited to 50%, allowing the transformer flux to reset with only two times the input voltage appearing across the power switch. Although during normal operation this circuit's duty cycle is well below 50%, the MIC2172 has a maximum duty cycle capability of 90%. If 90% was required during operation (start-up and high load currents), a complete reset of the transformer during the T1 1:1:1 D3 1N5819 VIN 12V R1* L1 100H D4 1N5819 C2* VOU T 5V, 1A R4 C5 3.74k 470F 1% D1* IN SW C1 22F MIC2171 GND FB COMP R2 1k C3 1F D2 1N5819 Q1 R5 1.24k 1% R 3 C4 * Voltage clipper Duty cycle limiter Figure 6. MIC2171 Forward Converter May 2007 10 M9999-051107 Micrel, Inc. MIC2171 Package Information 5-Pin TO-220 (T) 5-Pin TO-263 (U) May 2007 11 M9999-051107 Micrel, Inc. MIC2171 MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com The 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) 2005 Micrel, Incorporated. May 2007 12 M9999-051107