MIC2171. General Description. Features. Applications. Typical Application. 100kHz 2.5A Switching Regulator

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1 1kHz.5A Switching Regulator General Description The is a complete 1kHz SMPS current-mode controller with an internal 65.5A power switch. Although primarily intended for voltage step-up applications, the floating switch architecture of the makes it practical for step-down, inverting, and Cuk configurations as well as isolated topologies. Operating from 3 to 4, the draws only 7mA of quiescent current, making it attractive for battery operated supplies. The is available in a 5-pin TO- or TO-63 for 4 C to +85 C operation. Data sheets and support documentation can be found on Micrel s web site at: Features.5A, 65 internal switch rating 3 to 4 input voltage range Current-mode operation,.5a peak Internal cycle-by-cycle current limit Thermal shutdown Twice the frequency of the LM577 Low external parts count Operates in most switching topologies 7mA quiescent current (operating) Fits LT1171/LM577 TO- and TO-63 sockets Applications Laptop/palmtop computers Battery operated equipment Hand-held instruments Off-line converter up to 5W(requires external power switch) Pre-driver for higher power capability Typical Application +5 (4.75 min.) R3 1k COMP GND C3 1µF L1 15µH FB D1 1N58 C1* 47µF C 47µF * Locate near when supply leads > OUT +1,.5A R1 1.7k 1% R 1.4k 1% Figure 1. 5 to 1 Boost Converter 4 to 6 C1 47µF R3 1k C 1µF COMP GND FB R4* D 1N5818 C4 47µF * Optional voltage clipper (may be req d if T1 leakage inductance too high) C3* D1* T1 1:1.5 L PRI = 1µH Figure. 5Flyback Converter R1 3.74k 1% R 1.4k 1% OUT 5,.5A Micrel Inc. 18 Fortune Drive San Jose, CA USA tel +1 (48) fax + 1 (48) May 7 1 M

2 Ordering Information Part Number Standard RoHS Compliant* Temperature Range Package BT WT 4 to +85 C 5-Pin TO- BU WU 4 to +85 C 5-Pin TO-63 *RoHS compliant with "high-melting solder" exemption. Pin Configuration Tab GND GND FB 1 COMP Tab GND GND FB 1 COMP 5-Pin TO- (T) 5-Pin TO-63 (U) Pin Description Pin Number Pin Name Pin Function 1 COMP 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. FB Feedback: Inverting input of error amplifier. Connect to external resistive divider to set power supply output voltage. 3 GND Ground: Connect directly to the input filter capacitor for proper operation (see applications info). 4 Power Switch Collector: Collector of NPN switch. Connect to external inductor or input voltage depending on circuit topology. 5 Supply oltage: 3. to 4 May 7 M

3 Absolute Maximum Ratings Supply oltage ( )...4 Switch oltage ( )...65 Feedback oltage (transient, 1ms) ( FB )...±15 Lead Temperature (soldering, 1 sec.)... 3 C Storage Temperature (T s ) C to +15 C ESD Rating (1) Operating Ratings Operating Temperature Range... 4 C to +85 C Junction Temperature (T J ) C to +15 C Thermal Resistance TO--5 (θ JA ) ()...45 C/W TO-63-5 (θ JA ) (3)...45 C/W Electrical Characteristics = 5; T A = 5 C, bold values indicate 4 C< T A < +85 C, unless noted. Parameter Condition Min Typ Max Units Reference Section Feedback oltage ( FB ) COMP = Feedback oltage Line Regulation 3 4 COMP = %/ Feedback Bias Current (I FB ) FB = Error Amplifier Section Transconductance (g m ) I COMP = ±5µA oltage Gain (A ).9 COMP / Output Current COMP = Output Swing High Clamp, FB = 1 Low Clamp, FB = Compensation Pin Threshold Duty Cycle =.8.6 Output Switch Section ON Resistance I = A, FB = Current Limit Duty Cycle = 5%, T J 5 C Duty Cycle = 5%, T J < 5 C Duty Cycle = 8%, Note 4 Breakdown oltage (B) 3 4 I = 5mA Oscillator Section Frequency (f O ) na na µa/m µa/m µa µa Ω Ω A A A Duty Cycle [δ(max)] % Input Supply oltage Section Minimum Operating oltage.7 3. Quiescent Current (I Q ) 3 4, COMP =.6, I = 7 9 ma Supply Current Increase ( I ) I = A, COMP = 1.5, during switch on-time 9 ma Notes: 1. Devices are ESD sensitive. Handling precautions recommended.. 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 inches squared of horizontal PC board copper area. 4. For duty cycles (δ) between 5% and 95%, minimum guaranteed switch current is I CL = 1.66 (-δ) Amp (Pk). khz khz May 7 3 M

4 Typical Characteristics Minimum Operating oltage () Minimum Operating oltage Switch Current = A Temperature ( C) Feedback Bias Current (na) Feedback Bias Current Temperature ( C) Feedback oltage Change (m) Feedback oltage Line Regulation T J =-4 C T J = 15 C T J =5 C Operating () Supply Current (ma) Supply Current I = D.C.= 9% D.C.= 5% D.C.= % Operating oltage () Average Supply Current (ma) Supply Current δ = 9% δ = 5% Switch Current (A) Supply Current (ma) Supply Current 1 9 COMP = Temperature( C) Switch ON oltage () Switch On-oltage T =5 C J T = 4 C J T =15 C J. 1 3 Switch Current (A) Frequency (khz) Oscillator Frequency Temperature( C) Switch Current (A) Current Limit 4 C 5 C 15 C Duty Cycle (%) Transconductance (µa/m) Error Amplifier Gain Temperature( C) Transconductance(µS) Error Amplifier Gain Frequency (khz) Phase Shift ( ) Error Amplifier Phase Frequency (khz) May 7 4 M

5 Functional Diagram Reg..3 Anti-Sat. D1 1kHz Osc. Logic Driver Q1 FB Comparator 1.4 Ref. Error Amp. Current Amp. COMP GND Functional Description Refer to Block Diagram. Internal Power The operates when is.6. An internal.3 regulator supplies biasing to all internal circuitry including a precision 1.4 band gap reference. PWM Operation The 1kHz oscillator generates a signal with a duty cycle of approximately 9%. 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. Current-Mode Advantages The 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 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. Anti-Saturation The anti-saturation diode (D1) increases the usable duty cycle range of the by eliminating the base to collector stored charge which would delay Q1 s turnoff. Compensation Loop stability compensation of the 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. May 7 5 M

6 Application Information Soft Start A diode-coupled capacitor from COMP to circuit ground slows the output voltage rise at turn on (Figure 3). D1 D C1 COMP 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 is removed. Current Limit Q1 R1 C1 R GND FB COMP R3 C R1 C I CL.6/R Figure 4. Current Limit OUT Note: Input and output returns not common The maximum current limit of the 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. Thermal Management Although the 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, 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 the losses of the power switch driver circuitry. The dc losses are calculated from the supply voltage ( ) and device supply current (I Q ).The 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) = ( I Q ) + ( (min) x I x I ) P (bias+driver) = device operating losses (min) = supply voltage = I Q = typical quiescent supply current I CL = power switch current limit I = typical supply current increase As a practical example refer to Figure 1. = 5. I Q =.7A I CL =.1A δ = 66.% (.66) (min) = 5. (.1 x.37) = 4.18 P (bias+driver) = (5 x.7) + (4.18 x.1 x.9) P (bias+driver) =.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. P = (I ) R δ δ = duty cycle OUT + F (min) δ = OUT + F = I CL (R ) OUT = output voltage F = D1 forward voltage drop at I OUT From the Typical performance Characteristics: R =.37Ω P = (.1) P = 1.W P (total) = P (total) = 1.3W May 7 6 M

7 The junction temperature for any semiconductor is calculated using the following: T J = T A + P (total) θ JA T J = junction temperature T A = ambient temperature (maximum) P (total) = total power dissipation θ JA = junction to ambient thermal resistance For the practical example: T A = 7 C θ JA = 45 C/W (TO-) T J = 7 + (1.4 45) T J = 16 C This junction temperature is below the rated maximum of 15 C. Grounding Refer to Figure 5. Heavy lines indicate high current paths. GND FB C 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 or any switching regulator from functioning properly. Boost Conversion Refer to Figure 1 for a typical boost conversion application where a +5 logic supply is available but +1 at.5a 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. Discontinuous mode is preferred because the feedback control of the converter is simpler. When L1 discharges its current completely during the off-time, it is operating in discontinuous mode. L1 is operating in continuous mode if it does not discharge completely before the power switch is turned on again. 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. (1) I OUT ICL (min) OUT OUT + F (min) (1a) δ = OUT + F I CL = internal switch current limit I CL =.5A when δ < 5% I CL = 1.67 ( δ) when δ 5% (Refer to Electrical Characteristics.) I OUT = maximum output current (min) = minimum input voltage = δ = duty cycle OUT = required output voltage F = D1 forward voltage drop For the example in Figure 1. I OUT =.5A I CL = 1.67 (.66) =.4A (min) = 4.18 δ δ =.66 OUT = 1. F =.36 (@.6A, 7 C) I OUT 1 I OUT.58A This value is greater than the.5a output current requirement, so we can proceed to find the minimum inductance value of L1 for discontinuous operation at P OUT. () ( L1 P OUT δ ) f May 7 7 M

8 P OUT = 1.5 = 3W f = 1 15Hz (1kHz) For our practical example: ( ) L L1 1.4µH (use 15µH) Equation (3) solves for L1 s maximum current value. TON (3) IL1(peak) = L1 T ON = δ / f = sec IL1(peak) = I L1(peak) = 1.84A Use a 15µH inductor with a peak current rating of at least A. 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 shows a practical flyback converter design using the. 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 is located near a low impedance voltage source. Output Diode The output diode allows T1 to store energy in its primary inductance (D non-conducting) and release energy into C4 (D conducting). The low forward voltage drop of a Schottky diode minimizes power loss in D. Frequency Compensation A simple frequency compensation network consisting of R3 and C prevents output oscillations. High impedance output stages (transconductance type) in the often permit simplified loop-stability 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. oltage 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 s internal power switch. 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.8, discontinuous operation cannot be used. POUT (8) δ ICL ( (min) ) For a practical example let: (see Figure ) P OUT = 5..5A =.5W = 4. to 6. I CL =.5A when δ < 5% 1.67 ( δ) when δ 5% (min) = (I CL R (min) = 4.78 (min) = 3. δ.74 (74%), less than.8 so discontinuous is permitted. A few iterations of equation (8) may be required if the duty cycle is found to be greater than 5%. Calculate the maximum transformer turns ratio a, or N PRI /N SEC, that will guarantee safe operation of the power switch. May 7 8 M (9) a CE F CE SEC (max) a = transformer maximum turns ratio CE = power switch collector to emitter maximum voltage F CE = safety derating factor (.8 for most commercial and industrial applications) (max) = maximum input voltage SEC = transformer secondary voltage ( OUT + F )

9 For the practical example: CE = 65 max. for the F CE =.8 SEC = a 5.6 a 8. (N PRI /N SEC ) Next, calculate the maximum primary inductance required to store the needed output energy with a power switch duty cycle of 55%..5f (min) TON (1) LPRI POUT L PRI = maximum primary inductance f = device switching frequency (1kHz) (min) = minimum input voltage T ON = power switch on time (3.) (7.4 1 ) LPRI.5 L PRI 11.4µH Use a 1µH 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. SEC OUT OFF.5f T (11) L SEC P L SEC = maximum secondary inductance T OFF = power switch off time (5.41) (.6 1 L SEC.5 L SEC 7.9µH Finally, recalculate the transformer turns ratio to insure that it is less than the value earlier found in equation (9). (1) a L L PRI SEC 6 6 ) 11.4 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. (min) TON (13) I PEAK(pri) = LPRI so: IPEAK(pri) = LPRI I PEAK(pri) =.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.5a. (max) + (OUTa) (14) BR FBRa BR = output rectifier maximum peak reverse voltage rating a = transformer turns ratio (1.) F BR = reverse voltage safety derating factor (.8) 6. + (5. 1.) BR.8 1. BR 1.5 A 1N5817 will safely handle voltage and current requirements in this example. Forward Converters Micrel s 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 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 LM57x or MIC457x family of buck switchers). However, for step-down conversion requiring a transformer (forward), the is a good choice. A 1 to 5 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- May 7 9 M

10 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 D. For most forward converters the duty cycle is limited to 5%, 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 5%, the MIC17 has a maximum duty cycle capability of 9%. If 9% was required during operation (start-up and high load currents), a complete reset of the transformer during the off-time would require the voltage across the power switch to be ten times the input voltage. This would limit the input voltage to 6 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 exceeds a duty cycle of 5%, T1 s reset winding current turns Q1 on. This action reduces the duty cycle of the until T1 is able to reset during each cycle. 1 R1* T1 1:1:1 C* D3 1N5819 L11µH D4 1N5819 C5 47µF OUT 5, 1A R4 3.74k 1% D1* C1 µf GND FB COMP R 1k C3 1µF D 1N5819 Q1 C4 R3 R5 1.4k 1% * oltage clipper Duty cycle limiter Figure 6. Forward Converter May 7 1 M

11 Package Information 5-Pin TO- (T) 5-Pin TO-63 (U) May 7 11 M

12 MICREL, C. 18 FORTUNE DRIE SAN JOSE, CA USA TEL +1 (48) FAX +1 (48) WEB 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. 5 Micrel, Incorporated. May 7 1 M