MIC2172/3172 Micrel, Inc. MIC2172/3172 100kHz 1.25A Switching Regulators slave’s. The master MIC2172’s oscillator frequency is increased up to 135kHz by connecting a resistor from SYNC to ground (see applications information). General Description The MIC2172 and MIC3172 are complete 100kHz SMPS current-mode controllers with internal 65V 1.25A power switches. The MIC2172 features external frequency synchronization or frequency adjustment, while the MIC3172 features an enable/shutdown control input. The MIC2172/3172 is available in an 8-pin plastic DIP or SOIC for –40°C to +85°C operation. Features Although primarily intended for voltage step-up applications, the floating switch architecture of the MIC2172/3172 makes it practical for step-down, inverting, and Cuk configurations as well as isolated topologies. • • • • • • • • • • • • • Operating from 3V to 40V, the MIC2172/3172 draws only 7mA of quiescent current making it attractive for battery operated supplies. The MIC3172 is for applications that require on/off control of the regulator. The MIC3172 is externally shutdown by applying a TTL low signal to EN (enable). When disabled, the MIC3172 draws only leakage current (typically less than 1µA). EN must be high for normal operation. For applications not requiring control, EN must be tied to VIN or TTL high. 1.25A, 65V internal switch rating 3V to 40V input voltage range Current-mode operation Internal cycle-by-cycle current limit Thermal shutdown Low external parts count Operates in most switching topologies 7mA quiescent current (operating) <1µA quiescent current, shutdown mode (MIC3172) TTL shutdown compatibility (MIC3172) External frequency synchronization (MIC2172) External frequency trim (MIC2172) Fits most LT1172 sockets (see applications info) Applications The MIC2172 is for applications requiring two or more SMPS regulators that operate from the same input supply. The MIC2172 features a SYNC input which allows locking of its internal oscillator to an external reference. This makes it possible to avoid the audible beat frequencies that result from the unequal oscillator frequencies of independent SMPS regulators. • • • • Laptop/palmtop computers Toys Hand-held instruments Off-line converter up to 50W (requires external power switch) • Predriver for higher power capability • Master/slave configurations (MIC2172) A reference signal can be supplied by one MIC2172 designated as a master. To insure locking of the slave’s oscillators, the reference oscillator frequency must be higher than the Typical Applications VIN 4V to 6V +5V (4.75V min.) SYNC VSW MIC2172 R3 1k VOUT +12V, 0.14A D1 VIN N/C COMP GND FB P1 P2 S C3 1µF R1 10.7k 1% 1N5822 R4* C1 22µF C1* 22µF L1 27µH R2 C2 1.24k 470µF 1% * Locate near MIC2172 when supply leads > 2" VSW EN MIC3172 R3 1k C3* D2 1N5818 D1* VIN Enable Shutdown VOUT 5V, 0.25A T1 C4 470µF R1 3.74k 1% 1:1.25 LPRI = 100µH COMP GND FB P1 P2 S C2 1µF R2 1.24k 1% * Optional voltage clipper (may be req’d if T1 leakage inductance too high) Figure 1. MIC2172 5V to 12V Boost Converter Figure 2. MIC3172 5V Flyback 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 April 2005 1 M9999-042205 MIC2172/3172 Micrel, Inc. Ordering Information Part Number Standard Pb-Free Temperature Range Package 8-pin plastic DIP MIC2172BN MIC2172YN –40°C to +85°C MIC2172BM MIC2172YM –40°C to +85°C 8-lead SOIC MIC3172BN MIC3172YN –40°C to +85°C 8-pin plastic DIP MIC3172BM MIC3172YM –40°C to +85°C 8-lead SOIC Pin Configuration MIC2172*/3172 † MIC2172*/3172 † S GND 1 8 P GND 1 S GND 1 8 P GND 1 COMP 2 7 VSW COMP 2 7 VSW FB 3 *SYNC/†EN 4 6 P GND 2 FB 3 5 VI N *SYNC/†EN 8-pin DIP (N) 4 6 P GND 2 5 VI N 8-lead SOIC (M) Pin Description Pin Number Pin Name 1 S GND Signal Ground: Internal analog circuit ground. Connect directly to the input filter capacitor for proper operation (see applications info). Keep separate from power grounds. 2 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. 3 FB 4 (MIC2172) SYNC 4 (MIC3172) EN Enable: Apply TTL high or connect to VIN to enable the regulator. Apply TTL low or connect to ground to disable the regulator. Device draws only leakage current (<1µA) when disabled. 5 VIN Supply Voltage: 3.0V to 40V 6 P GND 2 7 VSW 8 P GND 1 M9999-042205 Pin Function Feedback: Inverting input of error amplifier. Connect to external resistive divider to set power supply output voltage. Synchronization/Frequency Adjust: Capacitively coupled input signal greater than device’s free running frequency (up to 135kHz) will lock device’s oscillator on falling edge. Oscillator frequency can be trimmed up to 135kHz by adding a resistor to ground. If unused, pin must float (no connection). Power Ground #2: One of two NPN power switch emitters with 0.3Ω current sense resistor in series. Required. Connect to external inductor or input voltage ground depending on circuit topology. Power Switch Collector: Collector of NPN switch. Connect to external inductor or input voltage depending on circuit topology. Power Ground #1: One of two NPN power switch emitters with 0.3Ω current sense resistor in series. Optional. For maximum power capability connect to P GND 2. Floating pin reduces current limit by a factor of two. 2 April 2005 MIC2172/3172 Micrel, Inc. Absolute Maximum Ratings MIC2172 Input Voltage ................................................................. 40V Switch Voltage .............................................................. 65V Sync Current .............................................................. 50mA Feedback Voltage (Transient, 1ms) ........................... ±15V Operating Temperature Range 8-pin PDIP ................................................. –40 to +85°C 8-pin SOIC ................................................ –40 to +85°C Electrical Characteristics MIC2172 Parameter Conditions Reference Section Pin 2 tied to pin 3 Junction Temperature .............................. –55°C to +150°C Thermal Resistance θJA 8-pin PDIP ................................................. 130°C/W θJA 8-pin SOIC ................................................. 120°C/W Storage Temperature ............................... –65°C to +150°C Soldering (10 sec.) .................................................. +300°C Note 1, 3. Unless otherwise specified, VIN = 5V. Feedback Voltage (VFB) Feedback Voltage Line Regulation Min Typ Max Units 1.220 1.214 1.240 1.264 1.274 V V 0.03 %/V 310 750 1100 nA nA 3V ≤ VIN ≤ 40V Feedback Bias Current (IFB) Error Amplifier Section Transconductance (∆ICOMP/∆VFB) ∆ICOMP = ±25µA 3.0 2.4 3.9 6.0 7.0 µA/mV µA/mV Voltage Gain (∆VCOMP/∆VFB) 0.9V ≤ VCOMP ≤ 1.4V 500 800 2000 V/V Output Current VCOMP = 1.5V 125 100 175 350 400 µA µA Output Swing High Clamp, VFB = 1V Low Clamp, VFB = 1.5V 1.8 0.25 2.1 0.35 2.3 0.52 V V Compensation Pin Threshold Duty Cycle = 0 0.8 0.6 0.9 1.08 1.25 V V 0.76 1 1.1 Ω Ω 3 3.5 2.5 A A A Output Switch Section ON Resistance ISW = 1A, VFB = 0.8V Current Limit Duty Cycle = 50%, TJ ≥ 25°C Duty Cycle = 50%, TJ < 25°C Duty Cycle = 80% Note 2 Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V ISW = 5mA April 2005 1.25 1.25 1 65 3 75 V M9999-042205 MIC2172/3172 Parameter Micrel, Inc. Conditions Min Typ Max Units Frequency (fO) 88 85 100 112 115 kHz kHz Duty Cycle [δ(max)] 80 89 95 % Oscillator Section Sync Coupling Capacitor Required for Frequency Lock VPP = 3.0V VPP = 40V 22 2.2 51 4.7 120 10 pF pF Peak-to-Peak Voltage Required for Frequency Lock CCOUPLING = 12pF 2.2 12 30 V 2.7 3.0 V Input Supply Voltage Section Minimum Operating Voltage Quiescent Current (IQ) 3V ≤ VIN ≤ 40V, VCOMP = 0.6V, ISW = 0 7 9 mA Supply Current Increase (∆IIN) ∆ISW = 1A, VCOMP = 1.5V 9 20 mA Bold type denotes specifications applicable to the full operating temperature range. Note 1 Devices are ESD sensitive. Handling precautions required. Note 2 For duty cycles (δ) between 50% and 95%, minimum guaranteed switch current is given by ICL = 0.833 (2-δ) for the MIC3172. Note 3 Specification for packaged product only. Absolute Maximum Ratings MIC3172 Input Voltage ................................................................. 40V Switch Voltage .............................................................. 65V Enable Voltage .............................................................. 40V Feedback Voltage (Transient, 1ms) ........................... ±15V Operating Temperature Range 8-pin PDIP ................................................. –40 to +85°C 8-pin SOIC ................................................ –40 to +85°C 8-pin CerDIP ........................................... –55 to +125°C Electrical Characteristics MIC3172 Parameter Conditions Reference Section Pin 2 tied to pin 3 Junction Temperature ................................ –55°C to 150°C Thermal Resistance θJA 8-pin PDIP ................................................. 130°C/W θJA 8-pin SOIC ................................................. 120°C/W θJA 8-pin CerDIP .............................................. 100°C/W Storage Temperature ................................. –65°C to 150°C Soldering (10 sec.) .................................................... 300°C Note 1, 3. Unless otherwise specified, VIN = 5V. Feedback Voltage (VFB) Feedback Voltage Line Regulation 3V ≤ VIN ≤ 40V Typ Max Units 1.224 1.214 1.240 1.264 1.274 V V 0.07 Feedback Bias Current (IFB) M9999-042205 Min 310 4 %/V 750 1100 nA nA April 2005 MIC2172/3172 Parameter Micrel, Inc. Conditions Min Typ Max Units Transconductance (∆ICOMP/∆VFB) ∆ICOMP = ±25µA 3.0 2.4 3.9 6.0 7.0 µA/mV µA/mV Voltage Gain (∆VCOMP/∆VFB) 0.9V ≤ VCOMP ≤ 1.4V 500 800 2000 V/V Output Current VCOMP = 1.5V 125 100 175 350 400 µA µA Output Swing High Clamp, VFB = 1V Low Clamp, VFB = 1.5V 1.8 0.25 2.1 0.35 2.3 0.52 V V Compensation Pin Threshold Duty Cycle = 0 0.8 0.6 0.9 1.08 1.25 V V 0.76 1 1.1 Ω Ω 3 3.5 2.5 A A A Error Amplifier Section Output Switch Section ON Resistance ISW = 1A, VFB = 0.8V Current Limit Duty Cycle = 50%, TJ ≥ 25°C Duty Cycle = 50%, TJ < 25°C Duty Cycle = 80% Note 2 Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V ISW = 5mA 1.25 1.25 1 65 75 V Frequency (fO) 88 85 100 112 115 kHz kHz Duty Cycle [δ(max)] 80 89 95 % 2.7 3.0 V 7 0.1 9 5 mA µA 9 20 mA 0.4 1.2 2.4 V –1 0 2 1 10 µA µA Oscillator Section Input Supply Voltage Section and Enable Section Minimum Operating Voltage Quiescent Current (IQ) 3V ≤ VIN ≤ 40V, VCOMP = 0.6V, ISW = 0 Shutdown, VEN = 0V Quiescent Current Increase (∆IIN) ∆ISW = 1A, VCOMP = 1.5V Enable Input Threshold Enable Input Current VEN = 0V VEN = 2.4V Bold type denotes specifications applicable to the full operating temperature range. Note 1 Devices are ESD sensitive. Handling precautions required. Note 2 For duty cycles (δ) between 50% and 95%, minimum guaranteed switch current is given by ICL = 0.833 (2-δ) for the MIC3172. Note 3 Specification for packaged product only. April 2005 5 M9999-042205 MIC2172/3172 Micrel, Inc. Typical Performance Characteristics MIC2172 Minimum Operating Voltage 2.8 2.7 2.6 Switch Current = 1A 2.5 2.4 -50 0 50 100 Temperature (°C) 700 600 500 400 300 200 100 0 -100 150 8 ISW = 0 7 Supply Current (µA) D.C. = 90% 10 9 D.C. = 50% 8 7 D.C. = 0% 10 20 30 VIN Operating Voltage (V) -5 TJ = -40°C 0 10 20 30 VIN Operating (V) 4 3 2 1.6 40 MIC3172 5 Supply Current -50 0 50 100 Temperature (°C) 1.3 ON 1.2 1.1 OFF 1 0.9 0.8 -100 150 -50 0 50 100 Temperature (°C) 150 Current Limit Switch ON Voltage 4 40 30 δ = 90% 20 10 δ = 50% 1.2 1.0 0.8 Switch Current (A) 1.4 Switch ON Voltage (V) Average Supply Current (mA) -3 -4 Enable Thresholds 6 0 -100 40 50 TJ = –40°C TJ = 25°C 0.6 TJ = 125°C 0.4 3 –40°C 2 25°C 125°C 1 0.2 0 0.0 0.5 1.0 1.5 Switch Current (A) 0.0 0.0 2.0 Supply Current 0 1.5 Oscillator Frequency VCOMP = 0.6V 7 6 5 4 3 2 1 0 -100 M9999-042205 20 40 60 80 Duty Cycle (%) 150 100 Oscillator Frequency 140 MIC2172 130 100 90 80 70 -50 0 50 100 Temperature (°C) 0 110 Frequency (kHz) 9 8 0.5 1.0 Switch Current (A) 120 10 Supply Current (mA) TJ = 25°C -1 -2 1.4 MIC3172 VIN = 40V 1 0 1 0 150 fOSC (kHz) Supply Current (mA) 15 12 11 TJ = 125°C 3 2 Supply Current (Shutdown Mode) Supply Current 14 13 -50 0 50 100 Temperature (°C) 5 4 Enable Pin Voltage (mV) 2.3 -100 Feedback Voltage Change (mV) 800 Feedback Bias Current (nA) Minimum Operating Voltage (V) 2.9 6 5 Feedback Voltage Line Regulation Feedback Bias Current 60 -50 120 110 100 0 50 100 Temperature (°C) 6 150 90 1 10 100 RADJ (kΩ) 1000 April 2005 MIC2172/3172 Micrel, Inc. Typical Performance Characteristics Error Amplifier Gain 7000 4.5 4.0 6000 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -100 -50 0 50 100 Temperature (°C) 0 5000 4000 3000 2000 30 60 90 120 150 1000 0 150 Error Amplifier Phase -30 Phase Shift (°) Transconductance (µS) Transconductance (µA/mV) Error Amplifier Gain 5.0 180 1 10 100 1000 Frequency (kHz) 210 10000 1 10 100 1000 Frequency (kHz) 10000 Block Diagram MIC2172 VSW Pin 7 VI N Pin 5 Reg. Anti-Sat. 100kHz Osc. SYNC Pin 4 D1 2.3V Logic Driver Q1 Comparator FB Pin 3 1.24V Ref. S GND Pin 1 April 2005 Current Amp. Error Amp. P P GND GND 1 2 Pin 6 Pin 8 COMP Pin 2 7 M9999-042205 MIC2172/3172 Micrel, Inc. Block Diagram MIC3172 VSW Pin 7 VI N Pin 5 Reg. Anti-Sat. 100kHz Osc. EN Pin 4 D1 2.3V Logic Driver Q1 Comparator FB Pin 3 1.24V Ref. S GND Pin 1 Current Amp. Error Amp. P P GND GND 1 2 Pin 6 Pin 8 COMP Pin 2 technique. Feedback loop compensation is greatly simplified because inductor current sensing removes a pole from the closed loop response. Inherent cycle-by-cycle 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. Functional Description Refer to “Block Diagram MIC2172” and “Block Diagram MIC3172.” Internal Power The MIC2172/3172 operates when VIN is ≥ 2.6V (and VEN ≥ 2.0V for the MIC3172). An internal 2.3V regulator supplies biasing to all internal circuitry including a precision 1.24V band gap reference. Anti-Saturation The enable control (MIC3172 only) enables or disables the internal regulator which supplies power to all other internal circuitry. The anti-saturation diode (D1) increases the usable duty cycle range of the MIC2172/3172 by eliminating the base to collector stored charge which would delay Q1’s turnoff. PWM Operation Compensation 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. Loop stability compensation of the MIC2172/3172 can be accomplished by connecting an appropriate network from either COMP to circuit ground (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 MIC2172/3172 operates in current mode rather than voltage mode. There are three distinct advantages to this M9999-042205 8 April 2005 MIC2172/3172 Micrel, Inc. By using the MIC3172, U1 and Q1 shown in figure 5 can be eliminated, reducing the total components count. Applications Information Using the MIC3172 Enable Control (New Designs) Synchronizing the MIC2172 For new designs requiring enable/shutdown control, connect EN to a TTL or CMOS control signal (figure 3). The very low driver current requirement ensures compatibility regardless of the driver or gate used. Using several unsynchronized switching regulators in the same circuit will cause beat frequencies to appear on the inputs and outputs. These beat frequencies can be very low making them difficult to filter. U1 4 Enable Shutdown Micrel’s MIC2172 can be synchronized to a single master frequency avoiding the possibility of undesirable beat frequencies in multiple regulator circuits. The master frequency can be an external oscillator or a designated master MIC2172. The master frequency should be 1.05 to 1.20 times the slave’s 100kHz nominal frequency to guarantee synchronization. EN Logic Gate MIC3172 Figure 3. MIC3172 TTL Enable/Shutdown Using the MIC3172 in LT1172 Applications The MIC3172 can be used in most original LT1172 applications by adapting the MIC3172’s enable/shutdown feature to the existing LT1172 circuit. U2 4 U1 MIC2172 4 Unlike the LT1172 which can be shutdown by reducing the voltage on pin 2 (VC) below 0.15V, the MIC3172 has a dedicated enable/shutdown pin. To replace the LT1172 with the MIC3172, determine if the LT1172’s shutdown feature is used. 5 SYNC 10kΩ VSW MIC2172 Slave U3 Master 4 Additional Slaves Slave Figure 6. Master/Slave Synchronization VIN N/C VSW MIC2172 If the shutdown feature is not being used, connect EN to VIN to continuously enable the MIC3172 or use an MIC2172 with SYNC open (figure 4). 4 5 SYNC Circuits without Shutdown VIN 5 VSW SYNC VIN 4 SYNC Figure 6 shows a typical application where several MIC2172s operate from the same supply voltage. U1’s oscillator frequency is increased above U2’s and U3’s by connecting a resistor from SYNC to ground. U2-SYNC and U3-SYNC are capacitively coupled to the master’s output (VSW). The slaves lock to the negative (falling edge) of U1’s output waveform. VIN EN MIC2172 MIC3172 Figure 4. MIC2172/3172 Always Enabled Circuits with Shutdown U1 If shutdown was used in the original LT1172 application, connect EN to a logic gate that produces a TTL logic-level output signal that matches the shutdown signal. The MIC3172 will be enabled by a logic-high input and shutdown with a logic-low input (figure 5). The actual components performing the functions of U1 and Q1 may vary according to the original application. 4 5 SYNC VSW MIC2172 External Signal Slave U2 4 5 VSW SYNC 4 add connection EN MIC2172 MIC3172 Additional Slaves Slave COMP U1 Enable Shutdown Existing Logic Gate Figure 7. External Synchronization Existing Q1 VN2222 or equiv. R1 Care must be exercised to insure that the master MIC2172 is always operating in continuous mode. C1 Figure 5. Adapting to the LT1172 Socket April 2005 9 M9999-042205 MIC2172/3172 Micrel, Inc. Figure 7 shows how one or more MIC2172s can be locked to an external reference frequency. The slaves lock to the negative (falling edge) of the external reference waveform. 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 (VIN) and device supply current (IQ). The MIC2172/3172 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. Soft Start A diode-coupled capacitor from COMP to circuit ground slows the output voltage rise at turn on (figure 8). VIN VIN MIC2172/3172 0.004 + δ P(bias+driver) = VIN IQ + VIN ISW 50 ( COMP D1 D2 where: R1 C2 C1 P(bias+driver) = device operating losses VIN = supply voltage IQ = quiescent supply current ISW = power switch current (see “ Design Hints: Switch Current Calculations”) δ = duty cycle Figure 8. 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. Current Limit For designs demanding less output current than the MIC2172/ 3172 is capable of delivering, P GND 1 can be left open reducing the current capability of Q1 by one-half. VIN δ= VSW MIC2172/3172 C1 R3 C2 I CL ≈ 0.6V/R2 Then: Note: Input and output returns not common. 0.004 + 0.6 P(bias+driver) = ( 5 × 0.006) + 5 0.625 50 R2 Figure 9. Current Limit P(bias+driver) = 0.068W Alternatively, the maximum current limit of the MIC2172/3172 can be reduced by adding a voltage clamp to the COMP output (figure 9). 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. 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. Thermal Management Although the MIC2172/3172 family contains thermal protection circuitry, for best reliability, avoid prolonged operation with junction temperatures near the rated maximum. PSW = (ISW)2 RSW δ From the Typical performance Characteristics: The junction temperature is determined by first calculating the power dissipation of the device. For the MIC2172/3172, M9999-042205 V OUT + VF VIN = 5.0V IQ = 0.006A ISW = 0.625A δ = 60% (0.6) VOUT FB GND P1 P2 S COMP R1 V OUT + VF ± VIN VOUT = output voltage VF = D1 forward voltage drop As a practical example refer to figure 1. VIN Q1 ) RSW = 1Ω 10 April 2005 MIC2172/3172 Micrel, Inc. Then: Applications and Design Hints PSW = (0.625)2 × 1 × 0.6 P(SW) = 0.234W P(total) = 0.068 + 0.234 P(total) = 0.302W The junction temperature for any semiconductor is calculated using the following: Access to both the collector and emitter(s) of the NPN power switch makes the MIC2172/3172 extremely versatile and suitable for use in most PWM power supply topologies. Boost Conversion Refer to figure 11 for a typical boost conversion application where a +5V logic supply is available but +12V at 0.14A is required. TJ = TA + P(total) θJA Where: +5V (4.75V min.) C1* 22µF L1 27µH TJ = junction temperature TA = ambient temperature (maximum) P(total) = total power dissipation θJA = junction to ambient thermal resistance For the practical example: VIN N/C SYNC TA = 70°C θJA = 130°C/W (for plastic DIP) VOUT +12V, 0.14A 1N5822 R1 10.7k 1% VSW MIC2172 COMP GND FB P1 P2 S C3 1µF R3 1k D1 R2 C2 1.24k 470µF 1% * Locate near MIC2172 when supply leads > 2" Then: TJ = 70 + 0.30 × 130 TJ = 109°C This junction temperature is below the rated maximum of 150°C. Figure 11. 5V to 12V Boost Converter 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. Grounding Refer to figure 10. Heavy lines indicate high current paths. When L1 discharges its current completely during the MIC2172/3172’s off-time, it is operating in discontinuous mode. VIN VIN EN * VSW L1 is operating in continuous mode if it does not discharge completely before the MIC2172/3172 power switch is turned on again. MIC2172/3172 GND P1 P2 S FB VC 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. Single point ground * MIC3172 only Figure 10. Single Point Ground A single point ground is strongly recommended for proper operation. The signal ground, compensation network ground, and feedback 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 MIC2172/3172 or any switching regulator from functioning properly. April 2005 (1) IOUT (1a) δ= ICL V δ 2 IN ≤ V OUT V OUT + VF ± VIN V OUT + VF Where: ICL = internal switch current limit ICL = 1.25A when δ < 50% ICL = 0.833 (2 – δ) when δ ≥ 50% (Refer to Electrical Characteristics.) IOUT = maximum output current VIN = minimum input voltage δ = duty cycle 11 M9999-042205 MIC2172/3172 Micrel, Inc. Switch Operation VOUT = required output voltage VF = D1 forward voltage drop For the example in figure 11. 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. IOUT = 0.14A ICL = 1.147A VIN = 4.75V (minimum) δ = 0.623 VOUT = 12.0V VF = 0.6V C1 (input capacitor) may be reduced or eliminated if the MIC3172 is located near a low impedance voltage source. Output Diode Then: IOUT The output diode allows T1 to store energy in its primary inductance (D2 nonconducting) and release energy into C4 (D2 conducting). The low forward voltage drop of a Schottky diode minimizes power loss in D2. 1.147 × 4.75 × 0.623 2 ≤ 12 Frequency Compensation IOUT ≤ 0.141A This value is greater than the 0.14A output current requirement so we can proceed to find the inductance value of L1. (2) L1 ≤ A simple frequency compensation network consisting of R3 and C2 prevents output oscillations. High impedance output stages (transconductance type) in the MIC2172/3172 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. (VIN δ )2 2 POUT f SW Where: POUT = 12 × 0.14 = 1.68W fSW = 1×105Hz (100kHz) For our practical example: 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 MIC3172’s power NPN Q1. 2 ( 4.75 × 0.623) 2 × 1.68 × 1× 105 Enable/Shutdown IL1 ≤ 26.062µH (use 27µH) Equation (3) solves for L1’s maximum current value. (3) IL1(peak) = The MIC3172 includes the enable/shutdown feature. When the device is shutdown, total supply current is less than 1µA. This is ideal for battery applications where portions of a system are powered only when needed. If this feature is not required, simply connect EN to VIN or to a TTL high voltage. VIN T ON L1 Where: TON = δ / fSW = Discontinuous Mode Design 6.23×10-6 sec 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. 4.75 × 6.23 × 10-6 27 × 10-6 IL1(peak) = 1.096A Use a 27µH inductor with a peak current rating of at least 1.4A. IL1(peak) = (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. δ ≥ 2 POUT ICL VIN(min) For a practical example let: POUT = 5.0V × 0.25A = 1.25W VIN = 4.0V to 6.0V ICL = 1.25A when δ < 50% 0.833 (2 – δ) when δ ≥ 50% Figure 12 shows a practical flyback converter design using the MIC3172. M9999-042205 12 April 2005 MIC2172/3172 Micrel, Inc. Then: 0.5 f SW VIN(min)2 T ON2 LPRI ≤ POUT (10) 2 × 1.25 1.25 × 4 δ ≥ 0.5 (50%) Use 0.55. δ ≥ Where: LPRI = maximum primary inductance fSW = device switching frequency (100kHz) VIN(min) = minimum input voltage TON = power switch on time The slightly higher duty cycle value is used to overcome circuit inefficiencies. 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 MIC2172/ 3172 power switch. (9) a ≤ Then: V CE FCE ± VIN(max) LPRI ≤ V SEC ( 0.5 × 1× 105 × 4.02 5.5 × 10-6 )2 1.25 LPRI ≤ 19.23µH Use an 18µH primary inductance to overcome circuit inefficiencies. Where: 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) For the practical example: 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. (11) VCE = 65V max. for the MIC2172/3172 FCE = 0.8 VSEC = 5.6V L SEC ≤ 0.5 f SW V SEC 2 T OFF 2 POUT Where: LSEC = maximum secondary inductance TOFF = power switch off time Then: Then: 65 × 0.8 ± 6.0 a ≤ 5.6 a ≤ 8.2143 L SEC ≤ Next, calculate the maximum primary inductance required to store the needed output energy with a power switch duty cycle of 55%. VIN 4V to 6V R4* VSW EN MIC3172 R3 1k LSEC ≤ 25.4µH C3* D1* VIN )2 1.25 VOUT 5V, 0.25A T1 C1 22µF Enable Shutdown ( 0.5 × 1× 105 × 5.6 2 × 4.5 × 10-6 D2 1N5818 C4 470µF R1 3.74k 1% 1:1.25 LPRI = 100µH COMP GND FB P1 P2 S R2 1.24k 1% C2 1µF * Optional voltage clipper (may be req’d if T1 leakage inductance too high) Figure 12. MIC3172 5V 0.25A Flyback Converter April 2005 13 M9999-042205 MIC2172/3172 Micrel, Inc. Finally, recalculate the transformer turns ratio to insure that it is less than the value earlier found in equation (9). (12) a ≤ a = transformer turns ratio (0.8) FBR = reverse voltage safety derating factor (0.8) Then: LPRI L SEC VBR ≥ Then: a ≤ VBR ≥ 15.625V A 1N5817 will safely handle voltage and current requirements in this example. 1.8 × 10-5 2.54 × 10-5 Forward Converters a ≤ 0.84 Use 0.8 (same as 1:1.25). 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. (13) IPEAK(pri) = VIN(min) T ON LPRI IPEAK(pri) = 4.0 × 5.5 × 10-6 18µ H Micrel’s MIC2172/3172 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 13 shows the MIC3172 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 LM457x family of buck switchers). However, for step-down conversion requiring a transformer (forward), the MIC2172/3172 is a good choice. So: A 12V to 5V step-down converter using transformer isolation (forward) is shown in figure 14. 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-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. IPEAK(pri) = 1.22A 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 0.25A. (14) VBR ≥ 6.0 + ( 5.0 × 0.8) 0.8 × 0.8 ( VIN(max) + V OUT a ) FBR a 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 Where: VBR = output rectifier maximum peak reverse voltage rating VIN D1 1N4148 VIN VSW EN C2 2.2µF C1* 100µF R3 470 MIC3172 R3† COMP GND FB P1 P2 S D3 1N4148 3.7k R2† 1.2k C3 1µF D2 C4 1µF L1 R4 10Ω 100µH C5 330µF 5V, 0.1A to 1A (ILOAD > 100mA) * Locate near MIC2172/3172 when supply leads > 2" † R3/R2 sets output voltage Figure 13. Step-Down or Buck Converter M9999-042205 14 April 2005 MIC2172/3172 Micrel, Inc. 50%, the MIC2172 (and MIC3172) 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 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. into saturation for a period determined by the Pri 1/C2 time constant. Once the voltage across C2 has reached its maximum circuit value, Q1’s collector current will no longer increase. Since T1 is in series with Q1, this drop in primary current causes the flux in T1 to change and because of the mutual coupling to the feedback winding further reduces primary current eventually turning Q1 off. The primary windings now change state with the feedback winding forcing Q2 on repeating the alternate half cycle exactly as with Q1. This action produces a sinusoidal voltage wave form; whose amplitude is proportional to the input voltage, across T1’s primary winding which is stepped up and capacitively coupled to the lamp. To prevent core saturation, the application given here uses a duty cycle limiter consisting of Q1, C4 and R3. Whenever the MIC3172 exceeds a duty cycle of 50%, T1’s reset winding current turns Q1 on. This action reduces the duty cycle of the MIC3172 until T1 is able to reset during each cycle. Fluorescent Lamp Supply Lamp Current Regulation An extremely useful application of the MIC3172 is generating an ac voltage for fluorescent lamps used as liquid crystal display back lighting in portable computers. Initial ionization (lighting) of the fluorescent lamp requires several times the ac voltage across it than is required to sustain current through the device. The current through the lamp is sampled and regulated by the MIC3172 to achieve a given intensity. The MIC3172 uses L1 to maintain a constant average current through the transistor emitters. This current controls the voltage amplitude of the Royer oscillator and maintains the lamp current. During the negative half cycle, lamp current is rectified by D3. During the positive half cycle, lamp current is rectified by D2 through R4 and R5. R3 and C5 filter the voltage dropped across R4 and R5 to the MIC3172’s feedback pin. The MIC3172 maintains a constant lamp current by adjusting its duty cycle to keep the feedback voltage at 1.24V. The intensity of the lamp is adjusted using potentiometer R5. The MIC3172 adjusts its duty cycle accordingly to bring the average voltage across R4 and R5 back to 1.24V. Figure 15 shows a complete power supply for lighting a fluorescent lamp. Transistors Q1 and Q2 together with capacitor C2 form a Royer oscillator. The Royer oscillator generates a sine wave whose frequency is determined by the series L/C circuit comprised of T1 and C2. Assuming that the MIC3172 and L1 are absent, and the transistors’ emitters are grounded, circuit operation is described in “Oscillator Operation.” Oscillator Operation Resistor R2 provides initial base current that turns transistor Q1 on and impresses the input voltage across one half of T1’s primary winding (Pri 1). T1’s feedback winding provides additional base drive (positive feedback) to Q1 forcing it well T1 1:1:1 D3 1N5819 L1 100µH VIN 12V R1* C2* R4 C5 3.74k 470µF 1% D1* VIN Enable Shutdown D4 1N5819 VOUT 5V, 1A EN VSW MIC3172 C1 22µF GND P1 P2 S FB COMP R2 1k C3 1µF D2 1N5819 Q1† R3 † R5 1.24k 1% C4 † * Voltage clipper † Duty cycle limiter Figure 14. 12V to 5V Forward Converter April 2005 15 M9999-042205 MIC2172/3172 Micrel, Inc. On/Off Control Efficiency Especially important for battery powered applications, the lamp can be remotely or automatically turned off using the MIC3172’s EN pin. The entire circuit draws less than 1µA while shutdown. To obtain maximum circuit efficiency careful selection of Q1 and Q2 for low collector to emitter saturation voltage is a must. Inductor L1 should be chosen for minimal core and copper losses at the switching frequency of the MIC3172, and T1 should be carefully constructed from magnetic materials optimized for the output power required at the Royer oscillator frequency. Suitable inductors may be obtained from Coiltronics, Inc., tel: (407) 241-7876. Cold Cathode Fluorescent Lamp FB T1 EN GND P1 P2 S C3 300µH Q2 FB COMP R1 C1 D2 1N4148 D3 1N4148 L1 VSW MIC3172 C2 Sec D1 VIN Pri 1 Q1 Pri 2 Enable (On) Shutdown (Off) C4 R2 VIN 4.5V to 20V R3 C5 L1: T1: C2: C4: R4 R5 Intensity Control Coiltronics CTX300-4P Coiltronics CTX110602 Polyfilm, WIMA FKP2 0.1µF to 0.68µF 15pF to 30pF, 3kV min. Figure 15. LCD Backlight Fluorescent Lamp Supply M9999-042205 16 April 2005 MIC2172/3172 Micrel, Inc. Package Information PIN 1 DIMENSIONS: INCH (MM) 0.380 (9.65) 0.370 (9.40) 0.255 (6.48) 0.245 (6.22) 0.135 (3.43) 0.125 (3.18) 0.300 (7.62) 0.013 (0.330) 0.010 (0.254) 0.018 (0.57) 0.130 (3.30) 0.100 (2.54) 0.380 (9.65) 0.320 (8.13) 0.0375 (0.952) 8-Pin Plastic DIP (N) 0.026 (0.65) MAX) PIN 1 0.157 (3.99) 0.150 (3.81) DIMENSIONS: INCHES (MM) 0.020 (0.51) 0.013 (0.33) 0.050 (1.27) TYP 0.064 (1.63) 0.045 (1.14) 45° 0.0098 (0.249) 0.0040 (0.102) 0.197 (5.0) 0.189 (4.8) 0°–8° SEATING PLANE 0.010 (0.25) 0.007 (0.18) 0.050 (1.27) 0.016 (0.40) 0.244 (6.20) 0.228 (5.79) 8-Pin SOP (M) 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 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. © 1997 Micrel Incorporated April 2005 17 M9999-042205