MIC3172BN

MIC2172_06 MICREL | Alldatasheet

Document overview

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• PDF pages: 20

Technical content

Features

• 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

• 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)

(408) 955-1690 Pin Description Pin Number Pin Name Pin Function 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 Feedback: Inverting input of error amplifier. Connect to external resistive divider to set power supply output voltage. 4 (MIC2172) SYNC 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). 4 (MIC3172) EN Enable: Apply TTL high or connect to V IN 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 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. 7 VSW Power Switch Collector: Collector of NPN switch. Connect to external inductor or input voltage depending on circuit topology. 8 P GND 1 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.

(408) 955-1690 Operating Temperature Range Thermal Resistance θ Note 1, 3. Unless otherwise specified, VIN = 5V. Parameter Condition Min Typ Max Units Reference Section Pin 2 tied to pin 3 Feedback Voltage (VFB) 1.220 1.214 1.240 1.264 1.274 V V Feedback Voltage Line Regulation 3V ≤ V IN ≤ 40V 0.03 %/V Feedback Bias Current (IFB) 310 750 1100 nA nA Error Amplifier Section Transconductance (∆I COMP/∆VFB) ∆ICOMP = ±25µA 3.0 2.4 3.9 6.0 7.0 µA/mV µA/mV Voltage Gain (∆V COMP/∆VFB) 0.9V ≤ VCOMP ≤ 1.4V 500 800 2000 V/V Output Current V COMP = 1.5V 125 100 175 350 400 µA µA Output Swing High Clamp, V FB = 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 Output Switch Section ON Resistance I SW = 1A, VFB = 0.8V 0.76 1 1.1 Ω Ω Current Limit Duty Cycle = 50%, T J ≥ 25°C Duty Cycle = 50%, TJ < 25°C Duty Cycle = 80%, Note 2 1.25 1.25 3.5 2.5 A A A Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V ISW = 5mA 65 75 V Notes: 1. Exceeding the absolute maximum rating may damage the device. 2. The device is not guaranteed to function outside its operating rating. 4. Specification for packaged product only.

(408) 955-1690 Typical Characteristics MIC2172 (cont) Parameter Condition Min Typ Max Units Oscillator Section Frequency (fO) 88 100 112 115 kHz kHz Duty Cycle [δ(max)] 80 89 95 % Sync Coupling Capacitor Required for Frequency Lock VPP = 3.0V VPP = 40V 2.2 4.7 120 pF pF Input Supply Voltage Section Minimum Operating Voltage 2.7 3.0 V 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 Note 1, 3. Unless otherwise specified, VIN = 5V. Parameter Condition Min Typ Max Units Reference Section Pin 2 tied to pin 3 Feedback Voltage (VFB) 1.224 1.214 1.240 1.264 1.274 V V Feedback Voltage Line Regulation 3V ≤ V IN ≤ 40V 0.07 %/V Feedback Bias Current (IFB) 310 750 1100 nA nA Error Amplifier Section Transconductance (∆I COMP/∆VFB) ∆ICOMP = ±25µA 3.0 2.4 3.9 6.0 7.0 µA/mV µA/mV Voltage Gain (∆V COMP/∆VFB) 0.9V ≤ VCOMP ≤ 1.4V 500 800 2000 V/V Output Current V COMP = 1.5V 125 100 175 350 400 µA µA Output Swing High Clamp, V FB = 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 Output Switch Section ON Resistance I SW = 1A, VFB = 0.8V 0.76 1 1.1 Ω Ω Current Limit Duty Cycle = 50%, T J ≥ 25°C Duty Cycle = 50%, TJ < 25°C Duty Cycle = 80%, Note 2 1.25 1.25 3.5 2.5 A A A Breakdown Voltage (BV) 3V ≤ VIN ≤ 40V ISW = 5mA 65 75 V

(408) 955-1690 Typical Characteristics MIC3172 (cont) Parameter Condition Min Typ Max Units Oscillator Section Frequency (fO) 88 100 112 115 kHz kHz Duty Cycle [δ(max)] 80 89 95 % Sync Coupling Capacitor Required for Frequency Lock VPP = 3.0V VPP = 40V 2.2 4.7 120 pF pF Input Supply Voltage Section and Enable Section Minimum Operating Voltage 2.7 3.0 V 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 Enable Input Threshold 0.4 1.2 2.4 V Enable Input Current V EN = 0V VEN = 2.4V –1 0 µA µA 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.

(408) 955-1690 Typical Characteristics

(408) 955-1690 Typical Characteristics (cont.)

(408) 955-1690 Functional Characteristics MIC2172 Block Diagram MIC3172 Block Diagram

April 2006 10 M9999-041806 (408) 955-1690 Functional Description Refer to “Block Diagram MIC2172” and “Block Diagram MIC3172.” Internal Power The MIC2172/3172 operates when V IN 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. The enable control (MIC3172 only) enables or disables the internal regulator which supplies power to all other internal circuitry. 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. Current Mode Advantages The MIC2172/3172 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-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. Anti-Saturation 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. Compensation 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.

April 2006 13 M9999-041806 (408) 955-1690 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. Thermal Management Although the MIC2172/3172 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 MIC2172/3172, 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 (V IN) and device supply current (I Q). 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. () () ⎥⎦ ⎡ ⎟ ⎛ ++=+ 50 δ0.004IVI VP SWINQINdriverbias where: 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 FOUT INFOUT VV VVVδ + ±+= V OUT = output voltage V F = D1 forward voltage drop As a practical example refer to figure 1. VIN = 5.0V IQ = 0.006A ISW = 0.625A δ = 60% (0.6) Then: () () () 0.068WP 0.60.0040.62550.0065P driverbias driverbias ⎡ ⎟ ⎛ ++×= 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 SW = (ISW)2 RSW δ From the Typical performance Characteristics: RSW = 1Ω Then: PSW = 0.234W P(total) = 0.068 + 0.234 P(total) = 0.302W The junction temperature for any semiconductor is calculated using the following: TJ = TA + P(total) θJA Where: TJ = junction temperature TA = ambient temperature (maximum) P(total) = total power dissipation θJA = junction to ambient thermal resistance For the practical example: TA = 70°C θJA = 130°C/W (for plastic DIP) Then: TJ = 70 + 0.30 ⋅ 130 TJ = 109°C This junction temperature is below the rated maximum of 150°C. Grounding Refer to figure 10. Heavy lines indicate high current paths.

April 2006 15 M9999-041806 (408) 955-1690 1011.682 0.6234.75L1 ××× IL1 ≤ 26.062µH (use 27µH) Equation (3) solves for L1’s maximum current value. T VI ONIN L1(peak) = (3) Where: TON = δ / fSW = 6.23×10-6 sec L1(peak) 1027 106.234.75I − ××= IL1(peak) = 1.096A Use a 27µH inductor with a peak current rating of at least 1.4A. 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 12 shows a practical flyback converter design using the MIC3172. 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 MIC3172 is located near a low impedance voltage source. Output Diode 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. Frequency Compensation 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. 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. Enable/Shutdown 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 V IN or to a TTL high voltage. 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. IN(min)CL OUT VI P 2δ ≥ (8) For a practical example let: POUT = 5.0V × 0.25A = 1.25W VIN = 4.0V to 6.0V ICL = 1.25A when δ < 50% Then: 41.25 1.252δ × 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. SEC IN(max)CECE V VF Va ±≤ (9) Where: a = transformer maximum turns ratio VCE = power switch collector to emitter maximum voltage

Figure 12. MIC3172 5V 0.25A Flyback Converter

up and capacitively coupled to the lamp. voltage across R4 and R5 back to 1.24V. less than 1µA while shutdown. Figure 13. Step-Down or Buck Regulator

April 2006 20 M9999-041806 (408) 955-1690

Package Information

8-Pin Plastic DIP (N) 8-Pin SOIC (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 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. © 2004 Micrel, Incorporated.