LNK302

LNK302/304-306 LinkSwitch-TN Family ® Lowest Component Count, Energy-Efficient Off-Line Switcher IC Product Highlights Cost Effective Linear/Cap Dropper Replacement • Lowest cost and component count buck converter solution • Fully integrated auto-restart for short-circuit and open loop fault protection – saves external component costs • LNK302 uses a simplified controller without auto-restart for very low system cost • 66 kHz operation with accurate current limit – allows low cost off-the-shelf 1 mH inductor for up to 120 mA output current • Tight tolerances and negligible temperature variation • High breakdown voltage of 700 V provides excellent input surge withstand • Frequency jittering dramatically reduces EMI (~10 dB) – minimizes EMI filter cost • High thermal shutdown temperature (+135 °C minimum) Much Higher Performance over Discrete Buck and Passive Solutions • Supports buck, buck-boost and flyback topologies • System level thermal overload, output short-circuit and open control loop protection • Excellent line and load regulation even with typical configuration • High bandwidth provides fast turn-on with no overshoot • Current limit operation rejects line ripple • Universal input voltage range (85 VAC to 265 VAC) • Built-in current limit and hysteretic thermal protection • Higher efficiency than passive solutions • Higher power factor than capacitor-fed solutions • Entirely manufacturable in SMD EcoSmart – Extremely Energy Efficient • Consumes typically only 50/80 mW in self-powered buck topology at 115/230 VAC input with no load (opto feedback) • Consumes typically only 7/12 mW in flyback topology with external bias at 115/230 VAC input with no load • Meets California Energy Commission (CEC), Energy Star, and EU requirements ® FB BP S + Wide Range HV DC Input D + DC Output LinkSwitch-TN PI-3492-111903 Figure 1. Typical Buck Converter Application (See Application Examples Section for Other Circuit Configurations). OUTPUT CURRENT TABLE1 PRODUCT4 LNK302P/G/D 230 VAC ±15% MDCM 2 85-265 VAC MDCM2 63 mA CCM3 80 mA CCM 3 63 mA 80 mA 170 mA 280 mA 360 mA LNK304P/G/D 120 mA LNK305P/G/D 175 mA LNK306P/G/D 225 mA Table 1. Output Current Table. 120 mA 170 mA 175 mA 280 mA 225 mA 360 mA Notes: 1. Typical output current in a non-isolated buck converter. Output power capability depends on respective output voltage. See Key Applications Considerations Section for complete description of assumptions, including fully discontinuous conduction mode (DCM) operation. 2. Mostly discontinuous conduction mode. 3. Continuous conduction mode. 4. Packages: P: DIP-8B, G: SMD-8B, D: SO-8C. Applications • Appliances and timers • LED drivers and industrial controls Description LinkSwitch-TN is specifically designed to replace all linear and capacitor-fed (cap dropper) non-isolated power supplies in the under 360 mA output current range at equal system cost while offering much higher performance and energy efficiency. LinkSwitch-TN devices integrate a 700 V power MOSFET, oscillator, simple On/Off control scheme, a high voltage switched current source, frequency jittering, cycle-by-cycle current limit and thermal shutdown circuitry onto a monolithic IC. The startup and operating power are derived directly from the voltage on the DRAIN pin, eliminating the need for a bias supply and associated circuitry in buck or flyback converters. The fully integrated auto-restart circuit in the LNK304-306 safely limits output power during fault conditions such as short-circuit or open loop, reducing component count and system-level load protection cost. A local supply provided by the IC allows use of a non-safety graded optocoupler acting as a level shifter to further enhance line and load regulation performance in buck and buck-boost converters, if required. December 2006 LNK302/304-306 BYPASS (BP) REGULATOR 5.8 V DRAIN (D) + BYPASS PIN UNDER-VOLTAGE 5.8 V 4.85 V 6.3 V - CURRENT LIMIT COMPARATOR + - VI LIMIT JITTER CLOCK DCMAX THERMAL SHUTDOWN OSCILLATOR FEEDBACK (FB) 1.65 V -VT S R Q Q LEADING EDGE BLANKING SOURCE (S) PI-3904-020805 Figure 2a. Functional Block Diagram (LNK302). BYPASS (BP) DRAIN (D) REGULATOR 5.8 V FAULT PRESENT AUTORESTART COUNTER CLOCK RESET 6.3 V 5.8 V 4.85 V BYPASS PIN UNDER-VOLTAGE + - CURRENT LIMIT COMPARATOR + - VI LIMIT JITTER CLOCK DCMAX OSCILLATOR FEEDBACK (FB) 1.65 V -VT S R Q Q LEADING EDGE BLANKING THERMAL SHUTDOWN SOURCE (S) PI-2367-021105 Figure 2b. Functional Block Diagram (LNK304-306). 22 -2 Rev. H 12/06 LNK302/304-306 Pin Functional Description DRAIN (D) Pin: Power MOSFET drain connection. Provides internal operating current for both start-up and steady-state operation. BYPASS (BP) Pin: Connection point for a 0.1 μF external bypass capacitor for the internally generated 5.8 V supply. FEEDBACK (FB) Pin: During normal operation, switching of the power MOSFET is controlled by this pin. MOSFET switching is terminated when a current greater than 49 μA is delivered into this pin. SOURCE (S) Pin: This pin is the power MOSFET source connection. It is also the ground reference for the BYPASS and FEEDBACK pins. The LinkSwitch-TN oscillator incorporates circuitry that introduces a small amount of frequency jitter, typically 4 kHz peak-to-peak, to minimize EMI emission. The modulation rate of the frequency jitter is set to 1 kHz to optimize EMI reduction for both average and quasi-peak emissions. The frequency jitter should be measured with the oscilloscope triggered at the falling edge of the DRAIN waveform. The waveform in Figure 4 illustrates the frequency jitter of the LinkSwitch-TN. Feedback Input Circuit The feedback input circuit at the FB pin consists of a low impedance source follower output set at 1.65 V. When the current delivered into this pin exceeds 49 μA, a low logic level (disable) is generated at the output of the feedback circuit. This output is sampled at the beginning of each cycle on the rising edge of the clock signal. If high, the power MOSFET is turned on for that cycle (enabled), otherwise the power MOSFET remains off (disabled). Since the sampling is done only at the beginning of each cycle, subsequent changes in the FB pin voltage or current during the remainder of the cycle are ignored. 5.8 V Regulator and 6.3 V Shunt Voltage Clamp The 5.8 V regulator charges the bypass capacitor connected to the BYPASS pin to 5.8 V by drawing a current from the voltage on the DRAIN, whenever the MOSFET is off. The BYPASS pin is the internal supply voltage node for the LinkSwitch-TN. When the MOSFET is on, the LinkSwitch-TN runs off of the energy stored in the bypass capacitor. Extremely low power consumption of the internal circuitry allows the LinkSwitch-TN to operate continuously from the current drawn from the DRAIN pin. A bypass capacitor value of 0.1 μF is sufficient for both high frequency decoupling and energy storage. In addition, there is a 6.3 V shunt regulator clamping the BYPASS pin at 6.3 V when current is provided to the BYPASS pin through an external resistor. This facilitates powering of LinkSwitch-TN externally through a bias winding to decrease the no-load consumption to about 50 mW. BYPASS Pin Under-Voltage The BYPASS pin under-voltage circuitry disables the power MOSFET when the BYPASS pin voltage drops below 4.85 V. Once the BYPASS pin voltage drops below 4.85 V, it must rise back to 5.8 V to enable (turn-on) the power MOSFET. Over-Temperature Protection The thermal shutdown circuitry senses the die temperature. The threshold is set at 142 °C typical with a 75 °C hysteresis. When the die temperature rises above this threshold (142 °C) the power MOSFET is disabled and remains disabled until the die temperature falls by 75 °C, at which point it is re-enabled. Current Limit The current limit circuit senses the current in the power MOSFET. When this current exceeds the internal threshold (ILIMIT), the 23 -3 Rev. H 12/06 P Package (DIP-8B) G Package (SMD-8B) S S BP FB D Package (SO-8C) 1 2 3 4 3a 8 7 S S BP FB D 1 2 8 7 6 S S S S 4 5 5 D 3b PI-3491-120706 Figure 3. Pin Configuration. LinkSwitch-TN Functional Description LinkSwitch-TN combines a high voltage power MOSFET switch with a power supply controller in one device. Unlike conventional PWM (pulse width modulator) controllers, LinkSwitch-TN uses a simple ON/OFF control to regulate the output voltage. The LinkSwitch-TN controller consists of an oscillator, feedback (sense and logic) circuit, 5.8 V regulator, BYPASS pin undervoltage circuit, over-temperature protection, frequency jittering, current limit circuit, leading edge blanking and a 700 V power MOSFET. The LinkSwitch-TN incorporates additional circuitry for auto-restart. Oscillator The typical oscillator frequency is internally set to an average of 66 kHz. Two signals are generated from the oscillator: the maximum duty cycle signal (DCMAX) and the clock signal that indicates the beginning of each cycle. LNK302/304-306 PI-3660-081303 600 500 VDRAIN 400 300 200 100 0 68 kHz 64 kHz 12 V, 120 mA non-isolated power supply used in appliance control such as rice cookers, dishwashers or other white goods. This circuit may also be applicable to other applications such as night-lights, LED drivers, electricity meters, and residential heating controllers, where a non-isolated supply is acceptable. The input stage comprises fusible resistor RF1, diodes D3 and D4, capacitors C4 and C5, and inductor L2. Resistor RF1 is a flame proof, fusible, wire wound resistor. It accomplishes several functions: a) Inrush current limitation to safe levels for rectifiers D3 and D4; b) Differential mode noise attenuation; c) Input fuse should any other component fail short-circuit (component fails safely open-circuit without emitting smoke, fire or incandescent material). 0 Time (μs) 20 Figure 4. Frequency Jitter. power MOSFET is turned off for the remainder of that cycle. The leading edge blanking circuit inhibits the current limit comparator for a short time (tLEB) after the power MOSFET is turned on. This leading edge blanking time has been set so that current spikes caused by capacitance and rectifier reverse recovery time will not cause premature termination of the switching pulse. Auto-Restart (LNK304-306 only) In the event of a fault condition such as output overload, output short, or an open loop condition, LinkSwitch-TN enters into autorestart operation. An internal counter clocked by the oscillator gets reset every time the FB pin is pulled high. If the FB pin is not pulled high for 50 ms, the power MOSFET switching is disabled for 800 ms. The auto-restart alternately enables and disables the switching of the power MOSFET until the fault condition is removed. The power processing stage is formed by the LinkSwitch-TN, freewheeling diode D1, output choke L1, and the output capacitor C2. The LNK304 was selected such that the power supply operates in the mostly discontinuous-mode (MDCM). Diode D1 is an ultra-fast diode with a reverse recovery time (trr) of approximately 75 ns, acceptable for MDCM operation. For continuous conduction mode (CCM) designs, a diode with a trr of ≤35 ns is recommended. Inductor L1 is a standard off-the- shelf inductor with appropriate RMS current rating (and acceptable temperature rise). Capacitor C2 is the output filter capacitor; its primary function is to limit the output voltage ripple. The output voltage ripple is a stronger function of the ESR of the output capacitor than the value of the capacitor itself. To a first order, the forward voltage drops of D1 and D2 are identical. Therefore, the voltage across C3 tracks the output voltage. The voltage developed across C3 is sensed and regulated via the resistor divider R1 and R3 connected to U1’s FB pin. The values of R1 and R3 are selected such that, at the desired output voltage, the voltage at the FB pin is 1.65 V. Regulation is maintained by skipping switching cycles. As the output voltage rises, the current into the FB pin will rise. If this exceeds IFB then subsequent cycles will be skipped until the current reduces below IFB. Thus, as the output load is reduced, more cycles will be skipped and if the load increases, fewer Applications Example A 1.44 W Universal Input Buck Converter The circuit shown in Figure 5 is a typical implementation of a R1 13.0 kΩ 1% R3 2.05 kΩ 1% C3 10 μF 35 V L1 1 mH 280 mA D1 UF4005 RF1 8.2 Ω 2W D3 1N4007 D4 1N4007 L2 1 mH FB D BP S C1 100 nF D2 1N4005GP 12 V, 120 mA C2 100 μF 16 V R4 3.3 kΩ RTN 85-265 VAC C4 4.7 μF 400 V C5 4.7 μF 400 V LinkSwitch-TN LNK304 PI-3757-112103 Figure 5. Universal Input, 12 V, 120 mA Constant Voltage Power Supply Using LinkSwitch-TN. 24 -4 Rev. H 12/06 LNK302/304-306 LinkSwitch-TN RF1 D3 L2 D FB R1 D2 BP AC INPUT C4 C5 + C1 R3 C3 L1 C2 D1 DC OUTPUT S S S S D4 Optimize hatched copper areas ( ) for heatsinking and EMI. PI-3750-121106 Figure 6a. Recommended Printed Circuit Layout for LinkSwitch-TN in a Buck Converter Configuration using P or G Package. RF1 D3 L2 D S S S S D1 C3 D2 C2 L1 + LinkSwitch-TN FB BP AC INPUT C4 C5 C1 R3 R1 DC OUTPUT D4 Optimize hatched copper areas ( ) for heatsinking and EMI. PI-4546-011807 Figure 6b. Recommended Printed Circuit Layout for LinkSwitch-TN in a Buck Converter Configuration using D Package to Bottom Side of the Board. cycles are skipped. To provide overload protection if no cycles are skipped during a 50 ms period, LinkSwitch-TN will enter auto-restart (LNK304-306), limiting the average output power to approximately 6% of the maximum overload power. Due to tracking errors between the output voltage and the voltage across C3 at light load or no load, a small pre-load may be required (R4). For the design in Figure 5, if regulation to zero load is required, then this value should be reduced to 2.4 kΩ. Key Application Considerations LinkSwitch-TN Design Considerations Output Current Table Data sheet maximum output current table (Table 1) represents the maximum practical continuous output current for both mostly discontinuous conduction mode (MDCM) and continuous conduction mode (CCM) of operation that can be delivered from a given LinkSwitch-TN device under the following assumed conditions: 1) Buck converter topology. 2) The minimum DC input voltage is ≥70 V. The value of input capacitance should be large enough to meet this criterion. 3) For CCM operation a KRP* of 0.4. 4) Output voltage of 12 VDC. 5) Efficiency of 75%. 6) A catch/freewheeling diode with trr ≤75 ns is used for MDCM operation and for CCM operation, a diode with trr ≤35 ns is used. 7) The part is board mounted with SOURCE pins soldered to a sufficient area of copper to keep the SOURCE pin temperature at or below 100 °C. *KRP is the ratio of ripple to peak inductor current. LinkSwitch-TN Selection and Selection Between MDCM and CCM Operation Select the LinkSwitch-TN device, freewheeling diode and output inductor that gives the lowest overall cost. In general, MDCM 25 -5 Rev. H 12/06 LNK302/304-306 TOPOLOGY High-Side Buck – Direct Feedback + VIN BASIC CIRCUIT SCHEMATIC KEY FEATURES 1. Output referenced to input 2. Positive output (VO) with respect to -VIN 3. Step down – VO < VIN 4. Low cost direct feedback (±10% typ.) + VO FB D BP S LinkSwitch-TN PI-3751-121003 High-Side Buck – Optocoupler Feedback FB BP S + VIN D + VO LinkSwitch-TN PI-3752-121003 1. Output referenced to input 2. Positive output (VO) with respect to -VIN 3. Step down – VO < VIN 4. Optocoupler feedback - Accuracy only limited by reference choice - Low cost non-safety rated opto - No pre-load required 5. Minimum no-load consumption Low-Side Buck – Optocoupler Feedback + LinkSwitch-TN VIN + VO BP S FB D PI-3753-111903 Low-Side Buck – Constant Current LED Driver + LinkSwitch-TN VIN IO VF + 1. Output referenced to input 2. Negative output (VO) with respect to +VIN 3. Step down – VO < VIN 4. Optocoupler feedback - Accuracy only limited by reference choice - Low cost non-safety rated opto - No pre-load required - Ideal for driving LEDs BP S FB D R= VF PI-3754-112103 IO High-Side Buck Boost – Direct Feedback + VIN FB D BP S LinkSwitch-TN VO PI-3755-121003 High-Side Buck Boost – Constant Current LED Driver 300 Ω 2 kΩ FB BP S RSENSE = 2V IO IO RSENSE + VIN D LinkSwitch-TN 10 μF 50 V 100 nF 1. Output referenced to input 2. Negative output (VO) with respect to +VIN 3. Step up/down – VO > VIN or VO < VIN 4. Low cost direct feedback (±10% typ.) 5. Fail-safe – output is not subjected to input voltage if the internal MOSFET fails 6. Ideal for driving LEDs – better accuracy and temperature stability than Low-side Buck constant current LED driver PI-3779-120803 Table 2. Common Circuit Configurations Using LinkSwitch-TN. (continued on next page) 26 -6 Rev. H 12/06 + LNK302/304-306 TOPOLOGY Low-Side Buck Boost – Optocoupler Feedback + LinkSwitch-TN VIN VO BASIC CIRCUIT SCHEMATIC KEY FEATURES 1. Output referenced to input 2. Positive output (VO) with respect to +VIN 3. Step up/down – VO > VIN or VO < VIN 4. Optocoupler feedback - Accuracy only limited by reference choice - Low cost non-safety rated opto - No pre-load required 5. Fail-safe – output is not subjected to input voltage if the internal MOSFET fails BP S FB D + PI-3756-111903 Table 2 (cont). Common Circuit Configurations Using LinkSwitch-TN. provides the lowest cost and highest efficiency converter. CCM designs require a larger inductor and ultra-fast (trr ≤35 ns) freewheeling diode in all cases. It is lower cost to use a larger LinkSwitch-TN in MDCM than a smaller LinkSwitch-TN in CCM because of the additional external component costs of a CCM design. However, if the highest output current is required, CCM should be employed following the guidelines below. Topology Options LinkSwitch-TN can be used in all common topologies, with or without an optocoupler and reference to improve output voltage tolerance and regulation. Table 2 provide a summary of these configurations. For more information see the Application Note – LinkSwitch-TN Design Guide. Component Selection Referring to Figure 5, the following considerations may be helpful in selecting components for a LinkSwitch-TN design. Freewheeling Diode D1 Diode D1 should be an ultra-fast type. For MDCM, reverse recovery time trr ≤75 ns should be used at a temperature of 70 °C or below. Slower diodes are not acceptable, as continuous mode operation will always occur during startup, causing high leading edge current spikes, terminating the switching cycle prematurely, and preventing the output from reaching regulation. If the ambient temperature is above 70 °C then a diode with trr ≤35 ns should be used. For CCM an ultra-fast diode with reverse recovery time trr ≤35 ns should be used. A slower diode may cause excessive leading edge current spikes, terminating the switching cycle prematurely and preventing full power delivery. Fast and slow diodes should never be used as the large reverse recovery currents can cause excessive power dissipation in the diode and/or exceed the maximum drain current specification of LinkSwitch-TN. Feedback Diode D2 Diode D2 can be a low-cost slow diode such as the 1N400X series, however it should be specified as a glass passivated type to guarantee a specified reverse recovery time. To a first order, the forward drops of D1 and D2 should match. Inductor L1 Choose any standard off-the-shelf inductor that meets the design requirements. A “drum” or “dog bone” “I” core inductor is recommended with a single ferrite element due to its low cost and very low audible noise properties. The typical inductance value and RMS current rating can be obtained from the LinkSwitch-TN design spreadsheet available within the PI Expert design suite from Power Integrations. Choose L1 greater than or equal to the typical calculated inductance with RMS current rating greater than or equal to calculated RMS inductor current. Capacitor C2 The primary function of capacitor C2 is to smooth the inductor current. The actual output ripple voltage is a function of this capacitor’s ESR. To a first order, the ESR of this capacitor should not exceed the rated ripple voltage divided by the typical current limit of the chosen LinkSwitch-TN. Feedback Resistors R1 and R3 The values of the resistors in the resistor divider formed by R1 and R3 are selected to maintain 1.65 V at the FB pin. It is recommended that R3 be chosen as a standard 1% resistor of 2 kΩ. This ensures good noise immunity by biasing the feedback network with a current of approximately 0.8 mA. Feedback Capacitor C3 Capacitor C3 can be a low cost general purpose capacitor. It provides a “sample and hold” function, charging to the output voltage during the off time of LinkSwitch-TN. Its value should be 10 μF to 22 μF; smaller values cause poorer regulation at light load conditions. 27 -7 Rev. H 12/06 LNK302/304-306 Pre-load Resistor R4 In high-side, direct feedback designs where the minimum load is