High Accurate non-isolated Buck LED Driver

Transcription

1 High Accurate non-isolated Buck LED Driver Features High efficiency (More than 90%) High precision output current regulation (-3%~+3%) when universal AC input voltage (85VAC~265VAC) Lowest cost and very less component count solution Non-Isolated Buck application Need not transformer to simplify design Operation in CCM Support no electrolytic capacitor Programmable output current setting 50kHz operation frequency allows low cost 1mH inductor for up to 30V & 300mA output current Tight tolerances and negligible temperature variation Frequency jittering dramatically reduces EMI, minimizes EMI filter cost Under voltage lockout (9.0V~15V) Low startup current (<10uA) Building in current compensation Building in 400nS LEB time for CS pin Audio noise free operation SOT-23-6L Pb free package Full Protection Features VDD 24V over-voltage protection Gate 18V clamped voltage Building in hysteresis OTP (150 ) Cycle-by-cycle current limiting Transformer anti-saturation FB pin short to GND FB pin open CS pin open LED open protection LED+ and LED- short protection Applications Non-isolated LED driver, Replace for LNK302~LNK306 Description CRM8020 is especially designed for non-isolated LED driver. The building in perfect current compensation function ensures the accurate output current. CRM8020 has very less external components because of high integrated and simple topology. Thanks to novel power supply, CRM8020 need not use transformer and it could simplify PCB design. CRM8020 integrates frequency jittering, cycle-by-cycle current limit, inductor anti-saturation and thermal shutdown circuitry onto a monolithic IC. The integrated auto-restart circuit and FB detection in the CRM8020 safely limits output power during fault conditions such as LED short to circuit or open loop, reducing component count and system-level load protection cost. V1.1 1

2 Application circuit CRM8020 Note: Diode D1 must be ultra-fast diode. Fig 1 CRM8020 typical application Package(SOT-23-6L) DRV 1 6 VDD CS 2 5 NC GND 3 4 FB Fig 2 CRM8020 package Pin Assignment Name I/O 1 Pin No. Description VDD Power 6 Power Supply. NC 5 No connect FB Input 4 Voltage detect and protection GND Power 3 Reference GND CS Input 2 Current sense pin, a resistor sense the MOSFET current. DRV Output 1 Drive external power MOSFET V1.1 2

3 Absolute Maximum Ratings CS, FB referenced to GND VDD, DRV referenced to GND -0.3V~6V -0.3V~30V Storage Temperature Range -55 to 150 Maximum junction temperature 125 ESD ESD Capability, HBM Model 3000V ESD Capability, Machine Model 300V T L Lead Temperature 20second 220 (Soldering) SOT-23-6L T STG Storage Temperature Range -55 to Block Diagram Fig 3 CRM8020 s block diagram V1.1 3

4 ELECTRICAL CHARACTERISTICS Unless otherwise specified: T A =25 and VDD=15V Symbol Parameter Test Conditions Supply Voltage Limits Min. Typ. Max. VDD ON VDD Turn on threshold V VDD OFF VDD Turn off threshold V VDD OVP VDD over voltage protection Unit V I ST Startup current VDD=12.0V 10 µa I OP Oscillator Operating current of VDD C DRV =1.0nF 1.0 ma F OSC Operation Frequency 50 KHz F OSC-JIT Jitter Frequency 4 KHz T OSC-JIT Jitter period 4 ms D MAX Max on duty cycle 85 % FB V REF Inner reference voltage 0.4 V VFB SCP Output SCP FB voltage 0.8 V VFB OVP Output OVP FB voltage 3.2 V T FB-SCP Out SCP delay time 18 ms CS T CS-LEB CS pin Leading edge blanking time 400 ns V PEAK Maximum peak voltage 0.5 V V PEAK2 Inductor anti-saturation 1.0 V DRV Pin T R Rise time of driver C DRV =1.0nF 200 ns T F Fall time of driver C DRV =1.0nF 200 ns V DRV-MAX DRV clamped voltage 18 V OTP T OTPP OTP protection temp. 150 T OTPR OTP Release temp. 125 o C o C V1.1 4

5 CRM8020 Functional Description The CRM8020 controller consists of an oscillator, UVLO, VDD-OVP, FB clamped (FBC), output OVP, output SCP, over-temperature protection (OTP), current limit circuit, CS/FB leading edge blanking (LEB), driver and Output current Regulation circuits. output. The VDD is working as a hiccup mode, the Figure 5 shows its operation, if the OVP condition is removed, the VCC level will get back to normal level and the output will automatically return to normal operation. UVLO An UVLO comparator is implemented in CRM8020 to detect the voltage on VDD pin. It would assure the supply voltage enough to turn on the PWM controller and further to driver the power MOSFET. As shown fig 4, a hysteresis is built in to prevent the shutdown from the voltage dip during startup. The turn on and turn off threshold level are set at 15.0V and 8.5V, respectively. Fig 4 VDD-OVP To prevent IC damage under high operation voltage, CRM8020 are implemented an OVP function on VDD, whenever the VDD voltage is larger than the OVP threshold voltage, typical is 24V, the output gate drive circuit will be shutdown simultaneously thus to stop the switching of the power MOSFET until the next UVLO. The VDD OVP function in CRM8020is an auto-recovery type protection. If the OVP condition, usually caused by the LED opened, is not released, the VDD will tripped the OVP level again and re-shutdown the Fig 5 Oscillator The typical oscillator frequency is internally set to an average of 50 khz. Two signals are generated from the oscillator: the maximum duty cycle signal (DC MAX ) and the clock signal that indicates the beginning of each cycle. The CRM8020 s 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 of power MOSFET. The waveform in Figure 6 illustrates the frequency jitter of the CRM8020. V1.1 5

6 reference GND of IC and FB pin CRM8020 Output Short Protection When the output short circuit happens, the positive plateau of FB pin voltage is also near zero. The IC will shut down and restart again once FB voltage falls below 0.8V and lasts for about 18mS. Real Current control Fig 6 The perfect detection and compensation control method allows the CRM8020 to accurately control the output LED current, the output LED mean current can be calculated as: I LED 0.2V = R R CS --- The sensing resistor connected between the MOSFET source and GND Output over Voltage Protection (OVP) Output over voltage protection can prevent the components from damage in the over voltage condition. The positive plateau of FB pin voltage is proportional to the output voltage. Once the FB pin voltage is higher than 3.2V, the OVP signal will be triggered and latched after three cycles, the gate driver will be turned off and the IC works at quiescent mode, the VDD voltage dropped below the VDD OFF which will make the IC shut down and the system restarts again. The output OVP setting point can be calculated as: V OUT _ OVP 3. 2 R FBH + R R FBL CS FBL V OUT_OVP Output voltage protection value R FBH The resistor connected between inductor and FB pin R FBL The resistor connected between Over-Temperature Protection The thermal shutdown circuitry senses the die temperature. The threshold is set at 150 C typical with a 25 C hysteresis. When the die temperature rises above this threshold (150 C), the CRM8020 turn off the power MOSFET by DRV and remains turning off until the die temperature falls by 25 C, at which point it is re-enabled. Current Limit The current limit circuit senses the current of inductor by CS pin. When this current exceeds the internal threshold, typical is 0.5V, the 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 (LEB) 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. Leading Edge Blanking In order to avoid the premature termination of the switching pulse due to the parasitic capacitance discharging at MOSFET turning on, an internal leading edge blanking (LEB) unit is employed between the CS pin and the current comparator input. During the blanking time, the path, CS pin to the current comparator input, is blocked, below fig shows the V1.1 6

7 leading edge blanking. CRM8020 Auto-Restart In the event of a fault condition such as output overload, output short, or an open loop condition, CRM8020 enters into auto-restart operation. Fig 7 Fault protection There are several critical protections were integrated in the CRM8020 to prevent the LED from being damaged. Those damages V1.1 7

8 Applications Example Fig 8, CRM8020 application circuit V1.1 8

9 A 10watt universal input buck converter s circuit was shown in Fig.8; it is a typical implementation of a non-isolated power supply with 33V&300mA used in appliance control. The input stage comprises fusible resistor RF1, bridge rectifier DB105, 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 DB105; 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). The power processing stage is formed by the CRM8020, freewheeling diode D1, output choke L1, and the output capacitor C2. Diode D1 is an ultra-fast diode with a reverse recovery time (trr) of approximately 75ns, acceptable for DCM operation. For continuous conduction mode (CCM) designs, a diode with trr 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. The values of R1 and R2 are selected such that, at the desired output voltage, the voltage at the FB pin should be between 1V and 3V. Regulation is maintained by CS pin and FB pin. If FB voltage drops below 0.8V continuously and lasts for 18ms period, CRM8020 will enter auto-restart. Component Selection Referring to Figure8, the following considerations may be helpful in selecting components for an CRM8020 design. Freewheeling Diode D1 should be an ultra-fast type. For DCM, reverse recovery time trr 75ns should be used at a temperature of 70 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 and then a diode with trr 35ns 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 CRM8020. Feedback Diode D2 Diode D2 offer power supply for IC when power MOSFET turn off, it can be a low-cost slow diode such as the 1N400X series; however it should be specified type to guarantee a specified reverse recovery time. 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. Choose L1 greater than or equal to the typical calculated inductance with RMS current rating greater than or equal to calculated RMS V1.1 9

10 inductor current. CRM8020 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. Feedback Resistors R1 and R2 The values of the resistors in the resistor divider formed by R1 and R2 are selected to meet output OVP and SCP at the FB pin. It is recommended that R1 and R3 be chosen as a standard 1% resistor. The maximal source current from FB is about 4mA. V1.1 10

11 Package Information SOT-23-6L 符号 单位 ( 微米 ) 最小最大 单位 ( 英寸 ) 最小最大 A A B b C D e H L V1.1 11