ÓÉ Foxit PDF Editor ±à¼°æȨËùÓÐ (c) by Foxit Software Company, 2004 ½öÓÃÓÚÆÀ¹À¡£ PR6234 High Precision CC/CV Primary-Side PWM Power Controller Features 汪工 ＴＥＬ：１３８２８７１９４１０ ＱＱ：１９２９７９４２３８ Up to 5% Precision for Constant Voltage Regulation and Constant Current Regulation at Universal AC input Primary-side Sensing and Regulation Without TL431 and Opto-coupler Flyback Topology In DCM Operation Soft-start Built-in Leading Edge Blanking (LEB) Frequency Jitter to Reduce System EMI Programmable CV and CC Regulation Adjustable Constant Current and Output Power Setting Built-in Secondary Constant Current Control with Primary Side Feedback Built-in Secondary Constant Voltage Sampling Controller Program Cable Drop Compensation Rich Protections For System Reliability including VDD Under Voltage Lockout with Hysteresis (UVLO) Cycle-by-Cycle Current Limiting and Peak Current Protection (OCP) Over Temperature Protection (OTP) VDD Over Voltage Protection (OVP) VDD Clamp Lead-free SOT23-6 package Applications Cell Phone /Digital Cameras Charger Small Power Adaptor Auxiliary Power for PC, TV etc. Linear Regulator/RCC Replacement General Description PR6234 is a high performance offline PWM power controller for low power AC/DC charger and adaptor applications. It operates in primary-side sensing and provides constant voltage (CV) and constant current (CC) regulation without TL431 and opto-coupler. A typical output CC/CV curve is shown as in the Fig.1. V1.1 Fig.1. Typical CC/CV Curve In CC mode, the current point and maximum output power setting can be adjusted by the sense resistor RS at CS pin. In CV mode, PR6234 captures the auxiliary flyback signal at Pin INV and then regulates the output voltage. IN this mode, multi-mode operations are utilized to achieve high performance and high efficiency. In addition, built-in cable drop compensation further enhances output accuracy. PR6234 offers power on soft-start control and ensures safe operation with complete protections against all the fault conditions. Built-in protection circuitry includes Cycle-by-Cycle current limiting, peak current protection, OTP，VDD OVP, VDD clamp and 1/10 PR6234 UVLO. High Precision CC/CV Primary-Side PWM Power Controller Excellent EMI performance is achieved with frequency jitter and soft-drive. Pin Assignment （SOT23-6 ） Pin Description Pin Num 1 2 3 Pin Name GND BASE CS I/O P O I 4 INV I 5 6 COMP VDD I P Description Ground Base driver with current limit for power BJT Current sense input The voltage feedback from auxiliary winding. Connected to resistor divider from auxiliary winding reflecting output voltage. PWM duty cycle is determined by EA output COMP and current sense signal CS. Loop Compensation for CV Stability Power Supply Typical Application V1.1 2/10 PR6234 High Precision CC/CV Primary-Side PWM Power Controller Block Diagram Simplified Internal Circuit Architecture Absolute Maximum Ratings Parameter Value VDD Voltage -0.3 to VDD_clamp VDD Zener Clamp Continuous Current 10 mA COMP Voltage -0.3 to 7V BASE Voltage -0.3 to 7V CS Input Voltage -0.3 to 7V INV Input Voltage -0.3 to 7V Min/Max Operating Junction Temperature TJ -20 ℃to 150℃ Min/Max Storage Temperature Tstg -55 ℃to 150℃ Lead Temperature (Soldering, 10secs) 260℃ Note: Stress beyond those listed under “absolute maximum ratings” may cause permanent damage to the device. Exposure to absolute maximum-rated conditions for extended periods may affect device reliability. V1.1 3/10 PR6234 High Precision CC/CV Primary-Side PWM Power Controller Electrical Characteristics (Ta=25°C unless otherwise noted, VDD = 16V) Symbol Parameter Test Conditions Min Typ Max Unit 5 20 uA 2.5 3.5 mA Supply Voltage (VDD) Section IDD_ST Standby current VDD=13V IDD_OP Operation Current Operation supply current INV=2V,CS=0V, VDD=20V UVLO(ON) VDD Under Voltage Lockout Enter VDD falling 7.5 8.5 10 V UVLO(OFF) VDD Under Voltage Lockout Exit VDD rising 13.5 14.5 16.0 V Over voltage voltage Ramp up VDD until gate clock is off 27.5 29.5 31.5 V IDD=10mA 30.5 32.5 34.5 V 870 900 930 mV OVP VDD_clamp protection Maximum VDD opertation voltage Current Sense Input Section TLEB LEB time 540 ns Vth_oc Over current threshold Td_oc OCP Propagation delay 150 ns Input Impedance 50 Kohm Soft start time 10 ms ZSENSE_IN T_ss CV Section Freq_Nom System Nominal frequency switch Freq_startup △f/Freq 60 INV=0V,Comp=5V Frequency jitter range 14 KHZ ±4 % Error Amplifier section Vref_EA Gdc I_COMP_MAX Reference voltage for EA 1.97 DC gain of the EA Max. Cable compensation current INV=2V,COMP=0V 2 2.03 V 60 dB 42 uA 65 mA 25 mA 1.5 ohm Base DriveSection Is_preon Is_max Rdson_I V1.1 Base sourceing current pre-on Base sourceing maximum current Base drive low side on resistor 4/10 PR6234 High Precision CC/CV Primary-Side PWM Power Controller Operation Description PR6234 is a cost effective PWM power controller for off-line low power AC/DC applications including battery chargers and adaptors. It’s designed for the flyback topology working in a discontinuous conduction mode (DCM). It operates in primary-side sensing and provides constant voltage (CV) and constant current (CC) regulation without TL431 and opto-coupler. Built-in secondary constant voltage sampling controller can achieve high precision CC/CV control. Startup Operation VDD is the power supply terminal for the PR6234. The startup resistor from the rectified high voltage DC rail supplies current to the VDD bypass capacitor. During startup, the PR6234 typically draws only lower than 20uA, so that VDD could be quickly charged up above UVLO threshold. A large value startup resistor can be used to minimize the power loss in standby mode. As soon as VDD is beyond the UVLO(OFF), the chip will begin to soft-start. It will ramp peak current voltage threshold gradually from nearly zero to 0.90V. This control mode is use to minimize the component electrical over-stress during power on startup. After switching start, the output voltage begins to rise. The VDD bypass capacitor must supply the PR6234 internal circuitry until the output voltage is high enough to sustain VDD through the auxiliary winding. Principle of CC Operation PR6234’s CC/CV control characteristic is shown as the Fig. 1. PR6234 is designed to operation in DCM mode for flyback system. Under normal operation, when INV is less than 2.0V, the system works in CC mode, otherwise the system works in CV mode. When the secondary output current reaches V1.1 a level set by the internal current limiting circuit, the PR6234 enters current limit condition and causes the secondary output voltage to drop. As the output voltage decreases, so does the flyback voltage in a proportional manner. An internal current shaping circuitry adjusts the switching frequency based on the flyback voltage so that the transferred power remains proportional to the output voltage, resulting in a constant secondary side output current profile. This is the CC principle. In charger applications, a discharged battery charging starts in the CC portion of the curve until it is nearly full charged and smoothly switches to operate in CV portion of the curve. In PR6234, the CC portion provides output current limiting. PR6234 regulates the output current constant regardless of the output voltage drop in CC operation mode. The CC point and maximum output power can be externally adjusted by external current sense resistor RS at CS pin as illustrated in Typical Application Diagram. The output power is adjusted through CC point change. The larger RS, the smaller CC point is, and the smaller output power becomes, and vice versa as shown in Fig.2. Fig.2. Adjustable output power by changing RS 5/10 PR6234 High Precision CC/CV Primary-Side PWM Power Controller Principle of CV Operation In constant voltage operation, the PR6234 captures the auxiliary flyback signal at INV pin through a resistor divider. The signal at INV pin is pre-amplified against the internal reference voltage. This error signal is then amplified by the internal error amplifier. When the secondary output voltage is above regulation, the error amplifier output COMP decreases to reduce the switch current. Otherwise, the error amplifier output voltage increases to ramp up the switch current to bring the secondary output back to regulation. In an AC/DC adapter, the normal operation occurs only on the CV portion of the curve. In CV operation, the output voltage is regulated through the primary side control. In the DCM flyback converter, the output voltage can be sensed via the auxiliary winding. During BJT turn-on time, the load current is supplied from the output filter capacitor CO, the current in the primary winding ramps up and the energy is stored in the magnetic core of the transformer. When BJT turns off, the energy stored in the magnetic core of the transformer is transferred to output. The auxiliary voltage reflects the output voltage as shown in fig.3 and it is given by VAUX = N AUX ⋅ (VO + ∆V ) NS (1) Where ΔV indicates the drop voltage of the output Diode. Via a resistor divider connected between the auxiliary winding and INV (pin 3), the auxiliary voltage is sampled at the end of the demagnetization and it is hold until the next sampling by the internal algorithm control circuit. The sampled voltage is compared with Vref (2.0V) and the error is amplified. The error amplifier output COMP controls the PWM duty cycle to regulate the output voltage, thus constant output voltage can be achieved. Program Cable drop Compensation The voltage drop due to cable loss will increase as the load current increases. It causes the output voltage to fall off. An internal cable drop compensation circuit is designed to compensate the drop due to the cable loss. When adding to the cable drop, the auxiliary voltage reflecting the output voltage will be corrected comparing with the equation (1). It’s given by VAUX = N AUX ⋅ (VO + ∆V + Vcable ) NS Where VCable indicates the drop of the cable. To compensate the cable drop, an offset voltage is generated at INV by an internal current I C flowing into the resister divider. The control circuit is shown as Fig.4. Fig.4. Fig.3. V1.1 (2) cable drop compensation circuit Auxiliary voltage waveform 6/10 PR6234 High Precision CC/CV Primary-Side PWM Power Controller The current I C is inversely proportional to the output COMP of the error amplifier. As a result, it is inversely proportional to the output load current. Thus the drop due to the cable loss can be compensated. The integrate equation is shown as below. R1 N Vref (1 + ) − I C R1 = AUX ⋅ (VO + ∆V + Vcable ) (3) R2 NS As the load current decreases from full-load to no-load, the offset voltage at INV will increase. It can also be programmed by adjusting the resistance of the divider to compensate the drop for various cable lines used. This feature allows for better output voltage accuracy by compensating for the output voltage droop due to the output cable resistance. In PR6234, cable drop compensation is implemented to achieve good load regulation in the CV mode. Switching frequency In PR6234, the switching frequency is adaptively controlled by the load conditions and the operation modes. The maximum operation switching frequency is set to 60 KHz internally. For flyback operating in DCM, The maximum output power is given by PoMAX = 1 LP FSW I P2 2 (3) Where LP indicates the inductance of primary winding and IP is the peak current of primary winding. The primary-side sensing topology must work in DCM. Refer to the equation 3, to prevent from working in continuous conduction mode (CCM), the switching frequency is locked by an internal loop such that the switching frequency is FSW = 1 2TDemag (4) Since TDemag is inversely proportional to the inductance, as a result, the product LP and V1.1 Fsw is constant, thus the maximum output power is limited. Current Sensing and Leading Edge Blanking PR6234 detects primary BJT current from the CS pin, which is not only for the peak current mode control but also for the cycle-by-cycle current limit. The maximum voltage threshold of the current sensing pin is set as 0.9V. Thus the BJT peak current can be calculated as: I peak (max) = 0.9V RS (4) A 540 ns leading-edge blanking (LEB) time is included in the input of CS pin to prevent the false-trigger caused by the current spike. So that the external R-C filter can be eliminated. The current sense input voltage and the EA output COMP determine the switch duty cycle, and then regulation the output voltage. EMI improvement To improve EMI of the PR6234 system, two methods are designed in the chip. One is the frequency jitter. This control is achieved by changing the operation frequency. The oscillation frequency is modulated so that the tone energy is spread out. The spread spectrum minimizes the conduction band EMI. The other one is soft drive. The internal power BJT in PR6234 is driven by a dedicated gate driver for power switch control. Too weak gate drive strength will result in higher conduction and switch loss of BJT, while too strong drive will produce EMI problem. A good tradeoff is achieved through the built-in totem pole driver design with right output strength control. The soft drive is designed to open the power BJT gradually. In this way, the EMI will be improved much better. 7/10 PR6234 High Precision CC/CV Primary-Side PWM Power Controller Protection Control PR6234 ensures safe operation with complete protection against all the fault conditions. When these protections are triggered, the BJT will turn off. PR6234 has several protections, such as Cycle-by-Cycle current limiting, peak current protection, over temperature protection (OTP), over voltage protection (OVP), VDD clamp, V1.1 power on soft start, and under voltage lockout on VDD (UVLO). VDD is supplied by transformer auxiliary winding output. The output of PR6234 is shut down when VDD drops below UVLO (ON) limit and then switcher enters power on start-up sequence thereafter. Every restart is a soft-start. 8/10 PR6234 High Precision CC/CV Primary-Side PWM Power Controller Characterization Plots The characteristic graphs are normalized at TA=25℃. UVLO(off)(V) VS TEMP(C) 9.5 15.5 9 15 UVLO(off) (V) U VL O ( o n ) ( V ) UVLO(on)(V) VS TEMP(C) 8.5 8 7.5 -40 -10 20 50 Temperature( C) 80 110 14.5 14 13.5 -40 125 -10 20 75 2.5 70 2 1.5 20 50 Temperature(C) 125 80 110 60 125 -10 20 50 Tempterature(C) 80 110 125 I_cable_compensation (uA) vs Vcomp(V) Rdson(ohm) vs Temperature( C) 45 I_cable_compensation (uA) 35 30 25 Rdson(ohm) 110 65 55 -40 1 -10 80 Freq_Max (KHz) VS TEMP(C) Freq_Max (KHz) Istartup (uA) Istartup (uA) VS TEMP(C) 3 -40 50 Temperature(C) 20 15 10 5 40 35 30 25 20 15 10 5 0 0 25 50 75 100 Tempterature( C) V1.1 125 150 0 1 2 Vcomp (V) 3 4 9/10 PR6234 High Precision CC/CV Primary-Side PWM Power Switch Package Dimensions SOT-23-6L Symbol Millimeters Min. Max. Min. Max. A 1.000 1.300 0.039 0.051 A1 0.000 0.150 0.000 0.006 A2 1.000 1.200 0.039 0.047 b 0.300 0.500 0.012 0.020 c 0.100 0.200 0.004 0.008 D 2.800 3.020 0.110 0.119 E 1.500 1.700 0.059 0.067 E1 2.600 3.000 0.102 0.118 e V1.1 Inches 0.095(BSC) 0.037(BSC) e1 1.800 2.000 0.071 0.079 L 0.300 0.600 0.012 0.024 θ˚ 0˚ 8˚ 0˚ 8˚ 10/10