UCC2839 UCC3839 Secondary Side Average Current Mode Controller FEATURES DESCRIPTION • Practical Secondary Side Control of Isolated Power Supplies The UCC3839 provides the control functions for secondary side average current mode control in isolated power supplies. Start up, pulse width modulation and MOSFET drive must be accomplished independently on the primary side. Communication from secondary to primary side is anticipated through an opto-isolator. • Provides a Self Regulating Bias Supply From a High Input Voltage Using an External N-Channel Depletion Mode FET • Onboard Precision, Fixed Gain, Differential Current Sense Amplifier • Wide Bandwidth Current Error Amplifier • 5V Reference • High Current, Programmable Gm Amplifier Optimized to Drive Opto-couplers Accordingly, the UCC3839 contains a fixed gain current sense amplifier, voltage and current error amplifiers, and a Gm type buffer/driver amplifier for the opto-isolator. Additional housekeeping functions include a precision 5V reference and a bias supply regulator. Power for the UCC3839 can be generated by peak rectifying the voltage of the secondary winding of the isolation transformer. From this unregulated voltage, the UCC3839’s bias supply regulator will generate its own 7.5V bias supply using an external, N-channel, depletion mode FET. The UCC3839 can be configured for traditional average current mode control where the output of the voltage error amplifier commands the current error amplifier. It can also be configured for output voltage regulation with average current mode short circuit current limiting, employing two parallel control loops regulating the output voltage and output current independently. BLOCK DIAGRAM UDG-97011 SLUS179A - APRIL 1999 - REVISED NOVEMBER 2001 UCC2839 UCC3839 CONNECTION DIAGRAMS ABSOLUTE MAXIMUM RATINGS Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15V Supply Current (LED not connected) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2mA (LED connected) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14mA Analog Inputs . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to 15V Power Dissipation at TA = 60°C (LED not connected). . . . . . . . . . . . . . . . . . . . . . . . . . 20mW (LED connected). . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55mW Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C Junction Temperature . . . . . . . . . . . . . . . . . . . –55°C to +150°C Lead Temperature (Soldering, 10sec.) . . . . . . . . . . . . . +300°C DIL-14, SOIC-14 (Top View) D or N Package Currents are positive into, negative out of the specified terminal. Consult Packaging Section of Databook for thermal limitations and considerations of package. ELECTRICAL CHARACTERISTICS: Unless otherwise specified, 0°C to 70°C for the UCC3839, –40°C to 85° for the UCC2839. VLINE = 10V, RG = 400Ω. TA = TJ. PARAMETER TEST CONDITIONS MIN TYP MAX UNITS Current Error Amplifier VIO 10 AVOL CMRR VCM = 0.5V to 5.5V mV 60 dB 60 dB PSRR VLINE = 10V to 20V 60 CAO High CA– = 1V, CA+ = 1.1V, ICAO = –100 A 4.8 7 dB V ICAO CA– = 1V, CA+ = 1.1V, CAO = 0.5V –500 –250 µA CAO Low CA– = 1V, CA+ = 0.9V, ICAO = 500µA GBW F = 100kHz, TA = 25°C 0.2 3 5 1.46 1.5 0.4 V MHz Voltage Error Amplifier VA– AVOL PSRR VLINE = 10V to 20V VAO High IVAO = –100µA to 100µA IVAO VA– = 1.45V, VAO = 0.5V VAO Low VA– = 1.55V, VAO = 0.5V, IVAO = 500µA GBW (Note 1) 1.525 V 60 dB 60 dB 4.8 5 –500 0.2 3 5 5.2 V –250 µA 0.4 V MHz Current Sense Amplifier CSO Zero CS+ = CS– = –0.3V to 5.5V, ICSO = –100µA to 100µA 0.85 1 1.15 V AV CS+ = 0, CS– = 0mV to –200mV 7.75 8 8.2 V/V 2 4 4.75 5 5.2 V 0 10 µA Current Sense Amplifier (cont.) Slew Rate CS+ = 0, CS– = 0mV to –0.5V CSO CS+ = –200mV, CS– = –700mV V/µs LED Driver ILED LED = 5.5V, CA– = 1V, CA+ = 1.1V, RG = 400 LED = 5.5V, CA– = 1V, CA+ = 0.9V, RG = 400 Gm LED = 5.5V, CAO = 1V to 3V, RG = 400 Slew Rate CAO = 2V to 2.5V, LED = 400Ω to 5.5V, RG = 400 2 9 10 11 mA 2.25 2.5 2.75 mS 1 4 V/µs UCC2839 UCC3839 ELECTRICAL CHARACTERISTICS: Unless otherwise specified, 0°C to 70°C for the UCC3839, –40°C to 85° for the UCC2839. VLINE = 10V, RG = 400Ω. TA = TJ. PARAMETER TEST CONDITIONS MIN TYP MAX UNITS 4.94 5 5.06 5.1 V 0.298 0.3 0.302 V/V 7 7.5 8 V 1.3 2 mA Precision Reference REF TJ = 25°C IREF = 0mA to 1mA, VLINE = 10V to 20V VA+/REF 4.9 V VDD Regulator VDD IDD = 0mA to –15mA, VLINE = 10V to 40V IVDD VLINE = 10V to 40V, CA– = 0V, CA+ = 1V, VA– = 2.9V, CS+ = CS– = 0, IREF = 0 Note 1: Ensured by design. Not 100% tested in production. PIN DESCRIPTIONS CA–: Current Error Amplifier Negative Input. REF: 5V Precision Reference Buffer Output. Minimum Decoupling Capacitance = 0.01µF CAO: Current Error Amplifier Output. Output source current is limited, and output sink current is guaranteed to be greater than the VAO output source current. Current loop compensation components are generally connected to CAO and CA–. VA–: Voltage Error Amplifier Negative Input. Voltage Error Amplifier is internally referenced to 1.5V VAO: Voltage Error Amplifier Output. In a two loop average current mode control configuration, VAO is connected to CA+ and is the current command signal. VAO is internally clamped not to exceed 5V for short circuit control. In a single loop voltage mode control configuration with a parallel average short circuit current control loop, VAO is connected directly to CAO. Output source current is limited, and output sink current is guaranteed to be greater than the CAO output source current. CA+: Current Error Amplifier Positive Input. CS–: Current Sense Amplifier Negative Input. CSO: Current Sense Amplifier Output. Internally set gain VOUT/VIN = 8 VIN = 0V results in CSO = 1V. CS+: Current Sense Amplifier Positive Input. GM: Gm (transconductance) Programming Pin. Resistor RGM = 400Ω to GND. VDD: 7.5V Regulator output. Supply for most of the chip. Minimum Decoupling Capacitance = 0.01µF GND: Chip Ground. VGATE: External FET Gate Control Voltage. LED: Output of LED Driver. Connect LED from VDD pin to LED. APPLICATION INFORMATION Fig. 1 shows a typical secondary side average current mode controller configuration using the UCC3839. In this configuration, output voltage is sensed and regulated by the voltage error amplifier. Its output, VAO provides the reference for the current error amplifier at the CA+ pin. VAO can be connected to CA+ directly or through a resistive divider depending on the particular application requirements. precision current sense amplifier of the chip. The onboard current sense amplifier has a gain of 8 and is intended for differential sensing of the shunt voltage with a common mode voltage range from 0V up to 5V. The output of the current sense amplifier, CSO is 1V for zero input which guarantees that the circuit can control currents down to 0A. The CSO signal is fed to the CA– input of the current error amplifier through a resistor. The current error amplifier takes the VAO and CSO signals and generates the error signal for the pulse width modulator. Average current mode control needs accurate output current information which is provided by a low value current sense resistor. The voltage proportional to the converter’s output current is sensed and amplified by the 3 UCC2839 UCC3839 APPLICATION INFORMATION (cont.) Since the PWM function is located on the primary side of the power converter the CAO signal must be sent across the safety isolation boundary. The UCC3839 anticipates an opto-coupler to provide isolation between primary and secondary. Therefore, CAO drives a transconductance amplifier that controls LED current in an opto-isolator. During start up and when CAO exceeds 4V, the current in the LED drops to zero. Maximum LED current is obtained during normal operation as CAO reaches its lowest potential. Its value is determined by the programming resistor value from the GM pin to circuit GND. controls the current in the opto-coupler providing the feedback signal for the PWM section on the primary side. Voltage regulation is still maintained by the voltage error amplifier until a user programmable output current is reached. At this time CAO will take control over the Gm amplifier and the output current of the converter will be regulated while the output voltage falls below its nominal value. This current level is set at the CA+ input by a resistive divider from the 5V reference of the chip. Since the chip is powered from a peak rectifier which maintains the bias supply for the UCC3839 even under short circuit conditions, both of these techniques can be used to eliminate the short circuit runaway problem in isolated power supplies using peak current mode control on the primary side. An alternative secondary side controller configuration is introduced in Fig. 2. In this circuit, the voltage and current control loops of the UCC3839 are connected parallel. It can be achieved by connecting the VAO and CAO pins together. The error amplifier with the lower output voltage UDG-97012 Figure 1. Secondary side average current mode controller. 4 UCC2839 UCC3839 APPLICATION INFORMATION (cont.) UDG-97014 Figure 2. Voltage mode with average current short circuit limit. UDG-97014 Figure 3. 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