UCC1581 UCC2581 UCC3581 Micropower Voltage Mode PWM FEATURES BLOCK DIAGRAM • Low 85µA Startup Current • Low 300µA Operating Current • Automatically Disabled Startup Preregulator • Programmable Minimum Duty Cycle with Cycle Skipping • Programmable Maximum Duty Cycle • Output Current 1A Peak Source and Sink • Programmable Soft Start • Programmable Oscillator Frequency • External Oscillator Synchronization Capability Note: Pin Connection shown for 14-pin Package UDG-95011-1 DESCRIPTION The UCC3581 voltage mode pulse width modulator is designed to control low power isolated DC - DC converters in applications such as Subscriber Line Power (ISDN I.430). Primarily used for single switch forward and flyback converters, the UCC3581 features BiCMOS circuitry for low startup and operating current, while maintaining the ability to drive power MOSFETs at frequencies up to 100kHz. The UCC3581 oscillator allows the flexibility to program both the frequency and the maximum duty cycle with two resistors and a capacitor. A TTL level input is also provided to allow synchronization to an external frequency source. The UCC3581 includes programmable soft start circuitry, overcurrent detection, a 7.5V linear preregulator to control chip VDD during startup, and an on-board 4.0V logic supply. The UCC3581 provides functions to maximize light load efficiency that are not normally found in PWM controllers. SLUS295 - MARCH 1999 A linear preregulator driver in conjunction with an external depletion mode N-MOSFET provides initial controller power. Once the bootstrap supply is functional, the preregulator is shut down to conserve power. During light load, power is saved by providing a programmable minimum duty cycle clamp. When a duty cycle below the minimum is called for, the modulator skips cycles to provide the correct average duty cycle required for output regulation. This effectively reduces the switching frequency, saving significant gate drive and power stage losses. The UCC3581 is available in 14-pin plastic and ceramic dual-in-line packages and in a 14-pin narrow body small outline IC package (SOIC). The UCC1581 is specified for operation from −55°C to +125°C, the UCC2581 is specified for operation from −40°C to +85°C, and the UCC3581 is specified for operation from 0°C to +70°C. UCC1581 UCC2581 UCC3581 CONNECTION DIAGRAMS ABSOLUTE MAXIMUM RATINGS Supply Voltage (IDD ≤ 10mA). . . . . . . . . . . . . . . . . . . . . . . . 15V Supply Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30mA VREF Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . –10mA OUT Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 1A Analog Inputs EN . . . . . . . . . . . . . . . . . . . . . . . . . . . . –0.3V to (VDD + 0.3V) VC, ISEN, SYNC, DCMIN. . . . . . . . . . –0.3V to (VREF + 0.3V) Power Dissipation at TD = 25°C (N, J, Q, L Package) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1W (D Package) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.65W Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C Junction Temperature . . . . . . . . . . . . . . . . . . . –55C to +150°C Lead Temperature (Soldering, 10 sec.) . . . . . . . . . . . . . +300°C DIL-14, SOIC-14 (Top View) N or J, D Packages Unless otherwise specified, all voltages are with respect to Ground. Currents positive into, negative out of the specified terminal. Consult Packaging Section of Databook for thermal limitations and considerations of packages. ORDERING INFORMATION UCC1581J UCC2581D UCC2581N UCC3581D UCC3581N TEMPERATURE RANGE –55°C to +125°C –40°C to +85°C –40°C to +85°C 0°C to +70°C 0°C to +70°C PACKAGE CDIP SOIC PDIP SOIC PDIP ELECTRICAL CHARACTERISTICS: Unless otherwise stated, these specifications apply for VDD = 10V, 0.1µF capacitor from VDD to GND, 1.0µF capacitor from REF to GND, RT1 = 680kΩ, RT2 = 12kΩ, CT = 750pF and TA = TJ. PARAMETER TEST CONDITIONS MIN TYP MAX UNITS 3.94 4.0 4.06 V 10 30 mV 7.3 7.9 V Reference Section Output Voltage I = –0.2mA Load Regulation –5.0mA < I < –0.2mA Undervoltage Lockout Section Start Threshold 6.7 Minimum Operating Voltage After Start 6.2 6.8 7.4 V Hysteresis 0.2 0.5 0.8 V Regulated VDD Voltage 7.0 7.5 8.0 V Regulated VDD to UVLO Delta 100 230 600 mV 8.2 V 21 kHz Linear Preregulator Section VDD Override Threshold Oscillator Section Frequency 25°C 18 Temperature Stability (Note 1) 3.0 % CT Peak Voltage (Note 1) 2.5 V CT Valley Voltage (Note 1) 1.0 SYNC VIH SYNC VIL 19.5 1.9 (Note 1) 2.1 V 2.3 1.8 V V PWM SECTION Maximum Duty Cycle Minimum Duty Cycle 80 (VC > 1.0V at start of cycle) DCMIN = 1.18V Input Bias Current 83 (VC < 1.0V) DCMIN = 0V 86 % 0 % 8 10.5 13 % (DCMIN), (Note 1) –150 20 150 nA (VC), (Note 1) –150 20 150 nA 2 UCC1581 UCC2581 UCC3581 ELECTRICAL CHARACTERISTICS: Unless otherwise stated, these specifications apply for VDD = 10V, 0.1µF capacitor from VDD to GND, 1.0µF capacitor from REF to GND, RT1 = 680kΩ, RT2 = 12kΩ, CT = 750pF and TA = TJ. PARAMETER TEST CONDITIONS MIN TYP MAX UNITS –150 20 150 nA 0.4 0.5 0.6 V Current Sense Section Input Bias Current Overcurrent Threshold Output Section OUT Low Level I = 100mA 0.6 1.2 V OUT High Level I = –100mA, VDD – OUT 0.6 1.2 V Rise/Fall Time (Note 1) 20 100 ns –9 –11.5 –14 µA VIH 1.9 2.0 2.1 V VIL 1.7 1.8 1.9 V Hysteresis 180 230 280 mV 5 10 15 µA 85 130 µA 300 600 µA 15 16.5 V 100 150 µA Soft start Section Soft start Current SS = 2V Chip Enable Section Source Current Overall Section Start-Up Current VDD < Start Threshold Operating Supply Current VC = 0V VDD Zener Shunt Voltage IDD = 10mA IDD Stand-by Shunt Voltage EN = 0V 13.5 Note 1: Guaranteed by design. Not 100% tested in production PIN DESCRIPTIONS CT: Oscillator timing capacitor pin. Minimum value is 100pF. 2 .0V . The oscillator charging current is 9. 2 • RT1 DCMIN: Input for programming minimum duty cycle where pulse skipping begins. This pin can be grounded to disable minimum duty cycle feature and pulse skipping. See Application Diagram Fig. 1. 2 .0V . The current into this pin is RT1 The value of RT1 should be between 220k and 1MΩ. EN: Enable input. This pin has an internal 10µA pull-up. A logic low input inhibits the PWM output and causes the soft start capacitor to be discharged. RT2: Resistor pin to program oscillator discharge time. The minimum value of RT2 is 10kΩ. See Application Diagram Fig. 1. GND: Circuit ground. SS: Soft start capacitor pin. The charging current out of SS is 3.75X the current in RT1. GT: Pin for controlling the gate of an external depletion mode N-MOSFET for the startup supply. The external N-MOSFET regulates VDD to 7.5V until the bootstrap supply comes up, then GT goes low. SYNC: Oscillator synchronization pin. Rising edge triggered CMOS/TTL compatible input with a 2.1V threshold. SYNC should be grounded if not used. The minimum pulse width of the SYNC signal is 100ns. ISEN: Input for overcurrent comparator. This function can be used for pulse-by-pulse current limiting. The threshold is 0.5V nominal. VC: Control voltage input to PWM comparator. The nominal control range of VC is 1.0V to 2.5V. OUT: Gate drive output to external N-MOSFET. VDD: Chip input power with an 15V internal clamp. VDD is regulated by startup FET to 7.5V until the bootstrap voltage comes up. VDD should be bypassed at the chip with a 0.1µF minimum capacitor. REF: 4.0V reference output. A minimum value bypass capacitor of 1.0µF is required for stability. RT1: Resistor pin to program oscillator charging current. 3 UCC1581 UCC2581 UCC3581 APPLICATION INFORMATION The UCC3581’s oscillator allows the user the flexibility to program the frequency and the duty cycle by adjusting two resistors and a capacitor. Application Diagram Fig. 1 shows these components as RT1, RT2, and CT. RT1 programs the timing capacitor charging current which results in a linear ramp charging CT. Discharge of CT is accomplished though RT2 which results in a standard RC discharge waveform. The oscillator on-time (CT charging) is calculated by the formula RT2 UCC3581 1 CT 2 GT 3 VDD RT2 14 SYNC 13 RT1 12 VIN CT BSS129 OR EQUIV. D2 1µF t ON = 0.082 • RT1 • CT . RT1 The off-time (CT discharging) is calculated by the formula t OFF = 0.95 • RT1 • CT . Q1 4 OUT 5 GND 6 REF EN 11 SS 10 CSS Resistor RT1 programs the charging current. The current is 2.0V . RT1 REF CT charging current is 9.2 times the current in RT1. RT1 can range from 220kΩ to 1MΩ. Minimum capacitor size is 100pF, and minimum RT2 size is 10k. DCMIN 9 1µF REF 7 ISEN VC 8 VIN U1 D1 A Block Diagram of the Oscillator is shown in Fig. 2. The oscillator also has an external synchronization pin. When a low to high level is detected, and if the oscillator’s output is in the high state (CT charging), the oscillator output immediately goes low and CT starts discharging. The sync input is rising edge sensitive and is ignored when the oscillator output is low. REF REF & E/A U1 RL T1 UDG-99043 Figure 1. Application diagram. UDG-96105 Figure 2. Oscillator. 4 UCC1581 UCC2581 UCC3581 APPLICATION INFORMATION (cont.) A Typical Micropower Application The externally bypassed 4.0V reference is controlled by undervoltage lockout and chip enable circuitry. The enable input is internally tied to a 10µA current source which allows the pin to be driven by an open collector driver. The part is also enabled if EN floats. The UCC3581 has a soft start function which requires a user supplied external timing capacitor. When in soft start mode, the soft start capacitor, CSS, is charged with a constant current source. The soft start current is 3.75X the current in RT1. The circuit shown in Fig. 3 illustrates the use of the UCC3581 in a micropower application. The isolated 5V flyback power supply uses a minimum of parts and operates over an 8:1 input voltage range (15VDC to 120VDC) while delivering a regulated 5V output with a load swing from 0W to 1W. It operates in the discontinuous mode at light load or high line, and continuous mode at heavier loads and lower line voltages. Higher input line voltages are possible by simply increasing the voltage ratings of C1, Q1, D1 and D2. There is an on-chip control amplifier, which when driving the gate of an external depletion mode N-MOSFET, acts as a 7.5V linear preregulator supplying VDD directly from the primary input power line. The preregulator may subsequently be fully disabled by a tertiary bootstrap winding providing a minimum of 8.2V to the VDD pin. The most notable feature of the design is its efficiency. With a load of 1 watt, the typical efficiency is 82%, dropping to 70% around 50mW. With a load of only 12.5mW, the efficiency remains as high as 50%. At this load, with an input of 50V, the total input current is only 500µA. Note that the power supply can be disabled by pulling the UCC3581 enable pin low, in which case the input current drops to less than 150µA. Computation of DCMIN DCMIN for a given duty cycle is calculated as follows. ∆V = i OSC • DC • (t ON + t OFF ) The UCC3581 achieves very low losses by means of low quiescent current and pulse skipping at light loads which reduces switching losses. The degree of pulse skipping is controlled by programming the minimum duty cycle. In this example, the frequency is 35kHz at maximum load and drops to <2kHz at 12.5mW load (minimum pulse width of around 6µsec, or 21% duty cycle at 35kHz). Another way losses are reduced is operating with a VDD of around 10V rather than the more common 12V to 16V. At such light primary currents, the MOSFET remains in full saturation with a gate drive voltage well below 10V. CT where • i = oscillator charge current = 9.2 . (2.0V/RT1) • DC = Duty Cycle, as a fraction of 1 • tON = 0.082 • RT1 • CT • tOFF = 0.95 • RT2 • CT • CT = Oscillator Capacitor The CT pin ramp slews from 1V to 2.5V. Therefore, add ∆V to 1V to get DCMIN voltage. Gate drive losses are minimized by choosing a MOSFET with low total gate charge, in this case only 8nC maximum. By choosing a large gate drive resistor, EMI is minimized by reducing peak currents. Due to pulse skipping, switching times are less critical for efficiency at light load. Example: For 10% duty cycle with RT1 = 680kΩ, RT2 = 12kΩ, and CT = 705pF, ∆V = i OSC • DC • = The shunt regulator (LM3411) and optocoupler (MOC8100) are also key to the efficiency at such light loads, and were chosen for their low operating current. The LM3411 has a quiescent current of only 150µA maximum (compared to 1mA for the more common TL431). In addition, because it is not a three terminal device, the LM3411’s quiescent current does not flow in the optocoupler LED. Since this bias current is not in the feedback control path, a higher value pull-up resistor can be used on the optocoupler output transistor, further reducing losses. (t ON + t OFF ) CT 2 .0V • (01 9 .2 • . ) • 4182 . • 10 −5 sec+ 8.55 • 10 −6 sec 680k 750 • 10 −12 ∆V = 0.18V Therefore, DCMIN = 1V + 0.18V = 1.18V 5 UCC1581 UCC2581 UCC3581 TYPICAL APPLICATION UDG-96104 Figure 3. Micropower power supply with 50% efficiency at 12.5mW load. 90.0% 50V 25V 80.0% 70.0% Efficiency (%) 75V 60.0% 100V 50.0% 40.0% 30.0% 20.0% 10.0% 0.0% 12.6 37.9 63.1 128.3 Output Load (mW) Figure 4. UCC3581 efficiency vs. line and load. 6 236.3 480.7 1054 UCC1581 UCC2581 UCC3581 APPLICATION INFORMATION (cont.) If the sync input is used, it should not be left in a high impedance state where noise could cause false triggering. If unused, it should be grounded. A rather large soft start capacitor was chosen to give a startup time of several hundred milliseconds, reducing the input surge current while the output is coming up. The transformer was designed with a standard Magnetics RM8 ferrite core using P material, gapped for an AL of 2 1600mH/1000Turn . The primary consists of 44 turns, while the 5V secondary has 10 turns and the bootstrap winding 18 turns. For simplicity, all the windings can be #28 AWG. A two section bobbin was used to provide high primary to secondary isolation. A much smaller design, with reduced isolation, could have been done for this low power level. Note that for stability, the UCC3581 VREF bypass capacitor needs to be at least 1µF. The VDD supply also needs some capacitance to hold it up between pulses at light load and high line, where the frequency may drop to less than 1kHz due to pulse skipping. Otherwise it may drop low enough for the startup MOSFET to be biased on, lowering efficiency. TYPICAL CHARACTERISTIC CURVES 140 4.10 470k/47k 4.08 120 FREQUENCY [kHz] 4.06 VREF [V] 4.04 4.02 4.00 3.98 3.96 3.94 100 680k/12k 80 60 40 1M/10k 220k/10k 20 3.92 3.90 -75 -50 -25 0 25 50 75 100 0 125 100 TEMPERATURE [°C] Figure 5. Reference voltage vs. temperature. 1000 CT [pF] Figure 6. Frequency vs. CT vs. RT1 and RT2. 7 10000 UCC1581 UCC2581 UCC3581 TYPICAL CHARACTERISTIC CURVES (cont.) 1600 100 1M/10K 1200 80 680K/12K 70 220K/10K 60 50 1000 800 NO LOAD 600 400 470K/47K 40 200 30 0 0 20 40 60 FREQUENCY [kHz] 80 100 0 Figure 7. Duty cycle vs. frequency vs. RT1 / RT2. 35 30 25 20 15 10 5 0 200 400 600 800 1000 20 40 60 FREQUENCY [kHz] 80 100 Figure 8. IDD vs. frequency RT1 = 680k, RT2 = 12k. 40 ISOFT START [uA] 1nF LOAD 1400 IDD [uA] DUTY CYCLE [%] 90 1200 RT1 [kW] Figure 9. Soft start current vs. RT1. UNITRODE CORPORATION 7 CONTINENTAL BLVD. • MERRIMACK, NH 03054 TEL. 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