XD494

XD494 DIP16 / XL494 SOP16 1 Features 3 Description • • The XD494 device incorporates all the functions required in the construction of a pulse-widthmodulation (PWM) control circuit on a single chip. Designed primarily for power-supply control, this device offers the flexibility to tailor the power-supply control circuitry to a specific application. 1 • • • • • Complete PWM Power-Control Circuitry Uncommitted Outputs for 200-mA Sink or Source Current Output Control Selects Single-Ended or Push-Pull Operation Internal Circuitry Prohibits Double Pulse at Either Output Variable Dead Time Provides Control Over Total Range Internal Regulator Provides a Stable 5-V Reference Supply With 5% Tolerance Circuit Architecture Allows Easy Synchronization The XD494 device contains two error amplifiers,an on-chip adjustable oscillator, a dead-time control (DTC) comparator, a pulse-steering control flip-flop, a 5-V, 5%-precision regulator, and output-control circuits. The error amplifiers exhibit a common-mode voltage range from –0.3 V to VCC – 2 V. The dead-time control comparator has a fixed offset that provides approximately 5% dead time. The on-chip oscillator can be bypassed by terminating RT to the reference output and providing a sawtooth input to CT, or it can drive the common circuits in synchronous multiple-rail power supplies. 2 Applications • • • • • • • • • • • Desktop PCs Microwave Ovens Power Supplies: AC/DC, Isolated, With PFC, > 90 W Server PSUs Solar Micro-Inverters Washing Machines: Low-End and High-End E-Bikes Power Supplies: AC/DC, Isolated, No PFC, < 90 W Power: Telecom/Server AC/DC Supplies: Dual Controller: Analog Smoke Detectors Solar Power Inverters The uncommitted output transistors provide either common-emitter or emitter-follower output capability. The XD494 device provides for push-pull orsingleended output operation, which can be selected through the output-control function. The architecture of this device prohibits the possibility of either output being pulsed twice during push-pull operation.The XD494C device is characterized foroperationfrom 0°C to 70°C. The XL494 device ischaracterizedfor operation from –40°C to 85°C. 5 Device Information(1) PART NUMBER 494 4 Pinout Drawing D, DB, N, NS, OR PW PACKAGE (TOP VIEW) 1IN+ 1IN− FEEDBACK DTC CT RT GND C1 1 16 2 15 3 14 4 13 5 12 6 11 7 10 8 9 1 1 2IN+ 2IN− REF OUTPUT CTRL VCC C2 E2 E1 PACKAGE (PIN) BODY SIZE SOIC (16) 9.90 mm × 3.91 mm PDIP (16) 19.30 mm × 6.35 mm SOP (16) 10.30 mm × 5.30 mm TSSOP (16) 5.00 mm × 4.40 mm XD494 DIP16 / XL494 SOP16 6 Pin Configuration and Functions D, DB, N, NS, OR PW PACKAGE (TOP VIEW) 1IN+ 1IN− FEEDBACK DTC CT RT GND C1 1 16 2 15 3 14 4 13 5 12 6 11 7 10 8 9 2IN+ 2IN− REF OUTPUT CTRL VCC C2 E2 E1 Pin Functions PIN NAME NO. TYPE DESCRIPTION 1IN+ 1 I Noninverting input to error amplifier 1 1IN- 2 I Inverting input to error amplifier 1 2IN+ 16 I Noninverting input to error amplifier 2 2IN- 15 I Inverting input to error amplifier 2 C1 8 O Collector terminal of BJT output 1 C2 11 O Collector terminal of BJT output 2 CT 5 — Capacitor terminal used to set oscillator frequency DTC 4 I Dead-time control comparator input E1 9 O Emitter terminal of BJT output 1 E2 10 O Emitter terminal of BJT output 2 FEEDBACK 3 I Input pin for feedback GND 7 — OUTPUT CTRL 13 I Selects single-ended/parallel output or push-pull operation REF 14 O 5-V reference regulator output RT 6 — Resistor terminal used to set oscillator frequency VCC 12 — Positive Supply Ground 2 XD494 DIP16 / XL494 SOP16 7 Specifications 7.1 Absolute Maximum Ratings over operating free-air temperature range (unless otherwise noted) (1) MIN MAX UNIT VCC Supply voltage (2) VI Amplifier input voltage VO Collector output voltage 41 V IO Collector output current 250 mA 260 °C 150 °C 41 V VCC + 0.3 V Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds Tstg (1) (2) Storage temperature range –65 Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions is not implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. All voltages are with respect to the network ground terminal. 7.2 ESD Ratings MAX V(ESD) Electrostatic discharge Human body model (HBM), per ANSI/ESDA/JEDEC JS-001, all pins 500 Charged device model (CDM), per JEDEC specification JESD22C101, all pins 200 UNIT V 7.3 Recommended Operating Conditions VCC Supply voltage VI Amplifier input voltage VO Collector output voltage MIN MAX 7 40 V –0.3 VCC – 2 V Collector output current (each transistor) Current into feedback terminal fOSC Oscillator frequency CT Timing capacitor RT Timing resistor TA Operating free-air temperature UNIT 40 V 200 mA 0.3 mA 1 300 kHz 0.47 10000 nF 1.8 500 kΩ XD494 0 70 XL494 –40 85 °C 7.4 Thermal Information over operating free-air temperature range (unless otherwise noted) PARAMETER RθJA (1) (2) Package thermal impedance (1) (2) XD494 UNIT D DB N NS PW 73 82 67 64 108 °C/W Maximum power dissipation is a function of TJ(max), θJA, and TA. The maximum allowable power dissipation at any allowable ambient temperature is PD = (TJ(max) – TA) / θJA. Operating at the absolute maximum TJ of 150°C can affect reliability. The package thermal impedance is calculated in accordance with JESD 51-7. 3 XD494 DIP16 / XL494 SOP16 7.5 Electrical Characteristics, Reference Section over recommended operating free-air temperature range, VCC = 15 V, f = 10 kHz (unless otherwise noted) TEST CONDITIONS (1) PARAMETER XD494, XL494 MIN TYP (2) MAX 4.75 UNIT Output voltage (REF) IO = 1 mA 5 5.25 Input regulation VCC = 7 V to 40 V 2 25 mV Output regulation IO = 1 mA to 10 mA 1 15 mV Output voltage change with temperature ΔTA = MIN to MAX 2 10 mV/V Short-circuit output current (3) REF = 0 V (1) (2) (3) 25 V mA For conditions shown as MIN or MAX, use the appropriate value specified under recommended operating conditions. All typical values, except for parameter changes with temperature, are at TA = 25°C. Duration of short circuit should not exceed one second. 7.6 Electrical Characteristics, Oscillator Section CT = 0.01 μF, RT = 12 kΩ (see Figure 5) TEST CONDITIONS (1) PARAMETER XD494, XL494 MIN TYP (2) Frequency Standard deviation of frequency (3) All values of VCC, CT, RT, and TA constant Frequency change with voltage VCC = 7 V to 40 V, TA = 25°C Frequency change with temperature (4) ΔTA = MIN to MAX (1) (2) (3) UNIT 10 kHz 100 Hz/kHz 1 Hz/kHz 10 Hz/kHz For conditions shown as MIN or MAX, use the appropriate value specified under recommended operating conditions. All typical values, except for parameter changes with temperature, are at TA = 25°C. Standard deviation is a measure of the statistical distribution about the mean as derived from the formula: N 2 xn - X n =1 å( s= (4) MAX ) N -1 Temperature coefficient of timing capacitor and timing resistor are not taken into account. 7.7 Electrical Characteristics, Error-Amplifier Section See Figure 6 PARAMETER TEST CONDITIONS XD494, XL494 MIN TYP (1) MAX UNIT Input offset voltage VO (FEEDBACK) = 2.5 V 2 10 Input offset current VO (FEEDBACK) = 2.5 V 25 250 nA Input bias current VO (FEEDBACK) = 2.5 V 0.2 1 μA Common-mode input voltage range VCC = 7 V to 40 V Open-loop voltage amplification ΔVO = 3 V, VO = 0.5 V to 3.5 V, RL = 2 kΩ Unity-gain bandwidth VO = 0.5 V to 3.5 V, RL = 2 kΩ Common-mode rejection ratio ΔVO = 40 V, TA = 25°C Output sink current (FEEDBACK) Output source current (FEEDBACK) (1) –0.3 to VCC – 2 V 95 dB 800 kHz 65 80 dB VID = –15 mV to –5 V, V (FEEDBACK) = 0.7 V 0.3 0.7 mA VID = 15 mV to 5 V, V (FEEDBACK) = 3.5 V –2 All typical values, except for parameter changes with temperature, are at TA = 25°C. 4 70 mV mA XD494 DIP16 / XL494 SOP16 7.8 Electrical Characteristics, Output Section PARAMETER TEST CONDITIONS Collector off-state current VCE = 40 V, VCC = 40 V Emitter off-state current VCC = VC = 40 V, VE = 0 Collector-emitter saturation voltage TYP (1) MAX UNIT 2 100 μA –100 μA Common emitter VE = 0, IC = 200 mA 1.1 1.3 Emitter follower VO(C1 or C2) = 15 V, IE = –200 mA 1.5 2.5 Output control input current (1) MIN VI = Vref 3.5 V mA All typical values, except for temperature coefficient, are at TA = 25°C. 7.9 Electrical Characteristics, Dead-Time Control Section See Figure 5 PARAMETER TEST CONDITIONS Input bias current (DEAD-TIME CTRL) VI = 0 to 5.25 V Maximum duty cycle, each output VI (DEAD-TIME CTRL) = 0, CT = 0.01 μF, RT = 12 kΩ Input threshold voltage (DEAD-TIME CTRL) (1) MIN TYP (1) MAX UNIT –2 –10 μA 45% Zero duty cycle Maximum duty cycle — 3 3.3 MIN TYP (1) MAX 4 4.5 0 V All typical values, except for temperature coefficient, are at TA = 25°C. 7.10 Electrical Characteristics, PWM Comparator Section See Figure 5 PARAMETER TEST CONDITIONS Input threshold voltage (FEEDBACK) Zero duty cyle Input sink current (FEEDBACK) V (FEEDBACK) = 0.7 V (1) 0.3 0.7 UNIT V mA All typical values, except for temperature coefficient, are at TA = 25°C. 7.11 Electrical Characteristics, Total Device PARAMETER MIN TYP (1) MAX VCC = 15 V 6 10 VCC = 40 V 9 15 TEST CONDITIONS Standby supply current RT = Vref, All other inputs and outputs open Average supply current VI (DEAD-TIME CTRL) = 2 V, See Figure 5 (1) 7.5 UNIT mA mA All typical values, except for temperature coefficient, are at TA = 25°C. 7.12 Switching Characteristics TA = 25°C PARAMETER Rise time Fall time Rise time Fall time (1) TEST CONDITIONS Common-emitter configuration, See Figure 7 Emitter-follower configuration, See Figure 8 All typical values, except for temperature coefficient, are at TA = 25°C. 5 MIN TYP (1) MAX UNIT 100 200 ns 25 100 ns 100 200 ns 40 100 ns XD494 DIP16 / XL494 SOP16 100 100 k VCC = 15 V TA = 25°C 40 k −2% 10 k 0.001 µF −1% 0.01 µF 0% 4k 0.1 µF 1k 400 (1) Df = 1% 100 CT = 1 µF 40 10 1k 80 70 60 50 40 30 20 10 0 4k 10 k 40 k 100 k 400 k VCC = 15 V ΔVO = 3 V TA = 25°C 90 A − Amplifier Voltage Amplification − dB f − Oscillator Frequency and Frequency Variation − Hz 7.13 Typical Characteristics 1M 1 10 100 1k 10 k 100 k 1M f − Frequency − Hz RT − Timing Resistance − Ω Frequency variation (Δf) is the change in oscillator frequency that occurs over the full temperature range. Figure 1. Oscillator Frequency and Frequency Variation vs Timing Resistance xxx xxx Figure 2. Amplifier Voltage Amplification vs Frequency 80 60 3 Gain − (dB) VO − Output Voltage − (V) 4 2 40 20 1 0 0 0 10 20 VI − Input Voltage − (mV) Figure 3. Error Amplifier Transfer Characteristics 0 10k 100k f − Frequency − (Hz) Figure 4. Error Amplifier Bode Plot 6 1M XD494 DIP16 / XL494 SOP16 8 Parameter Measurement Information VCC = 15 V 150 W 2W 12 VCC 4 Test Inputs 3 12 kW 6 5 0.01 mF 1 2 16 15 13 C1 DTC FEEDBACK E1 RT C2 CT 1IN+ 1IN− 2IN+ E2 8 150 W 2W Output 1 9 11 Output 2 10 Error Amplifiers 2IN− OUTPUT CTRL REF 14 GND 50 kW 7 TEST CIRCUIT VCC Voltage at C1 0V VCC Voltage at C2 0V Voltage at CT Threshold Voltage DTC 0V Threshold Voltage FEEDBACK 0.7 V Duty Cycle MAX 0% VOLTAGE WAVEFORMS Figure 5. Operational Test Circuit and Waveforms 7 0% XD494 DIP16 / XL494 SOP16 Parameter Measurement Information (continued) Amplifier Under Test + VI FEEDBACK − + Vref − Other Amplifier Figure 6. Amplifier Characteristics 15 V 68 W 2W Each Output Circuit tf Output tr 90% 90% CL = 15 pF (See Note A) 10% 10% TEST CIRCUIT OUTPUT VOLTAGE WAVEFORM NOTE A: CL includes probe and jig capacitance. Figure 7. Common-Emitter Configuration 15 V Each Output Circuit Output CL = 15 pF (See Note A) 90% 90% 68 W 2W 10% 10% tr TEST CIRCUIT OUTPUT VOLTAGE WAVEFORM NOTE A: CL includes probe and jig capacitance. Figure 8. Emitter-Follower Configuration 8 tf XD494 DIP16 / XL494 SOP16 9 Detailed Description 9.1 Overview The design of the XD494 not only incorporates the primary building blocks required to control a switchingpower supply, but also addresses many basic problems and reduces the amount of additional circuitry required in the total design. The XD494 is a fixed-frequency pulse-width-modulation (PWM) control circuit. Modulation of output pulses is accomplished by comparing the sawtooth waveform created by the internal oscillator on the timing capacitor (CT) to either of two control signals. The output stage is enabled during the time when the sawtooth voltage is greater than the voltage control signals. As the control signal increases, the time during which the sawtooth input is greater decreases; therefore, the output pulse duration decreases. A pulse-steering flip-flop alternately directs the modulated pulse to each of the two output transistors. 9.2 Functional Block Diagram OUTPUT CTRL (see Function Table) 13 RT 6 CT 5 Oscillator Q1 1D DTC 4 Dead-Time Control Comparator ≈0.1 V ≈0.7 V 1IN+ 1IN− 2 9 Q2 11 PWM Comparator 10 + 2IN+ 2IN− 15 − C2 E2 12 VCC + Reference Regulator − 14 7 FEEDBACK E1 Pulse-Steering Flip-Flop Error Amplifier 2 16 C1 C1 Error Amplifier 1 1 8 3 REF GND 0.7 mA 9.3 Feature Description 9.3.1 5-V Reference Regulator The XD494 internal 5-V reference regulator output is the REF pin. In addition to providing a stable reference,it acts as a preregulator and establishes a stable supply from which the output-control logic, pulse-steering flip-flop, oscillator, dead-time control comparator, and PWM comparator are powered. The regulator employs a band-gap circuit as its primary reference to maintain thermal stability of less than 100-mV variation over the operating freeair temperature range of 0°C to 70°C. Short-circuit protection is provided to protect the internal reference and preregulator; 10 mA of load current is available for additional bias circuits. The reference is internally programmed to an initial accuracy of ±5% and maintains a stability of less than 25-mV variation over an input voltage range of 7 V to 40 V. For input voltages less than 7 V, the regulator saturates within 1 V of the input and tracks it. 9 XD494 DIP16 / XL494 SOP16 Feature Description (continued) 9.3.2 Oscillator The oscillator provides a positive sawtooth waveform to the dead-time and PWM comparators for comparison to the various control signals. The frequency of the oscillator is programmed by selecting timing components RT and CT. The oscillator charges the external timing capacitor, CT, with a constant current, the value of which is determined by the external timing resistor, RT. This produces a linear-ramp voltage waveform. When the voltage across CT reaches 3 V, the oscillator circuit discharges it, and the charging cycle is reinitiated. The charging current is determined by the formula: 3V ICHARGE = RT (1) The period of the sawtooth waveform is: 3 V ´ CT T= ICHARGE (2) The frequency of the oscillator becomes: 1 fOSC = R T ´ CT (3) However, the oscillator frequency is equal to the output frequency only for single-ended applications. For pushpull applications, the output frequency is one-half the oscillator frequency. Single-ended applications: 1 f= R T ´ CT (4) Push-pull applications: 1 f= 2RT ´ CT (5) 9.3.3 Dead-time Control The dead-time control input provides control of the minimum dead time (off time). The output of the comparator inhibits switching transistors Q1 and Q2 when the voltage at the input is greater than the ramp voltage of the oscillator. An internal offset of 110 mV ensures a minimum dead time of ∼3% with the dead-time control input grounded. Applying a voltage to the dead-time control input can impose additional dead time. This provides a linear control of the dead time from its minimum of 3% to 100% as the input voltage is varied from 0 V to 3.3 V, respectively. With full-range control, the output can be controlled from external sources without disrupting the error amplifiers. The dead-time control input is a relatively high-impedance input (II < 10 μA) and should be used where additional control of the output duty cycle is required. However, for proper control, the input must be terminated. An open circuit is an undefined condition. 9.3.4 Comparator The comparator is biased from the 5-V reference regulator. This provides isolation from the input supply for improved stability. The input of the comparator does not exhibit hysteresis, so protection against false triggering near the threshold must be provided. The comparator has a response time of 400 ns from either of the controlsignal inputs to the output transistors, with only 100 mV of overdrive. This ensures positive control of the output within one-half cycle for operation within the recommended 300-kHz range. 10 XD494 DIP16 / XL494 SOP16 10 Application and Implementation 10.1 Application Information The following design example uses the XD494 to create a 5-V/10-A power supply. This application was takenfrom application note SLVA001. 10.2 Typical Application NTE331 32-V Input 140 mH VO Q2 R11 100 W R1 1 kW R2 4 kW 16 15 + 5-V REF R12 30 W NTE6013 NTE153 Q1 R8 5.1 k R10 270 W 14 13 − 12 11 10 R9 5.1 k 9 VREF XD494 Control Load + 1 Osc − 2 3 RF 51 kW 4 CT 0.001 mF 5 6 7 8 RT 50 kW R7 9.1 kW R5 510 W 5-V REF 5-V REF R3 5.1 kW R4 5.1 kW R6 1 kW C2 2.5 mF R13 0.1 W Figure 9. Switching and Control Sections 11 XD494 DIP16 / XL494 SOP16 Typical Application (continued) 10.2.1 Design Requirements • VI = 32 V • VO = 5 V • IO = 10 A • fOSC = 20-kHz switching frequency • VR = 20-mV peak-to-peak (VRIPPLE) • ΔIL = 1.5-A inductor current change 10.2.2 Detailed Design Procedure 10.2.2.1 Input Power Source The 32-V dc power source for this supply uses a 120-V input, 24-V output transformer rated at 75 VA. The 24-V secondary winding feeds a full-wave bridge rectifier, followed by a current-limiting resistor (0.3 Ω) and two filter capacitors (see Figure 10). Bridge Rectifiers 3 A/50 V 120 V 24 V 3A +32 V 0.3 W 20,000 μF + + 20,000 μF Figure 10. Input Power Source The output current and voltage are determined by Equation 6 and Equation 7: VRECTIFIER = VSECONDARY ´ 2 = 24 V ´ 2 = 34 V IRECTIFIER(AVG) » 6) VO 5V ´ IO » ´ 10 A = 1.6 A VI 32 V (7) The 3-A/50-V full-wave bridge rectifier meets these calculated conditions. Figure 9 shows the switching and control sections. 10.2.2.2 Control Circuits 10.2.2.2.1 Oscillator Connecting an external capacitor and resistor to pins 5 and 6 controls the XD494 oscillator frequency. Theoscillator is set to operate at 20 kHz, using the component values calculated by Equation 8 and Equation 9: 1 fOSC = R T ´ CT (8) Choose CT = 0.001 μF and calculate RT: RT + 1 f OSC CT + (20 10 3) 1 (0.001 10 *6) + 50 kW (9) 10.2.2.2.2 Error Amplifier The error amplifier compares a sample of the 5-V output to the reference and adjusts the PWM to maintain a constant output current (see Figure 11). 12 XD494 DIP16 / XL494 SOP16 Typical Application (continued) VO 14 13 VREF R3 5.1 kW R5 510 W + 2 − Error Amplifier R9 5.1 kW 3 R4 5.1 kW XD494 1 R7 51 kW R8 5.1 kW XD494 Figure 11. Error-Amplifier Section The XD494 internal 5-V reference is divided to 2.5 V by R3 and R4. The output-voltage error signal also is divided to 2.5 V by R8 and R9. If the output must be regulated to exactly 5.0 V, a 10-kΩ potentiometer can be used in place of R8 to provide an adjustment. To increase the stability of the error-amplifier circuit, the output of the error amplifier is fed back to the inverting input through RT, reducing the gain to 101. 10.2.2.2.3 Current-Limiting Amplifier The power supply was designed for a 10-A load current and an IL swing of 1.5 A, therefore, the short-circuit current should be: I ISC = IO + L = 10.75 A (10) 2 The current-limiting circuit is shown in Figure 12. 14 VO R2 3 kW LOAD + R11 0.1 kW VREF 16 15 R1 1 kW XD494 XD494 Figure 12. Current-Limiting Circuit Resistors R1 and R2 set the reference of about 1 V on the inverting input of the current-limiting amplifier. Resistor R13, in series with the load, applies 1 V to the noninverting terminal of the current-limiting amplifier when the load current reaches 10 A. The output-pulse width is reduced accordingly. The value of R13 is: 1V R13 = = 0.1W 10 A (11) 10.2.2.2.4 Soft Start and Dead Time To reduce stress on the switching transistors at start-up, the start-up surge that occurs as the output filter capacitor charges must be reduced. The availability of the dead-time control makes implementation of a soft-start circuit relatively simple (see Figure 13). 13 XD494 DIP16 / XL494 SOP16 Typical Application (continued) Oscillator Ramp 14 +5 V Osc C2 + RT 5 4 0.1 V R6 7 XD494 Pin 4 Voltage Oscillator Ramp Voltage ton PWM Output Figure 13. Soft-Start Circuit The soft-start circuit allows the pulse width at the output to increase slowly (see Figure 13) by applying a negative slope waveform to the dead-time control input (pin 4). Initially, capacitor C2 forces the dead-time control input to follow the 5-V regulator, which disables the outputs (100% dead time). As the capacitor charges through R6, the output pulse width slowly increases until the control loop takes command. With a resistor ratio of 1:10 for R6 and R7, the voltage at pin 4 after start-up is 0.1 × 5 V, or 0.5 V. The soft-start time generally is in the range of 25 to 100 clock cycles. If 50 clock cycles at a 20-kHz switching rate is selected, the soft-start time is: 1 1 t= = = 50 msper clock cycle f 20kHz (12) The value of the capacitor then is determined by: soft - start time 50 ms ´ 50 cycles C2 = = = 2.5 mF R6 1 kW (13) This helps eliminate any false signals that might be created by the control circuit as power is applied. 14 XD494 DIP16 / XL494 SOP16 Typical Application (continued) 10.2.2.3 Inductor Calculations The switching circuit used is shown in Figure 39. L S1 VI C1 D1 R1 VO Figure 14. Switching Circuit The size of the inductor (L) required is: d = duty cycle = VO/VI = 5 V/32 V = 0.156 f = 20 kHz (design objective) ton = time on (S1 closed) = (1/f) × d = 7.8 μs toff = time off (S1 open) = (1/f) – ton = 42.2 μs L ≉ (VI – VO ) × ton/ΔIL ≉ [(32 V – 5 V) × 7.8 μs]/1.5 A ≉ 140.4 μH 10.2.2.4 Output Capacitance Calculations Once the filter inductor has been calculated, the value of the output filter capacitor is calculated to meet the output ripple requirements. An electrolytic capacitor can be modeled as a series connection of an inductance, a resistance, and a capacitance. To provide good filtering, the ripple frequency must be far below the frequencies at which the series inductance becomes important. So, the two components of interest are the capacitance and the effective series resistance (ESR). The maximum ESR is calculated according to the relation between the specified peak-to-peak ripple voltage and the peak-to-peak ripple current. DVO(ripple) V = » 0.067 W ESR(max) = DIL 1.5 A (14) The minimum capacitance of C3 necessary to maintain the VO ripple voltage at less than the 100-mV design objective is calculated according to Equation 15: DIL 1.5 A C3 = = = 94 mF 8f DVO 8 ´ 20 ´ 103 ´ 0.1 V (15) A 220-mF, 60-V capacitor is selected because it has a maximum ESR of 0.074 Ω and a maximum ripple current of 2.8 A. 10.2.2.5 Transistor Power-Switch Calculations The transistor power switch was constructed with an NTE153 pnp drive transistor and an NTE331 npn output transistor. These two power devices were connected in a pnp hybrid Darlington circuit configuration (see Figure 15). 15 XD494 DIP16 / XL494 SOP16 NTE331 32 V Q2 R11 100 W R12 30 W DI IO + L = 10.8 A 2 Q1 NTE153 R10 270 W 11 10 9 Control XD494 8 Figure 15. Power-Switch Section The hybrid Darlington circuit must be saturated at a maximum output current of IO + ΔIL/2 or 10.8 A. The Darlington hFE at 10.8 A must be high enough not to exceed the 250-mA maximum output collector current of the XD494. Based on published NTE153 and NTE331 specifications, the required power-switch minimum drive was calculated by Equation 16 through Equation 18 to be 144 mA: hFE (Q1) at IC of 3 A = 15 (16) hFE (Q2) at IC of 10.0 A = 5 (17) I IO + L 2 ³ 144mA iB ³ hFE (Q2) ´ hFE (Q1) (18) The value of R10 was calculated by: V - [VBE (XD) + VCE (XD494)] 32 - (1.5 + 0.7) R10 £ I = iB 0.144 R10 £ 207 W (19) Based on these calculations, the nearest standard resistor value of 220 Ω was selected for R10. Resistors R11 and R12 permit the discharge of carriers in switching transistors when they are turned off. The power supply described demonstrates the flexibility of the XD494 PWM control circuit. Thispower-supply design demonstrates many of the power-supply control methods provided by the XD494, as well as theversatility of the control circuit. 16 XD494 DIP16 / XL494 SOP16 10.2.3 Application Curves for Output Characteristics VREF − Reference Voltage − (V) 6 5 4 3 2 1 0 0 1 2 3 4 5 6 7 VI − Input Voltage − (V) Figure 16. Reference Voltage vs Input Voltage 17 XD494 DIP16 / XL494 SOP16 18 17