NCP5173 1.5 A 560 kHz−1.0 MHz Boost Regulator The NCP5173 is a switching regulator with a high efficiency, 1.5 A integrated switch. It runs at a base frequency of 560 kHz and can be synchronized to an external clock up to 1.0 MHz. This part operates over a wide input voltage range, from 2.7 V to 30 V. The flexibility of the design allows the chip to operate in most power supply configurations, including boost, flyback, forward, inverting, and SEPIC. The IC uses current mode architecture, which allows excellent load and line regulation, as well as a practical means for limiting current. Combining high frequency operation with a highly integrated regulator circuit results in an extremely compact power supply solution. The circuit design includes provisions for features such as frequency synchronization, shutdown, and feedback controls for positive voltage regulation. http://onsemi.com MARKING DIAGRAM 5x6 QFN MN SUFFIX CASE 505AC NCP5173 AWLYYWW Features • • • • • • • • • • Integrated Power Switch: 1.5 A Guaranteed Wide Input Range: 2.7 V to 30 V High Frequency Allows for Small Components Minimum External Components Easy External Synchronization Built−in Overcurrent Protection Frequency Foldback Reduces Component Stress During an Overcurrent Condition Thermal Shutdown with Hysteresis Low 1.0 mm Maximum Profile Shut Down Current: 50 A Maximum Applications • Flat Panel Displays • Systems Requiring Low Profile Components Semiconductor Components Industries, LLC, 2003 October, 2003 − Rev. 0 1 1 NCP5173 = Specific Device Code A = Assembly Location WL = Wafer Lot YY = Year WW = Work Week NOTE: Thermal pad is electrically isolated from IC and all pins. ORDERING INFORMATION Device Package Shipping NCP5173MN QFN 95 Units/Rail NCP5173MNR2 QFN 2500 Tape & Reel Publication Order Number: NCP5173/D NCP5173 R2 3.72 k 2 C1 0.01 F 3 D1 VSW VC FB Test PGND NCP5173 1 AGND VOUT 8 5V MBRS120T3 7 6 L1 4 SS VCC SS 5 + 22 H C3 22 F 3.3 V R3 R1 5k + C2 22 F 1.28 k Figure 1. Applications Diagram MAXIMUM RATINGS* Rating Value Unit Junction Temperature Range, TJ −40 to +150 °C Storage Temperature Range, TSTORAGE −65 to +150 °C 35 °C/W 230 Peak °C 1.2 kV Package Thermal Resistance: Junction−to−Ambient, RJA Lead Temperature Soldering: Reflow (Note 1) ESD, Human Body Model 1. 60 second maximum above 183°C. *The maximum package power dissipation must be observed. MAXIMUM RATINGS Pin Name Pin Symbol VMAX VMIN ISOURCE ISINK IC Power Input VCC 30 V −0.3 V N/A 200 mA Shutdown/Sync SS 30 V −0.3 V 1.0 mA 1.0 mA Loop Compensation VC 6.0 V −0.3 V 10 mA 10 mA Voltage Feedback Input FB 10 V −0.3 V 1.0 mA 1.0 mA Test Pin Test 6.0 V −0.3 V 1.0 mA 1.0 mA Power Ground PGND 0.3 V −0.3 V 4A 10 mA Analog Ground AGND 0V 0V N/A 10 mA Switch Input VSW 40 V −0.3 V 10 mA 3.0 A http://onsemi.com 2 NCP5173 ELECTRICAL CHARACTERISTICS (2.7 V < VCC < 30 V; 0°C < TJ < 125°C; for all specifications unless otherwise stated.) Test Conditions Min Typ Max Unit VC tied to FB; measure at FB 1.246 1.276 1.300 V FB = VREF −1.0 0.1 1.0 A VC = FB − 0.01 0.03 %/V IVC = 25 A 300 550 800 Mho (Note 2) 200 500 − V/V VC Source Current FB = 1.0 V or NFB = −1.9 V, VC = 1.25 V 25 50 90 A VC Sink Current FB = 1.5 V or NFB = −3.1 V, VC = 1.25 V 200 625 1500 A VC High Clamp Voltage FB = 1.0 V or NFB = −1.9 V; VC sources 25 A 1.5 1.7 1.9 V VC Low Clamp Voltage FB = 1.5 V or NFB = −3.1 V, VC sinks 25 A 0.25 0.50 0.65 V VC Threshold Reduce VC from 1.5 V until switching stops 0.75 1.05 1.30 V FB = 1.0 V 460 560 620 kHz FB = 0 V 60 104 160 kHz − 82 90 − % Frequency drops to reduced operating frequency 0.36 0.40 0.44 V − 640 − 1000 kHz Rise time = 20 ns 2.5 − − V SS = 0 V SS = 3.0 V −15 − −3.0 3.0 − 8.0 A − 0.50 0.85 1.20 V 2.7 V ≤ VCC ≤ 12 V 12 V < VCC ≤ 30 V 12 12 80 36 350 200 s ISWITCH = 1.5 A (Note 2) ISWITCH = 1.0 A, 0°C TJ ≤ 85°C ISWITCH = 1.0 A, −40°C ≤ TJ 0°C ISWITCH = 10 mA − − − − 0.8 0.55 0.75 0.09 1.4 − − 0.45 V 50% Duty cycle (Note 2) 80% Duty cycle (Note 2) 1.6 1.5 1.9 1.7 2.4 2.2 A FB = 0 V, ISW = 4.0 A (Note 2) 200 250 300 ns 2.7 V ≤ VCC ≤ 12 V, 10 mA ≤ ISW ≤ 1.0 A 12 V < VCC ≤ 30 V, 10 mA ≤ ISW ≤ 1.0 A 2.7 V ≤ VCC ≤ 12 V, 10 mA ≤ ISW ≤ 1.5 A (Note 2) 12 V < VCC ≤ 30 V, 10 mA ≤ ISW ≤ 1.5 A (Note 2) − − − 10 − 17 30 100 30 mA/A − − 100 VSW = 40 V, VCC = 0V − 2.0 100 Characteristic Positive and Negative Error Amplifiers FB Reference Voltage FB Input Current FB Reference Voltage Line Regulation Error Amp Transconductance Error Amp Gain Oscillator Base Operating Frequency Reduced Operating Frequency Maximum Duty Cycle FB Frequency Shift Threshold Sync/Shutdown Sync Range Sync Pulse Transition Threshold SS Bias Current Shutdown Threshold Shutdown Delay Power Switch Switch Saturation Voltage Switch Current Limit Minimum Pulse Width ICC/ IVSW Switch Leakage 2. Guaranteed by design, not 100% tested in production. http://onsemi.com 3 A NCP5173 ELECTRICAL CHARACTERISTICS (continued) (2.7 V < VCC < 30 V; 0°C < TJ < 125°C; for all specifications unless otherwise stated.) Test Conditions Characteristic Min Typ Max Unit General Operating Current ISW = 0 − 5.5 8.0 mA Shutdown Mode Current VC < 0.8 V, SS = 0 V, 2.7 V ≤ VCC ≤ 12 V VC < 0.8 V, SS = 0 V, 12 V ≤ VCC ≤ 30 V − − 12 − 60 100 A Minimum Operation Input Voltage VSW switching, maximum ISW = 10 mA − 2.45 2.70 V Thermal Shutdown (Note 3) 150 180 210 °C Thermal Hysteresis (Note 3) − 25 − °C 3. Guaranteed by design, not 100% tested in production. PIN FUNCTION DESCRIPTION Pin Number Symbol Function 1 VC Loop compensation pin. The VC pin is the output of the error amplifier and is used for loop compensation, current limit and soft start. Loop compensation can be implemented by a simple RC network as shown in the application diagram on page 2 as R1 and C1. 2 FB Positive regulator feedback pin. This pin senses a positive output voltage and is referenced to 1.276 V. When the voltage at this pin falls below 0.4 V, chip switching frequency reduces to 20% of the nominal frequency. 3 Test These pins are connected to internal test logic and should either be left floating or tied to ground. Connection to a voltage between 2.0 V and 6.0 V shuts down the internal oscillator and leaves the power switch running. 4 SS Synchronization and shutdown pin. This pin may be used to synchronize the part to nearly twice the base frequency. A TTL low will shut the part down and put it into low current mode. If synchronization is not used, this pin should be either tied high or left floating for normal operation. 5 VCC Input power supply pin. This pin supplies power to the part and should have a bypass capacitor connected to AGND. 6 AGND Analog ground. This pin provides a clean ground for the controller circuitry and should not be in the path of large currents. The output voltage sensing resistors should be connected to this ground pin. This pin is connected to the IC substrate. 7 PGND Power ground. This pin is the ground connection for the emitter of the power switching transistor. Connection to a good ground plane is essential. 8 VSW High current switch pin. This pin connects internally to the collector of the power switch. The open voltage across the power switch can be as high as 40 V. To minimize radiation, use a trace as short as practical. http://onsemi.com 4 NCP5173 VCC Thermal Shutdown Shutdown 2.0 V Regulator VSW Oscillator Delay Timer S PWM Latch R Q Switch Driver Sync SS Frequency Shift 5:1 ×5 Slope Compensation 0.4 V Detector 63 m − FB Ramp Summer + 1.276 V Positive Error Amp PWM Comparator + AGND VC Figure 2. Block Diagram http://onsemi.com 5 − PGND NCP5173 TYPICAL PERFORMANCE CHARACTERISTICS 7.2 70 7.0 VCC = 12 V 40 30 6.2 VCC = 12 V 20 6.0 5.8 5.6 ISW = 1.5 A 50 (mA/A) Current (mA) 6.6 6.4 VCC = 30 V 60 VCC = 30 V 6.8 0 50 Temperature (°C) VCC = 2.7 V 10 VCC = 2.7 V 0 100 0 100 Figure 4. ICC/ IVSW vs. Temperature Figure 3. ICC (No Switching) vs. Temperature 1.9 1200 −40 °C 1000 1.8 85 °C 800 600 VIN (V) VCE(SAT) (mV) 50 Temperature (°C) 1.7 25 °C 400 1.6 200 0 500 1.5 1000 0 ISW (mA) 100 fOSC (% of Typical) fOSC (kHz) 100 Figure 6. Minimum Input Voltage vs. Temperature Figure 5. VCE(SAT) vs. ISW 570 565 560 555 550 545 540 535 530 525 520 50 Temperature (°C) VCC = (12 V) −40°C 75 85°C 50 25°C 25 0 0 50 Temperature (°C) 350 100 Figure 7. Switching Frequency vs. Temperature 380 400 VFB (mV) 420 Figure 8. Switching Frequency vs. VFB http://onsemi.com 6 450 NCP5173 TYPICAL PERFORMANCE CHARACTERISTICS 0.20 1.280 VCC = 12 V 0.18 1.276 IFB (A) Voltage (V) 1.278 1.274 VCC = 2.7 V 1.272 0.16 VCC = 12 V 0.14 0.12 VCC = 30 V 0.10 1.270 VCC = 2.7 V 0.08 1.268 0 50 Temperature (°C) 0 100 Figure 9. Reference Voltage vs. Temperature 50 Temperature (°C) Figure 10. IFB vs. Temperature 2.60 99 VCC = 30 V 2.50 Duty Cycle (%) Current (A) 98 2.40 VCC = 2.7 V 2.30 VCC = 12 V 2.20 0 100 VCC = 12 V 97 96 VCC = 2.7 V 95 94 VCC = 30 V 50 Temperature (°C) 93 100 0 50 Temperature (°C) 100 Figure 12. Maximum Duty Cycle vs. Temperature Figure 11. Current Limit vs. Temperature 1.1 1.7 Voltage (V) Voltage (V) 1.0 VC High Clamp Voltage 1.5 1.3 1.1 0.9 0.7 VC Threshold 0 50 Temperature (°C) 0.9 0.8 0.7 0.6 0.5 0.4 100 0 Figure 13. VC Threshold and High Clamp Voltage vs. Temperature 50 Temperature (°C) 100 Figure 14. Shutdown Threshold vs. Temperature http://onsemi.com 7 NCP5173 TYPICAL PERFORMANCE CHARACTERISTICS 40 160 25°C VCC = 2.7 V 140 30 85°C 100 ISS (A) Delay (s) 120 VCC = 12 V 80 20 −40°C 10 VCC = 30 V 60 0 40 0 50 Temperature (°C) −10 100 gm (mho) ICC (A) 7 9 600 −40°C 30 25°C 20 5 VSS (V) Figure 16. ISS vs. VSS Figure 15. Shutdown Delay vs. Temperature 40 3 1 85°C 550 500 10 0 450 10 VIN (V) 50 Temperature (°C) 100 Figure 18. Error Amplifier Transconductance vs. Temperature Figure 17. ICC vs. VIN During Shutdown 100 2.6 2.5 Current (A) 60 IOUT (A) 0 20 −20 2.4 2.3 2.2 2.1 −60 −255 −175 −125 −75 −25 VREF − VFB (mV) 0 2.0 25 0 50 Temperature (°C) 100 Figure 20. Switch Leakage vs. Temperature Figure 19. Error Amplifier IOUT vs. VFB http://onsemi.com 8 NCP5173 APPLICATIONS INFORMATION THEORY OF OPERATION The oscillator is trimmed to guarantee an 18% frequency accuracy. The output of the oscillator turns on the power switch at a frequency of 600 kHz, as shown in Figure 21. The power switch is turned off by the output of the PWM Comparator. A TTL−compatible sync input at the SS pin is capable of syncing up to 1.8 times the base oscillator frequency. As shown in Figure 22, in order to sync to a higher frequency, a positive transition turns on the power switch before the output of the oscillator goes high, thereby resetting the oscillator. The sync operation allows multiple power supplies to operate at the same frequency. A sustained logic low at the SS pin will shut down the IC and reduce the supply current. An additional feature includes frequency shift to 20% of the nominal frequency when the FB pin triggers the threshold. During power up, overload, or short circuit conditions, the minimum switch on−time is limited by the PWM comparator minimum pulse width. Extra switch off−time reduces the minimum duty cycle to protect external components and the IC itself. As previously mentioned, this block also produces a ramp for the slope compensation to improve regulator stability. Current Mode Control VCC Oscillator S VC − + Q L R D1 Power Switch VSW PWM Comparator In Out X5 CO Driver RLOAD SUMMER Slope Compensation 63 m Figure 21. Current Mode Control Scheme The NCP5173 incorporates a current mode control scheme, in which the PWM ramp signal is derived from the power switch current. This ramp signal is compared to the output of the error amplifier to control the on−time of the power switch. The oscillator is used as a fixed−frequency clock to ensure a constant operational frequency. The resulting control scheme features several advantages over conventional voltage mode control. First, derived directly from the inductor, the ramp signal responds immediately to line voltage changes. This eliminates the delay caused by the output filter and error amplifier, which is commonly found in voltage mode controllers. The second benefit comes from inherent pulse−by−pulse current limiting by merely clamping the peak switching current. Finally, since current mode commands an output current rather than voltage, the filter offers only a single pole to the feedback loop. This allows both a simpler compensation and a higher gain−bandwidth over a comparable voltage mode circuit. Without discrediting its apparent merits, current mode control comes with its own peculiar problems, mainly, subharmonic oscillation at duty cycles over 50%. The NCP5173 solves this problem by adopting a slope compensation scheme in which a fixed ramp generated by the oscillator is added to the current ramp. A proper slope rate is provided to improve circuit stability without sacrificing the advantages of current mode control. Error Amplifier VC 1.276 V + − FB 1M 120 pF Voltage Clamp C1 0.01 F R1 5 k positive error−amp Figure 23. Error Amplifier Equivalent Circuit The FB pin is directly connected to the inverting input of the positive error amplifier, whose non−inverting input is fed by the 1.276 V reference. The transconductance amplifier has a high output impedance of approximately 1.0 M, as shown in Figure 23. The VC pin is connected to the output of the error amplifier and is internally clamped between 0.5 V and 1.7 V. A typical connection at the VC pin includes a capacitor in series with a resistor to ground, forming a pole/zero for loop compensation. An external shunt can be connected between the VC pin and ground to reduce its clamp voltage. Consequently, the current limit of the internal power transistor current is reduced from its nominal value. Oscillator and Shutdown Sync Current Ramp VSW Figure 22. Timing Diagram of Sync and Shutdown http://onsemi.com 9 NCP5173 Switch Driver and Power Switch output through the inductor and diode. Once VCC reaches approximately 1.5 V, the internal power switch briefly turns on. This is a part of the NCP5173’s normal operation. The turn−on of the power switch accounts for the initial current swing. When the VC pin voltage rises above the threshold, the internal power switch starts to switch and a voltage pulse can be seen at the VSW pin. Detecting a low output voltage at the FB pin, the built−in frequency shift feature reduces the switching frequency to a fraction of its nominal value, reducing the minimum duty cycle, which is otherwise limited by the minimum on−time of the switch. The peak current during this phase is clamped by the internal current limit. When the FB pin voltage rises above 0.4 V, the frequency increases to its nominal value, and the peak current begins to decrease as the output approaches the regulation voltage. The overshoot of the output voltage is prevented by the active pull−on, by which the sink current of the error amplifier is increased once an overvoltage condition is detected. The overvoltage condition is defined as when the FB pin voltage is 50 mV greater than the reference voltage. The switch driver receives a control signal from the logic section to drive the output power switch. The switch is grounded through emitter resistors (63 m total) to the PGND pin. PGND is not connected to the IC substrate so that switching noise can be isolated from the analog ground. The peak switching current is clamped by an internal circuit. The clamp current is guaranteed to be greater than 1.5 A and varies with duty cycle due to slope compensation. The power switch can withstand a maximum voltage of 40 V on the collector (VSW pin). The saturation voltage of the switch is typically less than 1.0 V to minimize power dissipation. Short Circuit Condition When a short circuit condition happens in a boost circuit, the inductor current will increase during the whole switching cycle, causing excessive current to be drawn from the input power supply. Since control ICs don’t have the means to limit load current, an external current limit circuit (such as a fuse or relay) has to be implemented to protect the load, power supply and ICs. In other topologies, the frequency shift built into the IC prevents damage to the chip and external components. This feature reduces the minimum duty cycle and allows the transformer secondary to absorb excess energy before the switch turns back on. COMPONENT SELECTION Frequency Compensation The goal of frequency compensation is to achieve desirable transient response and DC regulation while ensuring the stability of the system. A typical compensation network, as shown in Figure 25, provides a frequency response of two poles and one zero. This frequency response is further illustrated in the Bode plot shown in Figure 26. IL VOUT VCC VC R1 VC NCP5173 C2 C1 GND Figure 24. Startup Waveforms of Circuit Shown in the Application Diagram. Load = 400 mA. Figure 25. A Typical Compensation Network The NCP5173 can be activated by either connecting the VCC pin to a voltage source or by enabling the SS pin. Startup waveforms shown in Figure 24 are measured in the boost converter demonstrated in the Application Diagram on the page 2 of this document. Recorded after the input voltage is turned on, this waveform shows the various phases during the power up transition. When the VCC voltage is below the minimum supply voltage, the VSW pin is in high impedance. Therefore, current conducts directly from the input power source to the The high DC gain in Figure 26 is desirable for achieving DC accuracy over line and load variations. The DC gain of a transconductance error amplifier can be calculated as follows: GainDC GM RO where: GM = error amplifier transconductance; RO = error amplifier output resistance ≈ 1.0 M. http://onsemi.com 10 NCP5173 The low frequency pole, fP1, is determined by the error amplifier output resistance and C1 as: 1 fP1 2C1R where: VF = output diode forward voltage. In the flyback topology, peak VSW voltage is governed by: VSW(MAX) VCC(MAX)(VOUTVF) N O The first zero generated by C1 and R1 is: where: N = transformer turns ratio, primary over secondary. When the power switch turns off, there exists a voltage spike superimposed on top of the steady−state voltage. Usually this voltage spike is caused by transformer leakage inductance charging stray capacitance between the VSW and PGND pins. To prevent the voltage at the VSW pin from exceeding the maximum rating, a transient voltage suppressor in series with a diode is paralleled with the primary windings. Another method of clamping switch voltage is to connect a transient voltage suppressor between the VSW pin and ground. 1 fZ1 2C1R1 The phase lead provided by this zero ensures that the loop has at least a 45° phase margin at the crossover frequency. Therefore, this zero should be placed close to the pole generated in the power stage which can be identified at frequency: 1 fP 2CORLOAD where: CO = equivalent output capacitance of the error amplifier ≈120 pF; RLOAD= load resistance. The high frequency pole, fP2, can be placed at the output filter’s ESR zero or at half the switching frequency. Placing the pole at this frequency will cut down on switching noise. The frequency of this pole is determined by the value of C2 and R1: Magnetic Component Selection When choosing a magnetic component, one must consider factors such as peak current, core and ferrite material, output voltage ripple, EMI, temperature range, physical size and cost. In boost circuits, the average inductor current is the product of output current and voltage gain (VOUT/VCC), assuming 100% energy transfer efficiency. In continuous conduction mode, inductor ripple current is: 1 fP2 2C2R1 V (V VCC) IRIPPLE CC OUT (f)(L)(VOUT) DC Gain One simple method to ensure adequate phase margin is to design the frequency response with a −20 dB per decade slope, until unity−gain crossover. The crossover frequency should be selected at the midpoint between fZ1 and fP2 where the phase margin is maximized. where: f = 560 kHz The peak inductor current is equal to average current plus half of the ripple current, which should not cause inductor saturation. The above equation can also be referenced when selecting the value of the inductor based on the tolerance of the ripple current in the circuits. Small ripple current provides the benefits of small input capacitors and greater output current capability. A core geometry like a rod or barrel is prone to generating high magnetic field radiation, but is relatively cheap and small. Other core geometries, such as toroids, provide a closed magnetic loop to prevent EMI. fP1 Gain (dB) fZ1 fP2 Input Capacitor Selection In boost circuits, the inductor becomes part of the input filter, as shown in Figure 28. In continuous mode, the input current waveform is triangular and does not contain a large pulsed current, as shown in Figure 27. This reduces the requirements imposed on the input capacitor selection. During continuous conduction mode, the peak to peak inductor ripple current is given in the previous section. As we can see from Figure 27, the product of the inductor current ripple and the input capacitor’s effective series resistance (ESR) determine the VCC ripple. In most applications, input capacitors in the range of 10 F to 100F with an ESR less than 0.3 work well up to a full 1.5 A switch current. Frequency (LOG) Figure 26. Bode Plot of the Compensation Network Shown in Figure 25 VSW Voltage Limit In the boost topology, VSW pin maximum voltage is set by the maximum output voltage plus the output diode forward voltage. The diode forward voltage is typically 0.5 V for Schottky diodes and 0.8 V for ultrafast recovery diodes: VSW(MAX) VOUT(MAX)VF http://onsemi.com 11 NCP5173 By examining the waveforms shown in Figure 29, we can see that the output voltage ripple comes from two major sources, namely capacitor ESR and the charging/ discharging of the output capacitor. In boost circuits, when the power switch turns off, IL flows into the output capacitor causing an instant V = IIN × ESR. At the same time, current IL − IOUT charges the capacitor and increases the output voltage gradually. When the power switch is turned on, IL is shunted to ground and IOUT discharges the output capacitor. When the IL ripple is small enough, IL can be treated as a constant and is equal to input current IIN. Summing up, the output voltage peak−peak ripple can be calculated by: VCC ripple IIN IL (I IOUT)(1 D) VOUT(RIPPLE) IN (COUT)(f) I D OUT IIN ESR (COUT)(f) Figure 27. Boost Input Voltage and Current Ripple Waveforms IL IIN VCC + − The equation can be expressed more conveniently in terms of VCC, VOUT and IOUT for design purposes as follows: I (V VCC) 1 VOUT(RIPPLE) OUT OUT (COUT)(f) (COUT)(f) CIN (I )(V )(ESR) OUT OUT VCC RESR The capacitor RMS ripple current is: IRIPPLE (IIN IOUT)2(1 D)(IOUT)2(D) IOUT Figure 28. Boost Circuit Effective Input Filter The situation is different in a flyback circuit. The input current is discontinuous and a significant pulsed current is seen by the input capacitors. Therefore, there are two requirements for capacitors in a flyback regulator: energy storage and filtering. To maintain a stable voltage supply to the chip, a storage capacitor larger than 20 F with low ESR is required. To reduce the noise generated by the inductor, insert a 1.0F ceramic capacitor between VCC and ground as close as possible to the chip. VCC VOUTVCC Although the above equations apply only for boost circuits, similar equations can be derived for flyback circuits. Reducing the Current Limit In some applications, the designer may prefer a lower limit on the switch current than 1.5 A. An external shunt can be connected between the VC pin and ground to reduce its clamp voltage. Consequently, the current limit of the internal power transistor current is reduced from its nominal value. The voltage on the VC pin can be evaluated with the equation: Output Capacitor Selection VC ISWREAV where: RE = .063 , the value of the internal emitter resistor; AV = 5.0 V/V, the gain of the current sense amplifier. Since RE and AV cannot be changed by the end user, the only available method for limiting switch current below 1.5 A is to clamp the VC pin at a lower voltage. If the maximum switch or inductor current is substituted into the equation above, the desired clamp voltage will result. VOUT ripple IL Figure 29. Typical Output Voltage Ripple http://onsemi.com 12 NCP5173 A simple diode clamp, as shown in Figure 30, clamps the VC voltage to a diode drop above the voltage on resistor R3. Unfortunately, such a simple circuit is not generally acceptable if VIN is loosely regulated. The improved circuit does not require a regulated voltage to operate properly. Unfortunately, a price must be paid for this convenience in the overall efficiency of the circuit. The designer should note that the input and output grounds are no longer common. Also, the addition of the current sense resistor, RSENSE, results in a considerable power loss which increases with the duty cycle. Resistor R2 and capacitor C3 form a low−pass filter to remove noise. VIN VCC R2 Subharmonic Oscillation Subharmonic oscillation (SHM) is a problem found in current−mode control systems, where instability results when duty cycle exceeds 50%. SHM only occurs in switching regulators with a continuous inductor current. This instability is not harmful to the converter and usually does not affect the output voltage regulation. SHM will increase the radiated EM noise from the converter and can cause, under certain circumstances, the inductor to emit high−frequency audible noise. SHM is an easily remedied problem. The rising slope of the inductor current is supplemented with internal “slope compensation” to prevent any duty cycle instability from carrying through to the next switching cycle. In the NCP5173, slope compensation is added during the entire switch on−time, typically in the amount of 180 mA/s. In some cases, SHM can rear its ugly head despite the presence of the onboard slope compensation. The simple cure to this problem is more slope compensation to avoid the unwanted oscillation. In that case, an external circuit, shown in Figure 32, can be added to increase the amount of slope compensation used. This circuit requires only a few components and is “tacked on” to the compensation network. VC D1 R3 R1 C1 C2 Figure 30. Current Limiting Using a Diode Clamp Another solution to the current limiting problem is to externally measure the current through the switch using a sense resistor. Such a circuit is illustrated in Figure 31. VCC PGND AGND VSW − + VIN VC VC R1 Q1 R2 VSW C1 C2 C3 RSENSE R1 Output Ground R2 C1 Figure 31. Current Limiting using a Current Sense Resistor C2 The switch current is limited to: VBE(Q1) ISWITCH(PEAK) RSENSE C3 where: VBE(Q1) = the base−emitter voltage drop of Q1, typically 0.65 V. R3 Figure 32. Technique for Increasing Slope Compensation http://onsemi.com 13 NCP5173 The dashed box contains the normal compensation circuitry to limit the bandwidth of the error amplifier. Resistors R2 and R3 form a voltage divider off of the VSW pin. In normal operation, VSW looks similar to a square wave, and is dependent on the converter topology. Formulas for calculating VSW in the boost and flyback topologies are given in the section “VSW Voltage Limit.” The voltage on VSW charges capacitor C3 when the switch is off, causing the voltage at the VC pin to shift upwards. When the switch turns on, C3 discharges through R3, producing a negative slope at the VC pin. This negative slope provides the slope compensation. The amount of slope compensation added by this circuit is: 1 R3 I V SW R R T 2 3 (1D) e R3C3fSW fSW (1 D)REAV VIN VCC SS SS VC D2 D1 R1 C1 C3 where: I/T = the amount of slope compensation added (A/s); VSW = the voltage at the switch node when the transistor is turned off (V); fSW = the switching frequency, typically 560 kHz; D = the duty cycle; RE = 0.063 , the value of the internal emitter resistor; AV = 5.0 V/V, the gain of the current sense amplifier. In selecting appropriate values for the slope compensation network, the designer is advised to choose a convenient capacitor, then select values for R2 and R3 such that the amount of slope compensation added is 100 mA/s. Then R2 may be increased or decreased as necessary. Of course, the series combination of R2 and R3 should be large enough to avoid drawing excessive current from VSW. Additionally, to ensure that the control loop stability is improved, the time constant formed by the additional components should be chosen such that: C2 Figure 33. Soft Start Resistor R1 and capacitors C1 and C2 form the compensation network. At turn on, the voltage at the VC pin starts to come up, charging capacitor C3 through Schottky diode D2, clamping the voltage at the VC pin such that switching begins when VC reaches the VC threshold, typically 1.05 V (refer to graphs for detail over temperature). VC VF(D2)VC3 Therefore, C3 slows the start−up of the circuit by limiting the voltage on the VC pin. The soft−start time increases with the size of C3. Diode D1 discharges C3 when SS is low. If the shutdown function is not used with this part, the cathode of D1 should be connected to VIN. R3C3 1 D fSW Calculating Junction Temperature Finally, it is worth mentioning that the added slope compensation is a trade−off between duty cycle stability and transient response. The more slope compensation a designer adds, the slower the transient response will be, due to the external circuitry interfering with the proper operation of the error amplifier. To ensure safe operation, the designer must calculate the on−chip power dissipation and determine its expected junction temperature. Internal thermal protection circuitry will turn the part off once the junction temperature exceeds 180°C ± 30°. However, repeated operation at such high temperatures will reduce operating life. Calculation of the junction temperature is an imprecise but simple task. First, the power losses must be quantified. There are three major sources of power loss on the NCP5173: • biasing of internal control circuitry, PBIAS • switch driver, PDRIVER • switch saturation, PSAT Soft Start Through the addition of an external circuit, a soft−start function can be added to the NCP5173. Soft−start circuitry prevents the VC pin from slamming high during start−up, thereby inhibiting the inductor current from rising at a high slope. This circuit, shown in Figure 33, requires a minimum number of components and allows the soft−start circuitry to activate any time the SS pin is used to restart the converter. http://onsemi.com 14 NCP5173 or junction−to−ambient thermal resistance. The on−chip junction temperature can be calculated if JA, the air temperature near the surface of the IC, and the on−chip power dissipation are known. The internal control circuitry, including the oscillator and linear regulator, requires a small amount of power even when the switch is turned off. The specifications section of this datasheet reveals that the typical operating current, IQ, due to this circuitry is 5.5 mA. Additional guidance can be found in the graph of operating current vs. temperature. This graph shows that IQ is strongly dependent on input voltage, VIN, and temperature. Then: TJ TA(PDJA) where: TJ = IC or FET junction temperature (°C); TA = ambient temperature (°C); PD = power dissipated by part in question (W); JA = junction−to−ambient thermal resistance (°C/W). PBIAS VINIQ Since the onboard switch is an NPN transistor, the base drive current must be factored in as well. This current is drawn from the VIN pin, in addition to the control circuitry current. The base drive current is listed in the specifications as ICC/ISW, or switch transconductance. As before, the designer will find additional guidance in the graphs. With that information, the designer can calculate: For the NCP5173, JA = 35°C/W. Once the designer has calculated TJ, the question of whether the IC can be used in an application is settled. If TJ exceeds 150°C, the absolute maximum allowable junction temperature, the NCP5173 is not suitable for that application. If TJ approaches 150°C, the designer should consider possible means of reducing the junction temperature. Perhaps another converter topology could be selected to reduce the switch current. Increasing the airflow across the surface of the chip might be considered to reduce TA. A copper “landing pad” can be connected to the ground pin − Designers are referred to ON Semiconductor applications note AND8036/D for more information on properly sizing a copper area. I PDRIVER VINISW CC D ISW where: ISW = the current through the switch; D = the duty cycle or percentage of switch on−time. ISW and D are dependent on the type of converter. In a boost converter, 1 ISW(AVG) ILOAD D Efficiency V VIN D OUT VOUT Circuit Layout Guidelines In any switching power supply, circuit layout is very important for proper operation. Rapidly switching currents combined with trace inductance generates voltage transitions that can cause problems. Therefore the following guidelines should be followed in the layout. In a flyback converter, V I 1 ISW(AVG) OUT LOAD VIN Efficiency D VOUT 1. N VOUT NSP VIN The switch saturation voltage, V(CE)SAT, is the last major source of on−chip power loss. V(CE)SAT is the collector−emitter voltage of the internal NPN transistor when it is driven into saturation by its base drive current. The value for V(CE)SAT can be obtained from the specifications or from the graphs, as “Switch Saturation Voltage.” Thus, PSAT V(CE)SATISW D Finally, the total on−chip power losses are: PD PBIASPDRIVERPSAT 2. Power dissipation in a semiconductor device results in the generation of heat in the junctions at the surface of the chip. This heat is transferred to the surface of the IC package, but a thermal gradient exists due to the resistive properties of the package molding compound. The magnitude of the thermal gradient is expressed in manufacturers’ data sheets as JA, 3. In boost circuits, high AC current circulates within the loop composed of the diode, output capacitor, and on−chip power transistor. The length of associated traces and leads should be kept as short as possible. In the flyback circuit, high AC current loops exist on both sides of the transformer. On the primary side, the loop consists of the input capacitor, transformer, and on−chip power transistor, while the transformer, rectifier diodes, and output capacitors form another loop on the secondary side. Just as in the boost circuit, all traces and leads containing large AC currents should be kept short. Separate the low current signal grounds from the power grounds. Use single point grounding or ground plane construction for the best results. Locate the voltage feedback resistors as near the IC as possible to keep the sensitive feedback wiring short. Connect feedback resistors to the low current analog ground. http://onsemi.com 15 NCP5173 MBRS140T3 VCC −12 V P6KE−15A + T1 1.0 F 22 F + 47 F GND VCC (5) GND 1N4148 + 1:2 47 F PGND (7) VSW (8) AGND (6) NCP5173 VC (1 ) +12 V MBRS140T3 FB (2) 47 nF 10.72 k 1.28 k 4.7 nF 2.0 k Figure 34. Additional Application Diagram, 2.7 to 13 V Input, 12 V/ 200 mA Output Flyback Converter GND VCC (5) VC (1 ) GND 5.0 k 2.2 F 1.1 k 22 F NCP5173 200 pF .01 F VIN Low ESR 15 H VSW (8) AGND (6) −5.0 VOUT FB (2) PGND (7) 300 Figure 35. Additional Application Diagram, −9.0 V to −28 V Input, −5.0 V/700 mA Output Inverted Buck Converter R1 R2 1.245 k/0.1 W, 1% 99.755 k/0.1 W, 1% GND C1 D1 C11 .01 R3 2.0 k 3 Test 4 SS VSW 8 PGND 7 NCP5173 C10 .1 1 V C 2 FB AGND VCC .1 50 V D1 D1 C3 .1 50 V D1 D1 .1 50 V D1 D1 100 VO 1N4148 1N4148 6 5 C2 C8 10 C9 .1 C7 .1 50 V C4 .1 50 V 1N4148 1N4148 1N4148 1N4148 1N4148 C5 .1 50 V C6 .1 50 V 4.0 V Figure 36. Additional Application Diagram, 4.0 V Input, 100 V/ 10 mA Output Boost Converter with Output Voltage Multiplier http://onsemi.com 16 GND NCP5173 PACKAGE DIMENSIONS 5x6 QFN MN SUFFIX CASE 505AC−01 ISSUE O D A B PIN ONE LOCATION 2X 0.15 C ÍÍÍ ÍÍÍ ÍÍÍ NOTES: 1. DIMENSIONING AND TOLERANCING PER ASME Y14.5M, 1994. 2. CONTROLLING DIMENSION: MILLIMETERS. 3. DIMENSION b APPLIES TO PLATED TERMINAL AND IS MEASURED BETWEEN 0.25 AND 0.30 mm FROM TERMINAL. 4. COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS. E DIM A A1 A2 A3 b D D2 E E2 e K L 0.15 C 2X 0.10 C (A3) A2 A 0.08 C A1 C SEATING PLANE D2 8X L e 1 4 E2 8X K 8 5 b 8X NOTE 3 0.10 C A B 0.05 C http://onsemi.com 17 MILLIMETERS MIN MAX 0.80 1.00 0.00 0.05 0.65 0.75 0.20 REF 0.35 0.50 6.00 BSC 3.95 4.25 5.00 BSC 2.95 3.25 1.27 BSC 0.20 −−− 0.45 0.55 NCP5173 ON Semiconductor and are registered trademarks of Semiconductor Components Industries, LLC (SCILLC). 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