CS5124/6 CS5124/6 High Performance, Integrated Current Mode PWM Controllers Features Description The CS5124/6 is a fixed frequency current mode controller designed specifically for DC-DC converters found in the telecommunications industry. The CS5124/6 integrates many commonly required current mode power supply features and allows the power supply designer to realize substantial cost and board space savings. The product matrix is as follows: CS5124: 400kHz w/VBIAS Pin, 195mV first current sense threshold CS5126: 200kHz w/SYNC Pin, 335mV first current sense threshold The CS5124/6 integrates the following features: Internal Oscillator, Slope Compensation, Sleep On/Off, Under Voltage Lock Out, Thermal Shutdown, Soft Start Timer, Low Voltage Current Sense for Resistive Sensing, Second Current Threshold for Pulse by Pulse Over Current Protection, a Direct Optocoupler Interface and Leading Edge Current Blanking. The CS5124/6 has supply range of 7.7V to 20V and is available in 8 pin SO narrow package. ■ Line UVLO Monitoring ■ Low Current Sense Voltage for Resistive Current Sensing ■ External Synchronization to Higher or Lower Frequency Oscillator (CS5126 Only) ■ Bias for Start up Circuitry (CS5124 Only) ■ Thermal Shutdown ■ Sleep On/Off Pin ■ Soft Start Timer ■ Leading Edge Blanking ■ Direct Optocoupler Interface ■ 90ns Propagation Delay Applications Diagram ■ 35ns Driver Rise and Fall Times ■ Sleep Mode 36-75VIN L1 10µH R1 510k R2 200k C1 0.1µF, 100V D1 CTX15-14514 T1 C2 1.5µF, 100V Q1 ZVN3310A R4 10Ω R5 17.4k MBRD360CT D4 VCC Gnd BIAS GATE UVLO Q2 IRFR220 C3 .022µF R6 1k R8 0.39Ω VCC R7 30.1k C5 47µF, 10V C6 C9 1000pF C7 0.1µF 1 Gnd BIAS GATE ISENSE UVLO U2 .01µF IS VFB SS CS5124 SS Package Options 8 Lead SO Narrow CS5124 R3 47Ω BAS16LT1 C4 0.47µF, 25V ENABLE 5VOUT VFB C8 1000pF CS5126 TPS5908 R9 10.0k VCC ISOLATED RTN 48VRTN 1 Gnd UVLO GATE SYNC ISENSE VFB SS 48V to 5V, 1A flyback converter using the CS5124 Cherry Semiconductor Corporation 2000 South County Trail, East Greenwich, RI 02818 Tel: (401)885-3600 Fax: (401)885-5786 Email: [email protected] Web Site: www.cherry-semi.com Rev. 3/12/99 1 A ® Company CS5124/6 Absolute Maximum Ratings Pin Symbol Lead Name VMAX VMIN ISOURCE ISINK 1.5A Peak 200mA DC 1mA 1mA 1mA 2mA 20mA 1mA 1mA VCC VCC Power Input 20V -0.3V 1mA SYNC (CS5126) VBIAS (CS5124) UVLO SS VFB ISENSE GROUND Clock Synchronization Input VCC Clamp Output UVLO Shutdown Input Soft Start Capacitor Input Voltage Feed Back Input Current Sense Input Ground 20V 20V 6V 6V 6V 6V 0V -0.3V -0.3V -0.3V -0.3V -0.3V -0.3V 0V GATE Gate Drive Output 20V -0.3V 1mA 1mA 1mA 1mA 3mA 1mA 1.5A peak 200mA DC 1.5A Peak 200mA DC 1.5A Peak 200mA DC Operating Junction Temperature, TJ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-40 to 135°C Storage Temperature Range, TS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .-40 to 150°C ESD (Human Body Model) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2kV ESD (Machine Model) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .200V Lead Temperature Soldering: Reflow (SMD styles only) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60 sec. max above 183°C, 230°C peak Electrical Characteristics: -40°C ≤ TJ ≤ 125°C, -40°C ≤ TA ≤ 105°C, 7.60V ≤ VCC ≤ 20V, UVLO = 3.0V, ISENSE = 0V, CV(CC) = 0.33µF, CGATE = 1nF (ESR = 10Ω), CSS = 470pF CV(FB) = 100pF, unless otherwise stated. PARAMETER TEST CONDITIONS MIN TYP MAX 10 500 210 13 750 275 mA µA µA 7.2 6.8 350 7.7 7.3 425 8.3 7.8 500 V V mV 1.5 (Note 1) 35 2.3 1.8 1.88 85 2.45 2.3 2.45 150 2.6 V V mV V (Note 1) 2.50 2.63 2.76 V Turn-on – Turnoff (-40°C ≤ TJ ≤ 100°C) (Note 1) Turn-on – Turnoff (100°C ≤ TJ ≤ 125°C) (Note 1) 170 185 200 mV 50 185 400 mV -1 5 7.5 1 12 µA V ■ General ICC Operating - VGATE not switching. ICC at VCC Low VCC = 6V ICC Sleep VUVL = 1V ■ Low VCC Lockout VCC Turn-on Threshold Voltage VCC Turn-off Threshold Voltage VCC Hysteresis ■ UVLO Sleep Threshold Voltage Sleep Threshold Voltage Sleep Hysteresis UVLO Turn-off Threshold Voltage UVLO Turn-on Threshold Voltage UVLO Hysteresis UVLO Hysteresis UVLO Input Bias Current UVLO Clamp UVLO decreasing UVLO increasing With UVLO sinking 1mA. 2 UNIT CS5124/6 Electrical Characteristics: -40°C ≤ TJ ≤ 125°C, -40°C ≤ TA ≤ 105°C, 7.60V ≤ VCC ≤ 20V, UVLO = 3.0V, ISENSE = 0V, CV(CC) = 0.33µF, CGATE = 1nF (ESR = 10Ω), CSS = 470pF CV(FB) = 100pF, unless otherwise stated. PARAMETER ■ VCC Clamp and BIAS Pin VCC Clamp Voltage BIAS Minimum Voltage BIAS Clamp TEST CONDITIONS TYP MAX CS5124 Only. Connect an NFET as follows: BIAS = G, VCC = S, VIN = D. 36V ≤ VIN ≤ 60V, 220nF ≤ 7.275 7.9 8.625 CSS ≤ 500nF, R = 500k Measure Voltage on BIAS with: 1.6 2.8 4 10V ≤ VCC ≤ 20V and 50µA ≤ IBIAS ≤ 1mA With BIAS pin sinking 1mA 12 15 20 ■ 200kHz Oscillator CS5126 Only Operating Frequency Max Duty Cycle Clamp Slope Compensation (Normal operation) Slope Compensation (Synchronized operation) (Note 1) SYNC Input Threshold Voltage SYNC Input Impedance Measured with SYNC = 1V &10V ■ 400kHz Oscillator Operating Frequency Max Duty Cycle Clamp Slope Compensation MIN UNIT V V V 175 78 12 200 82.5 18 225 85 23 kHz % mV/µs 7 1 50 12 2 120 16 3 230 mV/µs V kΩ 360 80.0 15 400 82.5 21 440 85.0 26 kHz % mV/µs 7 0.5 10 10.0 13 µA mA 1.40 1.62 1.80 V 4.7 200 4.9 275 400 V mV 170 250 60 195 275 90 215 315 130 mV mV ns 90 130 60 180 ns mV 300 485 335 525 360 575 mV mV 60 90 130 ns 110 175 125 210 ns mV CS5124 Only ■ Soft Start Soft Start Charge Current Soft Start Discharge Current VSS Voltage when VFB VFB = 300mV Begins to Rise Peak Soft Start Charge Voltage Valley Soft Start Discharge Voltage ■ Current Sense CS5124 Only First Current Sense Threshold At max duty cycle. Second Current Sense Threshold ISENSE to GATE Prop. Delay 0 to 700mV pulse into ISENSE (after blanking time) Leading Edge Blanking Time 0 to 400mV pulse into ISENSE Internal Offset (Note 1) ■ Current Sense First Current Sense Threshold Second Current Sense Threshold ISENSE to GATE Prop. Delay CS5126 Only At max duty cycle Leading Edge Blanking Time 0 to 800mV pulse into ISENSE (after blanking time) 0 to 550mV pulse into ISENSE Internal Offset (Note 1) 3 CS5124/6 Electrical Characteristics: -40°C ≤ TJ ≤ 125°C, -40°C ≤ TA ≤ 105°C, 7.60V ≤ VCC ≤ 20V, UVLO = 3.0V, ISENSE = 0V, CV(CC) = 0.33µF, CGATE = 1nF (ESR = 10Ω), CSS = 470pF CV(FB) = 100pF, unless otherwise stated. PARAMETER TEST CONDITIONS ■ Voltage Feedback VFB Pull-up Res. VFB Clamp Voltage VFB Clamp Voltage VFB Fault Voltage Threshold CS5124 Only CS5126 Only ■ Output Gate Drive Maximum Sleep Pull-down Voltage GATE High (AC) GATE Low (AC) GATE High Clamp Voltage Rise Time MIN TYP MAX 2.9 4.3 8.1 kΩ 2.63 2.40 2.90 2.65 3.15 2.90 V V 460 490 520 mV 1.2 2.0 V VCC-0.5 0.0 13.5 45 0.5 16.0 65 V V V ns 25 55 ns 150 125 25 165 150 35 °C °C °C VCC = 6.0V, IOUT = 1mA Series resistance < 1Ω (Note 1) Series resistance < 1Ω (Note 1) VCC = 20V Measure GATE rise time, 1V < GATE < 9V; VCC =12V Measure GATE fall time, 9V > GATE > 1V; VCC = 12V Fall Time ■ Thermal Shutdown Thermal Shutdown Temperature (Note 1) (GATE low) Thermal Enable Temperature (Note 1) (GATE switching) Thermal Hysteresis (Note 1) Notes 1. Not tested in production. Specification is guaranteed by design. VCC-1 11.0 135 100 15 UNIT Package Lead Description PACKAGE LEAD # LEAD SYMBOL 8 Lead SO Narrow CS5124 CS5126 1 1 2 - VCC BIAS - 3 SYNC 3 2 UVLO 4 4 SS 5 5 VFB 6 6 ISENSE 7 7 GATE 8 8 Gnd FUNCTION VCC Power Input Pin. VCC Clamp Output Pin. This pin will control the gate of an N-channel MOSFET that in turn regulates VCC. This pin is internally clamped at 15V when the IC is in sleep mode. Clock Synchronization Pin. A positive edge will terminate the current PWM cycle. Ground this pin when it is not used. Sleep and under voltage lockout pin. A voltage greater than 1.8V causes the chip to "wake up" however the GATE remains low. A voltage greater than 2.6V on this pin allows the output to switch. Soft Start Capacitor Pin. A capacitor placed between SS and GROUND is charged with 10µA and discharged with 10mA. The Soft Start capacitor controls both soft-start time and hiccup mode frequency. Voltage Feedback Pin. The collector of an optocoupler is typically tied to this pin. This pin is pulled up internally by a 4.3kΩ resistor to 5V and is clamped internally at 2.9V(2.65V). If VFB is pulled > 4V, the oscillator is disabled and GATE will stay high. If the VFB pin is pulled < 0.49V, GATE will stay low. Current Sense Pin. This pin is connected to the current sense resistor on the primary side. If VFB is floating, the GATE will go low if ISENSE = 195mV (335mV). If ISENSE > 275mV (525mV), Soft Start will be initiated. Gate Drive Output Pin. Capable of driving a 3nF load. GATE is nominally clamped to 13.5V. Ground Pin. 4 CS5124/6 Block Diagram {CS5126 ONLY} SYNC VCC VCC UVLO COMP VCC VREF = 5V G2 + F3 OSC DIS + Q R F1 RAMP - TSHUT {85 mV/us} 170mV us + V 490mV + + V 1.91 V/1.83V ÷ + SOFT START LATCH 2.9 R G5 S 2ND ICOMP R SS COMP 1000Ω ISENSE V BLANKING BLANK R + + Q SET DOMAIN - LINE AMP - VFB {525mV} 275mV - F2 VCC {1/5} 1/10 V {125mV} 60mV + 2.0V V + PWM COMP - - + + VFB COMP 10µA (CS5124 ONLY) 4500Ω - REMOTE (SLEEP) COMP + BIAS V5REF G3 V5REF V 2.62 V/2.45V UVLO GATE R G7 + 150°C/125°C + DRIVER Q RESET DOMAIN V5REF V+REFOK V S S ENABLE V 7.7 V/7.275V LINE UVLO COMP - G1 G6 {2.65V} 2.90V V5REF + + V + SS AMP - + 275mV V 1.32V + SS Gnd V Theory of Operation Powering the IC 10µA. As the capacitor charges, a buffered version of the capacitor voltage appears on the VFB pin and the VFB voltage begins to rise. As VFB rises the duty cycle increases until the supply comes into regulation. VCC can be powered directly from a regulated supply and requires 500µA of start-up current. The CS5124/6 includes a line bias pin (BIAS) that can be used to control a series pass transistor for operation over a wide input voltage. The BIAS pin will control the gate voltage of an Nchannel MOSFET placed between VIN and VCC to regulate VCC at 8V. Soft Start Soft Start is accomplished by clamping the VFB pin 1.32V below the SS pin during normal start up and during restart after a fault condition. When the CS5124/6 starts, the Soft Start capacitor is charged from a 10µA source from 0V to 4.9V. The VFB pin follows the Soft Start pin offset by –1.32V until the supply comes into regulation or until the Soft Start error amp is clamped at 2.9V (2.65V for the CS5126). During fault conditions the Soft Start capacitor is discharged at 10mA. VCC and UVLO Pins The UVLO pin has three different modes; low power shutdown, Line UVLO, and normal operation. To illustrate how the UVLO pin works; assume that VIN, as shown in the application schematic, is ramped up starting at 0V with the UVLO pin open. The SS and ISENSE pins also start at 0V. While the UVLO is below 1.8V, the IC will remain in a low current sleep mode and the BIAS pin of the CS5124 is internally clamped to a maximum of 15V. When the voltage on the UVLO pin rises to between 1.8V and 2.6V the reference for the VCC UVLO is enabled and VCC is regulated to 8V by the BIAS pin (CS5124 only), but the IC remains in a UVLO state and the output driver does not switch. When the UVLO pin exceeds 2.6V and the VCC pin exceeds 7.7V, the GATE pin is released from a low state and can begin switching based on the comparison of the ISENSE and VFB pins. The Soft Start capacitor begins charging from 0V at Fault Conditions The CS5124/6 recognizes the following faults: UVLO off, Thermal Shutdown, VREF(OK), and Second Current Threshold. Once a fault is recognized, fault latch F2 is set and the IC immediately shuts down the output driver and discharges the Soft Start capacitor. Soft Start will begin only after all faults have been removed and the Soft Start capacitor has been discharged to less than 0.275V. Each fault will be explained in the following sections. 5 CS5124/6 Theory of Operation: continued Under Voltage Lockout (UVLO) for the CS5126) threshold. If the ISENSE voltage exceeds the second threshold, F2 is set, the driver turns off, and the soft-start capacitor discharges. After the Soft Start capacitor has discharged to less than 0.275V Soft Start will begin. If the fault condition has been removed the supply will operate normally. If the fault remains the supply will operate in hiccup mode until the fault condition is removed. The UVLO pin is tied to typically the midpoint of a resistive divider between VIN and GROUND. During a start up sequence, this pin must be above 2.6V in order for the IC to begin normal operation. If the IC is running and this pin is pulled below 1.8V, F2 shuts down the output driver and discharges the Soft Start capacitor in order to insure proper start-up. If the UVLO pin is pulled high again before the Soft Start capacitor discharges, the IC will complete the Soft Start discharge and, if no other faults are present, will immediately restart the power supply. If the UVLO pin stays low, then it will enter either the low current sleep mode or the UVLO state depending on the level of the UVLO pin. VFB Comparator The VFB comparator detects when the output voltage is too high. When the regulated output voltage is too high, the feedback loop will drive VFB low. If VFB is less than 0.49V the output of the VFB comparator will go high and shut the output driver off. Oscillator Thermal Shutdown The internally trimmed, 400kHz (CS5124) or 200kHz (CS5126) provides the slope compensation ramp as well as the pulse for enabling the output driver. If the IC junction temperature exceeds approximately 150°C the thermal shutdown circuit sets F2, which shuts down the output driver and discharges the Soft Start capacitor. If no other faults are present the IC will initiate Soft Start when the IC junction temperature has been reduced by 25°C. PWM Comparator and Slope Compensation The CS5124/6 provides a fixed internal slope compensation ramp that is subtracted from the feedback signal. The pwm comparator compares peak primary current to a portion of the difference of the feedback voltage and slope compensation ramp. The 170mV/µs (85mV/µs for the CS5126) slope compensation ramp is subtracted from the voltage feedback signal internally. The difference signal is then divided by ten (five for the CS5126) before the PWM comparator to provide high noise rejection with a low voltage across the current sense network. (The effective ramp is 21mV/µs for the CS5124, and 18mV/µs for the CS5126). A 60mV (125mV for the CS5126) nominal offset on the positive input to the PWM comparator allows for operation with the ISENSE pin at, or even slightly below Gnd. VREF(OK) VREF(OK) is an internal monitor that insures the internal regulator is running before any switching occurs. This function does not trip the fault comparator like the other fault functions. To insure that Soft Start will occur at low line conditions the UVLO divider should be set up so that the VCC UVLO comparator turns on before the LINE UVLO comparator. Second Threshold Comparator Since the maximum dynamic range of the ISENSE signal in normal operation is 195mV (335mV for the CS5126), any voltage exceeding this threshold on the ISENSE pin is considered a fault and the PWM cycle is terminated. The 2nd ICOMP compares the ISENSE signal with a 275mV (525mV A 4.3kΩ pull-up resistor internally connected to a 5V nominal reference provides the bias current to for an opto-coupler connection to the VFB pin. Application Information UVLO and Thermal Shutdown Interaction BIAS Pin (CS5124 Only) The UVLO pin and thermal shutdown circuit share the same internal comparator. During high temperature operation (TJ >100°C) the UVLO pin will interact with the thermal shutdown circuit. This interaction increases the turnon threshold (and hysteresis) of the UVLO circuit. If the UVLO pin shuts down the IC during high temperature operation, higher hysteresis (see hysteresis specification) might be required to enable the IC. The bias pin can be used to control VCC as shown in the main application diagram. In order to provide adequate phase margin for the bias control loop, the pole created by the series pass transistor and the VCC bypass capacitor should be kept above 10kHz. The frequency of this pole can be calculated by Formula (1). Pole Transconductance of pass Transistor Frequency = 2 × π × CV(CC) 6 (1) The Line BIAS pin shows a significant change in the regulated VCC voltage when sinking large currents. This will show up as poor line regulation with a low value pull-up resistor. Typical regulated VCC vs BIAS pin sink current is shown in Figure 1. the rising edge of the Gate is shown in Figure 4. When this pin is held high or low the internal clock determines the oscillator frequency. SYNC 8.3 OSC GATE VCC 8.2 Figure 3. Synchronized Operation 8.1 8 140 7.9 130 5µ 10µA 20µA 50µA 100µA 200µA Phase Lag° Bias Current (IBIAS) Figure 1. Regulated VCC vs BIAS Sink Current Clock Synchronization Pin (CS5126 Only) 120 110 100 90 The CS5126 can be synchronized to signals ranging from 30% slower to several times faster than the internal oscillator frequency. If the part is synchronized to a fast signal, maximum duty cycle will be reduced as the frequency increases as shown in Figure 2. 80 70 200kHz 300kHz 400kHz 500kHz 600kHz Figure 4 : Typical Phase Lag between SYNC and GATE on. 0.82 Rail to rail gate driver operation can be obtained (up to 13.5V) over a range of MOSFET input capacitance if the gate resistor value is kept low. Figure 5 shows the high gate drive level vs. the series gate resistance with VCC = 8V driving an IRF220. 125°C 25°C 0.77 0.72 200kHz -40°C 300kHz 400kHz Frequency 500kHz 8.5 600kHz 8 Peak Voltage Maximum Duty Cycle Gate Drive Figure 2: CS5126 Maximum Duty Cycle vs Frequency (Synchronized Operation) 7.5 7 6.5 If the converter is initially free running and a sync signal is applied, the current oscillator cycle will terminate and the oscillator will lock on to the sync signal. The SYNC pin works with a positive edge triggered signal. When the sync signal transitions high the current PWM cycle terminates and a new cycle begins as shown in Figure 3. The typical phase lag between the rising edge of the SYNC signal and 6 0 0.3 0.5 2.5 5 11 Gate Resistor Value Figure 5. Gate Drive vs Gate Resistor Driving an IRF220 (VCC = 8V) 7 CS5124/6 Application Information: continued CS5124/6 Application Information: continued after a second threshold over-current condition will primarily be determined by the time required to charge the Soft Start cap from 0.275V nominal to 1.32V. A large negative dv/dt on the power MOSFET drain will couple current into the gate driver through the gate to drain capacitance. If this current is kept within absolute maximum ratings for the GATE pin it will not damage the IC. However if a high negative dv/dt coincides with the start of a PWM duty cycle, there will be small variations in oscillator frequency due to current in the controller substrate. If required, this can be avoided by choosing the transformer ratio and reset circuit so that a high dv/dt does not coincide with the start of a PWM cycle, or by clamping the negative voltage on the GATE pin with a schottky diode The second threshold will only be reached when a high dv/dt is present at the current sense pin. The signal must be fast enough to reach the second threshold before the first threshold turns off the driver. This will normally happen if the forward inductor saturates or when there is a shorted load. Excessive filtering of the current sense signal, a low value current sense resistor, or even an inductor that does not saturate during heavy output currents can prevent the second threshold from being reached. In this case the first current sense threshold will trip during each cycle of high output current conditions. The first threshold will limit output current but some components, especially the output rectifier, can overheat due to higher than normal average output current. First Current Sense Threshold During normal operation the peak primary current is controlled by the level of the VFB pin (as determined by the control loop) and the current sense network. Once the signal on the ISENSE pin exceeds the level determined by VFB pin the pwm cycle terminates. During high output currents the VFB pin will rise until it reaches the VFB clamp. The first current sense threshold determines the maximum signal allowed on the ISENSE pin before the PWM cycle is terminated. Under this condition the maximum peak current is determined by the VFB Clamp, the slope compensation ramp, the PWM comparator offset voltage and the PWM on time. The nominal first current threshold varies with on time and can be calculated from Formulas (2) & (3) below. CS5124 2.9V – 170mV/µs × TON 1st Threshold = – 60mV 10 Slope Compensation Current mode converters operating at duty cycles in excess of 50% require an artificial ramp to be added to the current waveform or subtracted from the feedback waveform. For the current loop to be stable the artificial ramp must be equivalent to at least 50% of the inductor current down slope and is typically chosen between 75 % to 100% of the inductor down current down slope. To choose an inductor value such that the internal slope compensation ramp will be equal to a certain fraction of the inductor down current slope use the Formula (4). (2) NSECONDARY 1 × (VOUT + VRECTIFIER) × × NPRIMARY Internal Ramp CS5126 2.65V – 85mV/µs × TON 1st Threshold = – 125mV 5 RI(SENSE) × Slope Value Factor = Inductor Value (H) (3) (4) Calculating the nominal inductor value for an artificial ramp equivalent to 100% of the current inductor down slope at CS5126 nominal conditions, a 5V output, a 200mΩ current sense resistor and a 4:1 transformer ratio yields When the output current is high enough for the ISENSE pin to exceed the first threshold, the pwm cycle terminates early and the converter begins to function more like a current source. The current sense network must be chosen so that the peak current during normal operation does not exceed the first current sense threshold. 1 20mV/µs × (5V + 0.3V) × 1 × 0.2Ω × 1 = 13.2 µH 4 To check that the slope compensation ramp will be greater than 50% of the inductor down under all conditions, substitute the minimum internal slope compensation value and use 0.5 for the slope compensation value. Then check that the actual inductor value will always be greater than the inductor value calculated. Second Current Sense Threshold The second threshold is intended to protect the converter from over-heating by switching to a low duty cycle mode when there are abnormally high fast rise currents in the converter. If the second current sense threshold is tripped, the converter will shut off and restart in Soft Start mode until the high current condition is removed. The dead time During synchronized operation of the CS5126 the slope compensation ramp is reduced by 33%. If the CS5126 will 8 be used in synchronized operation, the inductor value should be recalculated to work with the slope compensation ramp reduced to 67% of the normal value. converter and performs the same function as the flyback winding above. A flyback winding from a forward transformer can also be used to power VCC. Ideally the transformer volt-second product of a forward converter would be constant over the range of line voltages and load currents; and the transformer inductance could be chosen to store the required level of energy during each cycle to power VCC. Even though the flyback energy is not directly regulated it would remain constant. Unfortunately in a real converter there are many non-ideal effects that degrade regulation. Transformer inductance varies, converter frequency varies, energy stored in primary leakage inductance varies with output current, stray transformer capacitances and various parasitics all effect the level of energy available for VCC. If too little energy is provided to VCC, the bootstrapping circuit must provide power and efficiency will be reduced. If too much energy is provided VCC rises and may damage the controller. If this approach is taken the circuit must be carefully designed and component values must be controlled for good regulation. Powering the CS5124/6 from a Transformer Winding There are numerous ways to power the CS5124/6 from a transformer winding to enable the converter to be operated at high efficiency over a wide input range. Two ways are shown in the application circuits. The CS5124 application circuit (main application diagram) is a flyback converter that uses a second flyback winding to power VCC. R4 improves VCC regulation with load changes by snubbing the turn off spike. Once the turn off spike has subsided the voltage of this winding is voltage proportional to the voltage on the main flyback winding. This voltage is regulated because the main winding is clamped by the regulated output voltage. In the CS5126 application circuit (below) an extra winding is added to the forward inductor to power VCC. This winding is phased to conduct during the off time of the forward Additional Application Diagram 36-75VIN CTX15-14526 L1 T1 CTX15-14527 10µH C3 0.2µF, 100V R2 200k C2 1.5µF, 100V 5VOUT R1 39k T2 Q1 F2T493 C1 1.5µF, 100V R6 17.4k D3 11V C5 1µF, 25V ENABLE 10k C4 1000pF R10 10k C11 0.1µF R4 0.2Ω 1/4W C6 390pF MMBD6100L VCC Gnd UVLO GATE IS SYNC SS R9 SYNC MBRB2060CT Q2 IRF634 D2 C12 .01µF R7 2k R3 30.1k C8 47µF C7 47µF C9 U2 .01µF VFB CS5126 C10 1000pF TPS5908 R8 10.0k ISOLATED RTN 48VRTN 48V to 5V, 5A forward converter using the CS5126 9 CS5124/6 Application Information: continued CS5124/6 Package Specification PACKAGE DIMENSIONS IN mm (INCHES) PACKAGE THERMAL DATA Thermal Data RΘJC typ RΘJA typ D Lead Count 8 Lead SO Narrow Metric Max Min 5.00 4.80 English Max Min .197 .189 8L SO Narrow 45 165 ˚C/W C/W Surface Mount Narrow Body (D); 150 mil wide 4.00 (.157) 3.80 (.150) 6.20 (.244) 5.80 (.228) 0.51 (.020) 0.33 (.013) 1.27 (.050) BSC 1.75 (.069) MAX 1.57 (.062) 1.37 (.054) 1.27 (.050) 0.40 (.016) 0.25 (.010) 0.19 (.008) D 0.25 (0.10) 0.10 (.004) REF: JEDEC MS-012 Ordering Information Part Number CS5124XD8 CS5124XDR8 CS5126XD8 CS5126XDR8 Rev. 3/12/99 Description 8 Lead SO Narrow 8 Lead SO Narrow (tape & reel) 8 Lead SO Narrow 8 Lead SO Narrow (tape & reel) Cherry Semiconductor Corporation reserves the right to make changes to the specifications without notice. Please contact Cherry Semiconductor Corporation for the latest available information. 10 © 1999 Cherry Semiconductor Corporation