ML4880

me ed Com °C to 70°C ent Extend –20 Equipm ndheld ble Ha ta for Por RING ange ATU Temperature R FE rcial July 2000 ML4880* Portable PC/PCMCIA Power Controller GENERAL DESCRIPTION The ML4880 Portable PC and PCMCIA Power Controller is a complete solution for DC/DC power conversion for portable computing systems with single or multiple PCMCIA slots. The device provides two synchronous buck controllers to implement mixed voltage systems and a flyback controller for 12V VPP generation for PCMCIA slots. The flyback architecture enables generation of high currents (150mA or more per slot) on the 12V bus for multiple slot PCMCIA applications. Each regulator can be independently switched off to fully isolate the load from the power supply. The PFM architecture will automatically adjust switching frequency at light loads in order to maintain power conversion efficiencies in excess of 90% over a wide output power range. FEATURES s s Regulation to ± 3% maximum: provides 2% PCMCIA switch matrix margin Two synchronous buck controllers for 3.3/3V, 5V generation, and a flyback for high current, 12V generation from 5.5V to 18V input Regulator power conversion efficiencies > 90% Pulse frequency modulation for high efficiency operation Independent regulator shutdown for full load isolation Adjustable current limit Wide input voltage range (5.5V to 18V) s s s s s (* Indicates Part is End Of Life as of July 1, 2000) BLOCK DIAGRAM VIN 15 24 VREG CLAMP GATE CLAMP CLAMP A P DRV A 2 16 VIN BUCK REGULATOR A BIAS CIRCUITS 13 + N DRV A ISENSE A PWR GND A 23 14 4 VIN 5.5 – 18V 6 GND VOUT A 2.5 – 3.5V 7 GND SHDN A SHDN B SHDN C SHUTDOWN LOGIC – VFB A 9 TO SYSTEM POWER MANAGEMENT 11 18 10 VIN CLAMP GATE CLAMP CLAMP B 1 P DRV B 19 FLYBACK REGULATOR C + 17 N DRV C ISENSE C BUCK REGULATOR B + N DRV B ISENSE B PWR GND B 22 20 5 3 VOUT C 6V – 15V 12 VOUT B 4.5 – 5.0V VFB C PWR GND C – VFB B 8 – 21 1 ML4880 PIN CONFIGURATION ML4880 24-Pin SOIC (S24) CLAMP B CLAMP A ISENSE C ISENSE A ISENSE B GND GND VFB B VFB A SHDN C SHDN A VFB C 1 2 3 4 5 6 7 8 9 10 11 12 24 23 22 21 20 19 18 17 16 15 14 13 CLAMP PWR GND A PWR GND B PWR GND C NDRV B PDRV B SHDN B NDRV C VIN VREG NDRV A PDRV A TOP VIEW PIN DESCRIPTION PIN# NAME FUNCTION PIN# NAME FUNCTION 1 2 3 4 5 6 7 8 9 CLAMP B CLAMP A ISENSE C ISENSE A ISENSE B GND GND VFB B VFB A Gate clamp, regulator B Gate clamp, regulator A Current sense, regulator C Current sense, regulator A Current sense, regulator B Ground Ground Feedback node, buck regulator B Feedback node, buck regulator A Shutdown input, regulator C Shutdown input, regulator A Feedback node, regulator C 13 PDRV A 14 NDRV A 15 VREG 16 VIN 17 NDRV C 18 SHDN B 19 PDRV B 20 NDRV B P-channel drive, regulator A N-channel drive, regulator A Linear regulator output Power supply input voltage N-channel drive, regulator C Shutdown input, regulator B P-channel drive, regulator B N-channel drive, regulator B 21 PWR GND C Power Ground, regulator C 22 PWR GND B Power Ground, regulator B 23 PWR GND A Power Ground, regulator A 24 CLAMP Charge pump capacitor input 10 SHDN C 11 SHDN A 12 VFB C 2 ML4880 ABSOLUTE MAXIMUM RATINGS Absolute maximum ratings are those values beyond which the device could be permanently damaged. Absolute maximum ratings are stress ratings only and functional device operation is not implied. VIN ............................................................................................... 20V Peak Driver Output Current ........................................ 2A VFB Voltage ......................................... GND – 0.3V to 6V ISENSE Voltage ..................................................... ± 500mV All Other Analog Inputs ......... GND – 0.3V to VIN + 0.3V All Digital Inputs .................. GND – 0.3V to VREG + 0.3V Junction Temperature ............................................. 150°C Storage Temperature Range .................... –65°C to 150°C Lead Temperature (Soldering, 10 sec) ..................... 150°C Thermal Resistance (θJA) ...................................... 80°C/W OPERATING CONDITIONS VIN Range ..................................................... 5.5V to 18V Temperature Range ML4880CS ................................................ 0°C to 70°C ML4880ES ............................................. –20°C to 70°C ELECTRICAL CHARACTERISTICS Unless otherwise specified, VIN = 14V, C(VREG) = 10µF, TA = Operating Temperature Range (Note 1) PARAMETER Shutdown Inputs Input Low Voltage Input High Voltage Input Bias Current Buck Regulator A Duty Cycle Ratio VFB A Threshold Voltage ISENSE A Threshold Voltage Transition Time Buck Regulator B Duty Cycle Ratio VFB B Threshold Voltage ISENSE B Threshold Voltage Transition Time Flyback Regulator C Duty Cycle Ratio VIN = 5.5V, ISENSEC = VFBC = 0V VIN = 18V, ISENSEC = VFBC = 0V VFB C Threshold Voltage ISENSE C Threshold Voltage Transition Time Supply Linear Regulator Output Voltage Linear Regulator Load Regulation VIN Current 7V ≤ VIN ≤ 18V I(VREG) = 0 to 10mA SHDN A/B/C = 5V SHDN A/B/C = 0V Note 1: Limits are guaranteed by 100% testing, sampling or correlation with worst case test conditions. CONDITIONS MIN TYP MAX UNITS 0.8 2.0 –1 1 V V µA VIN = 5.5V, ISENSE A = VFB A = 0V 86 1.21 –140 1.25 –200 80 96 1.29 –250 100 % V mV ns CL = 1000pF, 0 to VREG VIN = 5.5V, ISENSE B = VFB B = 0V 92 1.21 –140 CL = 1000pF, 0 to VREG 86 61 1.21 –140 CL = 1000pF, 0 to VREG 6.25 6.10 98 1.25 –200 80 1.29 –250 100 % V mV ns 94 77 1.25 –200 80 1.29 –250 100 % % V mV ns 6.99 6.99 300 350 350 400 V V µA µA 3 ML4880 FUNCTIONAL DESCRIPTION The ML4880 converts a 5.5V to 18V input to three outputs via two synchronous buck controllers and a flyback controller. The two buck controllers utilize a unique current mode PFM control architecture that generate output voltages in the range of 2.5V to 3.5V (output A), and 4.5V to 5V (output B). The output current is set by external components, and can exceed 2A on each supply. The flyback controller also uses a current mode PFM control scheme and can be used to generate a 6V to 15V output. Again, the output current of the flyback is dependent on external components, and output currents of 500mA are obtainable. Even at light loads, the PFM architecture maintains high conversion efficiencies over a wide range of input voltages. If it is necessary to further extend battery life, the user can shutdown and fully disconnect each load from the input independently. BIAS CIRCUITS The bias circuits are comprised of a linear regulator and a precision voltage reference. The linear regulator produces a supply voltage (VREG) used by the control circuits. The VREG pin should be bypassed to GND with a 4.7µF to 10µF capacitor. The precision voltage reference is used by the feedback circuit of each controller to maintain an accurate output voltage. SHUTDOWN LOGIC Each controller has a separate shutdown pin. By applying a logic high to the appropriate pin, the transconductance amplifier and current comparator of each controller (shown in Figures 1 and 3) can be disabled. This prevents switching from occurring and disconnects the load from the input. All other circuitry within the ML4880 remains active during shutdown. GATE CLAMP The gate clamp circuit provides a method of preventing the buck regulator P-channel MOSFET switches from accidentally turning on when the input is suddenly switched to a higher voltage. This condition can occur during start-up or when an adapter voltage is applied. As shown in the block diagram, the P DRV drivers are capacitively coupled to the gates. Assume that P DRV is in the OFF state, or at VREG, and the gate voltage is held at VIN through the resistor. The gatesource voltage of the MOSFET is 0V and the switch stays off. If a higher input voltage (VIN + 5V for example) is suddenly applied with P DRV still in the OFF state, the source voltage of the MOSFET would jump up to VIN + 5V, but the gate would still be at VIN. The gate-source voltage becomes –5V and the P-channel MOSFET would turn on even though P DRV is still in the OFF state. In order to prevent the MOSFET from turning on, the gate clamp circuit senses the increase in the input voltage and pumps charge into the gate side of the coupling capacitor, quickly charging the capacitor to the new input voltage level. The CLAMP pin requires a capacitor to GND in order for the gate clamp circuit to function properly. The capacitor value should be 1.5 to 2 times the value used for the coupling capacitor. BUCK CONTROLLERS A block diagram of the buck controllers is shown in Figure 1. The circuit utilizes a constant ON-time PFM control architecture. The circuit determines the OFF-time by waiting for the inductor current to drop to a level set by the feedback voltage (VFB). VIN TON CIN OSCILLATOR/ ONE SHOT P DRV L SHOOT THRU PROTECTION VOUT COUT N DRV SYNCHRONOUS RECTIFIER COMPARATOR VSR Rgm VC CURRENT COMPARATOR + – TRANSCONDUCTANCE AMPLIFIER – + 1.25V VFB + – –18mV ISENSE IL RSENSE R1 R2 Figure 1. Buck Controller Functional Diagram 4 ML4880 The oscillator/one shot block generates a constant ONtime and a minimum OFF-time. The OFF-time is extended for as long as the output of the current comparator stays low. Note that the inductor current flows in the current sense resistor during the OFF-time. Therefore, a minimum OFF-time is required to allow for the finite circuit delays in sensing the inductor current. The ON-time is triggered when the current comparator’s output goes high. However, unlike conventional fixed ON-time controllers, the ML4880’s one shot has an inverse relationship with the input voltage as shown in Figure 4. Figure 5 plots the inductor voltage-ON-time product. Note that the voltsecond product is nearly constant at voltages above 7V input. This results in an inductor current ripple of: ∆IL = TON × (VIN − VOUT ) L which in turn raises the inductor current trip level, shortening the OFF-time. At some level of increasing the output load, the transconductance amplifier can no longer continue to increase its output current. When this occurs, the voltage across Rgm reaches a maximum and the inductor current cannot increase. If the inductor current tries to increase, the voltage developed across the current sense resistor would become more negative, causing the non-inverting input of the current comparator to be negative, which extends the OFF-time and reduces the inductor current. When the output voltage is too high, the transconductance amplifier’s output current will eventually become negative. However, since the inductor current flows in only one direction (assuming no shoot through current) the non-inverting input of the current comparator will also stay negative. This extends the OFFtime allowing the inductor current to decrease to zero and causing the converter to stop operation until the output voltage drops enough to increase the output current of the transconductance amp above zero. In summary, the three operation modes can be defined by the voltage at the ISENSE pin at the end of the OFF-time: VSENSE ≥ 0V 0V > VSENSE > –140mV Discontinuous current mode Continuous current mode (1) It is important to note that the ripple current does not vary in proportion with VIN, but remains nearly constant over a wide input voltage range. The transconductance amplifier generates a current from the voltage difference between the reference and the feedback voltage, VFB. This current produces a voltage across Rgm that adds to the negative voltage that is developed across the current sense resistor. When the current level in the inductor drops low enough (a less negative sense voltage) to cause the voltage at the noninverting input of the current comparator to go positive, the comparator trips and starts a new ON cycle. In other words, the current programming comparator controls the length of the OFF-time by waiting until the inductor current decreases to a value determined by the transconductance amplifier. This technique allows the feedback transconductance amplifier’s output current to steer the current level in the inductor. The higher the transconductance amplifier’s output current, the higher the inductor current. For example, when the output voltage drops due to a load increase, the transconductance amplifier will increase its output current and generate a larger voltage across Rgm, –140mV > VSENSE > –250mV Current limit The synchronous rectifier comparator and the two NOR gates make up the synchronous rectifier control circuit. The synchronous control does not influence the operation of the main control loop, and operation with a Schottky diode in place of the synchronous rectifier is possible, but at a lower conversion efficiency. The synchronous rectifier (N DRV) is turned on during the minimum OFF-time or whenever the ISENSE pin goes below –18mV. N DRV will remain on until a new ON-time is started or until the ISENSE pin goes above –18mV. When the ISENSE pin goes above –18mV, the current in the inductor has gone to zero VC TON TOFF(MIN) VSR IL Figure 2. One Shot and Synchronous Rectifier Timing Diagram 5 ML4880 or the buck regulator is operating in a discontinuous current mode (DCM). Therefore, the synchronous rectifier comparator is used only for DCM operation. A timing diagram is shown in Figure 2. FLYBACK CONTROLLER A block diagram of the flyback regulator is shown in Figure 3. The circuit also utilizes a constant ON-time PFM control architecture. As with the buck controllers, the flyback controller determines the OFF-time by waiting for the inductor current to drop to a level set by the feedback voltage VFB. As a result of the control scheme, the valley of the inductor current ripple is programmed instead of the peak, and features an automatic transition into and out of burst mode which does not require additional control circuitry. The flyback controller’s oscillator/one shot block has a constant ON-time and a minimum OFF-time similar to that of the buck controllers. The OFF-time is extended for as long as the current comparator’s output stays low. Again, since the inductor current flows in the current sense resistor during the OFF-time, a minimum OFF-time is required to allow for the finite circuit delays in sensing the inductor current. The ON-time is triggered when the current comparator’s output goes high. However, unlike conventional fixed ON-time controllers, the ML4880’s one shot has an inverse relationship with the input voltage as shown in Figure 4. Figure 5 plots the inductor voltageON-time product. Note that the volt-second product is nearly constant over the input voltage range. This results in an inductor current ripple of: It is important to note that the ripple current does not vary in proportion with VIN, but remains nearly constant over a wide input voltage range. The transconductance amplifier and current comparator operate in the same manner as the buck regulator. The current programming comparator controls the length of the OFF-time by waiting until the inductor current decreases to a value determined by the transconductance amplifier before starting a new ON-time. Like the buck controllers, when the flyback’s output voltage drops due to a load increase, the transconductance amplifier will increase its output current and generate a larger voltage across Rgm, which in turn causes the current comparator’s output to go positive earlier or shortens the OFF-time. Conversely, when the output voltage is too high, the transconductance amplifier’s output current will eventually become negative. This holds the current comparator’s output low and extends the OFF-time causing the converter to stop operation until the output voltage drops enough to increase the output current of the transconductance amp above zero. In summary, the flyback has three operation modes that can be defined by the voltage at the SENSE pin at the end of the OFF-time: VSENSE ≥ 0V 0V > VSENSE > –140mV Discontinuous current mode Continuous current mode T × VIN ∆IL = ON L –140mV > VSENSE > –250mV Current limit (2) VIN OSCILLATOR/ ONE SHOT TON N DRV Rgm VC + ISENSE – CURRENT COMPARATOR – TRANSCONDUCTANCE AMPLIFIER + 1.25V VFB Figure 3. Flyback Controller Functional Diagram 6 ML4880 DESIGN CONSIDERATIONS A typical design can be implemented by using the following design procedure. Note that this procedure is not intended to give final values, but to give a good starting point, and provide the relationships necessary to make trade-off decisions. Some experimentation will be necessary to optimize values and to verify that the design operates over worst case conditions. DESIGN SPECIFICATIONS It is important to start with a clear definition of the design specifications. Make sure the specifications reflect worst case conditions. Key specifications include the minimum and maximum input voltage and the output voltage and load current for each output. BUCK REGULATORS - INDUCTOR AND SENSE RESISTOR SELECTION Figure 6 shows the inductor current of the step down regulators. The inductor current is made up of two components: the DC current level set by the transconductance amplifier, ISENSE, and the inductor ripple current, ∆IL. The figure also shows that IOUT is the summation of ISENSE and 1/2 ∆IL: L= Inductor ripple currents in the range of 30% to 70% of the maximum output current are typical. As a good starting point set the inductor ripple current to 50% of the maximum output current: ∆IL = TON × (VIN − VOUT ) = FIRC × IOUT(MAX) L (4) where FIRC = ratio of inductor ripple current to the maximum output current, or: TON × (VIN − VOUT ) 0.5 × IOUT(MAX) (5) Calculate the inductance using the volt-seconds value given in Figure 5 at the maximum input voltage. Choose the nearest standard value, realizing the trade-offs mentioned before. Then, using the inductance value chosen, determine the actual inductor ripple current at the maximum and minimum input voltage using Equation 4 and Figure 5. The sense resistor value can be determined using the inductor ripple current value calculated above and Equation 3 rearranged as follows: RSENSE = VSENSE(MIN) 0.14 = 1 ∆I IOUT(MAX) − I − 1 ∆I 2 L(MIN) OUT(MAX) 2 L(MIN) IOUT V T × (VIN − VOUT ) = ISENSE + 1 ∆IL = SENSE + ON 2 RSENSE 2×L (3) (6) Therefore, the selection of the inductance value determines how much of the output current is made up of the ripple current. Higher inductor ripple current allows smaller inductor values, but results in higher peak currents, lower efficiency, and higher output voltage ripple. 18 5V BUCK 16 45 40 12V FLYBACK 14 VOLT-SECONDS (V-µs) 35 12 30 TON (µs) 10 3.3V BUCK 12V FLYBACK 6 25 3.3V BUCK 20 8 15 4 10 5V BUCK 2 0 5 10 VIN (V) 15 20 5 0 5 10 VIN (V) 15 20 Figure 4. TON vs. VIN Figure 5. Volt-seconds vs VIN 7 ML4880 Having determined the values for the inductor and sense resistor, we can now specify the inductor peak current rating. This value is calculated at current limit and at the maximum input voltage, and is given by: BUCK REGULATORS — MOSFET SELECTION The switching MOSFETs must be logic level types with the ON resistance specified at VGS = 4.5V. In general, the ON resistance - gate charge product provides a good figure of merit by which to compare various MOSFETs, the lower the figure the better. The internal gate drivers of the ML4880 can drive over 100nC of total gate charge, but 60nC to 70nC is a more practical limit to ensure good switching times. The drain-source breakdown voltage rating is determined by the input voltage. For input voltages up to 12V, a drain to source rating of 20V is acceptable. For input voltages up to 18V, a drain to source rating of 30V is recommended. For a more reliable design, look for MOSFETs that are avalanche rated. In high current applications, the MOSFET’s power dissipation often becomes a major design factor. The I2R losses generate the largest portion of heat in the MOSFET package. Make sure that the MOSFETs are within their rated junction temperature at the maximum ambient temperature by calculating the temperature rise using the thermal resistance specifications. The worst case power dissipation for the P-MOS switch occurs at the minimum input voltage and is determined as follows: IL(PK @ MAX) = ISENSE(MAX) + ∆IL(MAX) = VSENSE(MAX) + ∆IL(MAX) = 0.25V + ∆IL(MAX) RSENSE RSENSE (7) For reliable operation, the inductor current rating should exceed the value calculated by 10%-20%. For future reference, determine the peak inductor current at the minimum input voltage: IL(PK @ MIN) = ISENSE(MAX) + ∆IL(MIN) = VSENSE(MAX) + ∆IL(MIN) = 0.25V + ∆IL(MIN) RSENSE RSENSE (8) Now the sense resistor’s power rating can be determined. The sense resistor must be able to carry the peak current in the inductor during the OFF-time: PR 2 = IRMS(OFF) × RSENSE SENSE (9) where, 2 2   (ISENSE(MAX) ) + (ISENSE(MAX) ) (IL(PK @ MAX) ) + (IL(PK @ MAX) )   V  2 IRMS(OFF) = 1− OUT   3  VIN(MAX)  P(P − MOS) = IRMS(ON) × RDS(ON) where 2 (11) One final parameter should be specified: the winding resistance of the inductor. In general the winding resistance should be as low as possible, preferably in the low mΩ range. Since the inductor is in series with the load at all times, the copper losses can be approximated by: PCU = I2 OUT × RL A good rule of thumb is to allow 2mΩ of winding resistance per µH of inductance. IL VIN – VOUT L VOUT L 2 2   (ISENSE(MAX) ) + (ISENSE(MAX) ) (IL(PK @ MIN) ) + (IL(PK @ MIN) )  V 2  IRMS(ON) =  OUT   3  VIN(MIN)  The worst case power dissipation for the N-MOS switch occurs at the maximum input voltage and is determined using: P(N− MOS) = IRMS(OFF) × RDS(ON) 2 (10) (12) ILPK ∆IL ISENSE = IOUT = ISENSE + 1/2 ∆IL VSENSE RSENSE TON TOFF t Figure 6. Buck Regulator Inductor Current 8 ML4880 BUCK REGULATORS — INPUT CAPACITOR SELECTION The choice of the input capacitor is based on it’s ripple current and voltage ratings rather than its capacitance value. The input capacitor should be a low ESR type and located as close to the source of the P-MOS switch as possible. The input capacitor’s ripple current is determined by the load current and the input voltage, with the worst case condition occurring at VIN = 2 × VOUT: (V − VOUT )   V  IRMS(C ) ≈ ISENSE(MAX) + 1 ∆IL(MAX)   OUT IN  (13) IN   2 VIN  The selected capacitor must meet both the capacitance and ESR requirements. As a final check, make sure the output capacitor can handle the ripple current, IRMS(COUT): IRMS(C OUT ) ≈ ∆IL(MAX) 12 (16) BUCK REGULATORS - OUTPUT VOLTAGE The output of buck regulator A is adjustable and can be set to any voltage between 2.5V and 3.5V by connecting a resistor divider to the feedback pin as shown in Figure 1. Similarly, the output voltage of buck regulator B is adjustable and has a usable range of 4.5V to 5.0V. The resistor values R1 and R2 can be calculated using the following equation: The capacitor’s voltage rating is based on the maximum input voltage, VIN(MAX). Capacitor manufacturers typically recommend derating the capacitor voltage rating by 20% to 50% for aluminum electrolytic types and 50% to 70% for tantalum types. In high current applications it may necessary to add a 10µF bulk capacitor and a small 0.1µF ceramic capacitor to bypass VIN (pin 16) right at the ML4880. BUCK REGULATORS — OUTPUT CAPACITOR SELECTION The output capacitors determine the loop stability and the output ripple voltage. Use only low ESR capacitors intended for switching power supply applications, such as AVX TPS, Sprague 593D Sanyo OS-CON, or Nichicon PL series. To ensure stability, the minimum capacitance value is given by: COUT   T  ≥  4.3   ON(MAX)   VOUT   RSENSE    VOUT = 1.25V ×  R1+ R2  R2  (17) The value of R2 should be 125kΩ or less to minimize bias current errors. It is important to note that the accuracy of these resistors directly affects the accuracy of the output. Use precision resistors and set the nominal voltage approximately 1% to 2% high with no load in order to make up for the drops that normally occur when supplying heavy loads. This offset results in the best overall output accuracy over line and load. FLYBACK REGULATOR — INDUCTOR AND SENSE RESISTOR SELECTION Figure 7 shows the inductor current of the flyback regulator. The inductor current is made up of two components: the DC current level set by the transconductance amplifier, ISENSE, and the inductor ripple current, ∆IL. Therefore, the selection of the inductance value determines how much of the output current is made up of the inductor ripple current. Higher inductor ripple current allows smaller inductor values, but results in higher peak currents, lower efficiency, and higher output voltage ripple. (14) The maximum ESR value can be estimated using: ESR ≤ ∆VOUT ∆IL(MAX) (15) IL VIN L VOUT L ∆IL(PEAK) ∆IL = VIN TON L VSENSE ISENSE = RSENSE IOUT = (ISENSE + 1/2 ∆IL) VIN VIN + VOUT TON TOFF t Figure 7. Flyback Regulator Inductor Current 9 ML4880 The relationship of the inductor current to the output current is given by the following equation:    VSENSE TON × VIN   VIN VIN   = + IOUT = ISENSE + 1 ∆IL    2   VIN + VOUT   RSENSE 2 × L   VIN + VOUT     (18) The inductance value can be determined by setting the ripple current portion of Equation 18 to some percentage of the output current. Inductor ripple currents in the range of 50% to 80% of the maximum output current are typical. As a good starting point set the inductor ripple current to 65% of the maximum output current:  V + VOUT  T × VIN ∆IL = ON = FIRC × IOUT(MAX) ×  IN  L VIN   The winding resistance and the turns ratio of the coupled inductor need to be specified. In general the winding resistance should be as low as possible, preferably in the low milliohm range. A good rule of thumb is to allow 2 mW of winding resistance per mH of inductance. The turns ratio is specified at 1:1 for simplicity and availability. FLYBACK REGULATOR - MOSFET SELECTION The internal gate drive of the ML4880 limits the selection of the switching FET to logic level types with the ON resistance specified at VGS = 4.5V. Also, look for a FET with a maximum total gate charge of around 70nC. The drain to source breakdown voltage rating is determined as follows: VDS = (VIN(MAX) + VOUT ) × FDS (19) where FIRC = ratio of inductor ripple current to the maximum output current, or: (24)  T × VIN(MIN)  VIN(MIN) L = ON V 0.65 × IOUT(MAX)  IN(MIN) + VOUT   (20) A safety derating factor, FDS, in the range of 1.5 to 2.0 is recommended due to the voltage spikes caused by the leakage inductance of a transformer. For a more reliable design, look for MOSFETs that are avalanche rated. The maximum ON resistance of the MOSFET can be determined using the worst case power dissipation for the package type chosen. This assumes that the major portion of heat generated in the package is due to I2R losses. RDS(ON) = PPKG 2 IRMS(ON) Calculate the inductance using the volt-seconds value given in Figure 5 at the minimum input voltage. Choose the nearest standard value, and determine the actual inductor ripple current at the maximum and minimum input voltage using Equation 19 and Figure 5. Next, determine the sense resistor value using the inductor ripple current value calculated above using: RSENSE = VSENSE(MIN)  VIN(MIN) + VOUT  1 IOUT(MAX)   − 2 ∆IL(MIN) VIN(MIN)   (25) where, 2 2   (ISENSE(MAX) ) + (ISENSE(MAX) ) (IL(PK @ MAX) ) + (IL(PK @ MAX) )   VOUT 2   IRMS(ON) =   3  VIN(MIN) + VOUT  = 0.14  VIN(MIN) + VOUT  1 IOUT(MAX)   − 2 ∆IL(MIN) VIN(MIN)   (21) FLYBACK REGULATOR — RECTIFIER SELECTION The output diode must be able to carry the worst case output current as well as withstand the worst case reverse voltage. The worst case output current occurs at current limit and at the maximum input voltage. Therefore, the average forward current rating of the output diode can be determined by the following:  1  I T × VIN(MAX)   VIN(MAX) ID(AVG) = OUT(MAX) =  0.25 + ON    V 2×L FID   IN(MAX) + VOUT   FID   RSENSE Having determined the values for the inductor and sense resistor, the inductor peak current rating can now be specified. This value is calculated at current limit and at the maximum input voltage, and is given by: IL(PK @ MAX) = ISENSE(MAX) + ∆IL(MAX) = VSENSE(MAX) + ∆IL(MAX) = 0.25V + ∆IL(MAX) RSENSE RSENSE (22) For reliable operation, the inductor current rating should exceed the value calculated by 10%-20%. Now the sense resistor’s power rating can be determined. The sense resistor must be able to carry the peak current in the inductor, but only during the OFF-time: PR SENSE where, FID = derating factor (26) = IRMS(OFF) × RSENSE 2 (23) The derating factor is typically in the range of 0.5 to 0.9 depending on reliability goals. Typically a value of 0.8 is used for most applications. The reverse voltage rating is determined by the sum of the output voltage and the maximum input voltage: where,   (ISENSE(MAX) ) + (ISENSE(MAX) ) (IL(PK @ MAX) ) + (IL(PK @ MAX) )   VIN(MAX)   2 IRMS(OFF) =   3  VIN(MAX) + VOUT  2 2 10 ML4880 VR = where, VOUT + VIN(MAX) FVR (27)   VOUT = 1.25V ×  R1+ R2  R2  (32) FVR = derating factor Again a derating factor of 0.8 is typical for most applications. FLYBACK REGULATOR - INPUT CAPACITOR SELECTION The input capacitor should be a low ESR type and located as close to the primary winding of the transformer as possible. The capacitor’s ability to handle ripple current is the selection criteria used. The input capacitor’s ripple current is determined by the load current and the input voltage, with the worst case condition occurring at VIN = VOUT. IRMS(C × VIN   V  ≈ ISENSE(MAX) + 1 ∆IL(MAX)   OUT   VOUT + VIN  (28)  2  The value of R2 should be 125kΩ or less to minimize bias current errors. It is important to note that the accuracy of these resistors directly affects the accuracy of the output. Use precision resistors and set the nominal voltage approximately 1% to 2% high in order to make up for the ML4880’s load regulation. This offset results in the best overall output accuracy over line and load. LAYOUT Proximity of passive devices and adequate power and ground planes are critical for reliable operation of the circuit. In general, use the top layer for the high current connections and the bottom layer for the quiet connections such as GND, feedback and current sense. Some more specific guidelines follow. 1. The connection from the current sense resistor to the ISENSE pin should be made by a separate trace and located as close to the lead of the resistor as possible. The trace length from the sense resistor to the ML4880 should be kept as short as possible and away from switching components and their traces. 2. The trace lengths from the buck regulator’s input capacitor to the switching MOSFET, from the MOSFETs to the inductor, from the synchronous rectifier MOSFET to the SENSE resistor, and from the inductor to the output capacitor should all be as short as possible. The flyback regulator’s trace length from the input capacitor to the inductor, from the inductor to the MOSFET, from the sense resistor to the inductor, and from the diode to the output capacitor should also be kept short to minimize noise and ground bounce. 3. The high current ground paths need to be kept separate from the signal ground paths. The GND connection should be made at a single-point star ground. It is very important that the ground for the ML4880 ground pins (pins 6 and 7) be made using a separate trace. 4. Note: Concentrating on keeping the current sense and high current connections short as well as keeping the switching components and traces away from the sensitive analog components and traces during layout will eliminate the majority of problems created by a poor layout. 5. The VIN and VREG bypass capacitors need to be located close to the ML4880 for adequate filtering of the IC’s internal bias voltage. 6. Remote sensing the output for improved load regulation can be implemented with the ML4880. Any output can be remote sensed by using the top of the external resistor divider as the remote sense point. IN) The capacitor must also be able to handle the maximum input voltage. For reliable operation derate the capacitor voltage rating by 20% to 50% for aluminum electrolytic types and 50% to 70% for tantalum types. FLYBACK REGULATOR - OUTPUT CAPACITOR SELECTION The output capacitors determine the flyback regulator’s loop stability and the output ripple voltage. Use only low ESR capacitors intended for switching power supply applications. To ensure stability, the minimum capacitance value is given by:   I  T  COUT ≥  8.6   ON(MAX)   OUT(MAX)   VOUT   RSENSE   ∆IL(MIN)  The maximum ESR value can be estimated using: ESR ≤ ∆VOUT ∆IL(MAX) (29) (30) The selected capacitor must meet both the capacitance and ESR requirements. As a final check, make sure the output capacitor can handle the ripple current, IRMS(COUT). IRMS(C OUT ) ≈ IRMS(C ) IN (31) FLYBACK REGULATOR - OUTPUT VOLTAGE The output of the flyback regulator is adjustable and can be set to any voltage between 6.0V and 15.0V by connecting a resistor divider to the feedback pin as shown in Figure 3. The resistor values R1 and R2 can be calculated using the following equation: 11 VREG 5.5V to 18V VIN 16 VIN N DRV A 14 4 23 9 ISENSE A PWR GND A 3.3 SHDN 5.0 SHDN 12 SHDN CLAMP B 1 19 C16 20 47nF 5 22 8 D5* Q4 IRF7406 D4 IN4148 P DRV B D1 MBRS340T3 T1 39µH 3 ISENSE C PWR GND C PWR GND B VFB B 21 12 ISENSE B VIN 17 N DRV C N DRV B 10 SHDN C 18 SHDN B 11 SHDN A VFB A P DRV A 13 C2 33µF 25V C3 33µF 25V C4 10µF 35V C5 0.1µF C12 47nF CLAMP GND Figure 8. Typical Application Circuit ML4480 VIN R11 100k R4 887k 1% Q1 A&B Si9955 6 7 C10 10nF GND 12 VIN C11 68nF D2 IN4148 R7 100k Q2 IRF7406 2 Q3 IRF7403 D3* C13 10nF R9 205k 1% L1 33µH C14 100µF 10V C15 100µF 10V 3.3V, 2A C6 0.1µF 15 CLAMP A 24 C7 10µF 35V R8 80m R10 124k 1% L2 33µH C18 100µF 10V C19 100µF 10V VFB C 5.0V, 2A Q5 IRF7403 C17 10nF R13 383k 1% R12 80m R6 0.12 R14 124k 1% R5 100k 1% ML4880 C1 33µF 25V 12V, 0.5A C8 68µF 20V C9 68µF 20V *Optional ML4880 SUGGESTED VENDORS See Table 1 for a list of suggested component manufacturers. TABLE 1: COMPONENT SUPPLIERS COMPONENT SENSE RESISTORS INDUCTORS MANUFACTURER Dale IRC Coilcraft Coiltronics Dale Premier Magnetics Pulse Engineering Sumida XFMRS, Inc. CAPACITORS AVX Sprague Sanyo Nichicon Panasonic Diodes, Inc. International Rectifier Motorola National Semiconductor Siliconix PART NUMBER WSL Series LRC Series DO/DT Series OCTA-PAC Series LPE-6562 Series LPT-4545 Series VTK & HTK Series LCI & HCI Series CD & CDR Series Surface Mt Series TPS Series 593D Series SC & SA Series PL Series HFQ Series SK Series IRF7XXX Series MMSF Series MMDF Series NDS94XX NDS99XX Littlefoot Series Litefoot Series PHONE (402) 563-6506 (512) 992-7900 (708) 639-6400 (305) 781-8900 (605) 665-9301 (714) 362-4211 (619) 674-8100 (708) 956-0666 (317) 834-1066 (207) 282-5111 (708) 956-0667 (619) 661-6835 (708) 843-7500 (201) 348-5227 (805) 446-4800 (310) 322-3331 (602) 302-8056 (800) 272-9959 (408) 970-5700 SEMICONDUCTORS 13 ML4880 PHYSICAL DIMENSIONS inches (millimeters) Package: S24 24-Pin SOIC 0.600 - 0.614 (15.24 - 15.60) 24 0.291 - 0.301 0.398 - 0.412 (7.39 - 7.65) (10.11 - 10.47) PIN 1 ID 1 0.024 - 0.034 (0.61 - 0.86) (4 PLACES) 0.050 BSC (1.27 BSC) 0.095 - 0.107 (2.41 - 2.72) 0º - 8º 0.090 - 0.094 (2.28 - 2.39) 0.012 - 0.020 (0.30 - 0.51) SEATING PLANE 0.005 - 0.013 (0.13 - 0.33) 0.022 - 0.042 (0.56 - 1.07) 0.009 - 0.013 (0.22 - 0.33) 14 ML4880 ORDERING INFORMATION PART NUMBER ML4880CS ML4880ES TEMPERATURE RANGE 0°C to 70°C –20°C to 70°C PACKAGE 24-PIN SOIC (S24) (End Of Life) 24-PIN SOIC (S24) (Obsolete) © Micro Linear 1997 is a registered trademark of Micro Linear Corporation Products described in this document may be covered by one or more of the following patents, U.S.: 4,897,611; 4,964,026; 5,027,116; 5,281,862; 5,283,483; 5,418,502; 5,508,570; 5,510,727; 5,523,940; 5,546,017; 5,559,470; 5,565,761; 5,592,128; 5,594,376; Japan: 2598946; 2619299. Other patents are pending. Micro Linear reserves the right to make changes to any product herein to improve reliability, function or design. Micro Linear does not assume any liability arising out of the application or use of any product described herein, neither does it convey any license under its patent right nor the rights of others. The circuits contained in this data sheet are offered as possible applications only. Micro Linear makes no warranties or representations as to whether the illustrated circuits infringe any intellectual property rights of others, and will accept no responsibility or liability for use of any application herein. The customer is urged to consult with appropriate legal counsel before deciding on a particular application. 2092 Concourse Drive San Jose, CA 95131 Tel: 408/433-5200 Fax: 408/432-0295 DS4880-01 04/28/97 Printed in U.S.A. 16