e2PowerEdgeTM Economic Energy using Low VCEsat BJT’s Steve Sheard Marketing Engineer ON Semiconductor [email protected] Introduction ON Semiconductor’s e2PowerEdgeTM family of low VCEsat Bipolar Junction Transistors (BJT) are miniature surface mount devices featuring ultra low saturation voltage VCEsat and high current gain capability. These are designed for use in low voltage, high speed switching applications where affordable efficient energy control is important. Typical application are power management in any circuit that needs low losses. In portable and battery powered products such as cellular and cordless phones, PDAs, computers, printers, digital cameras, digital camcorders, DVD players and MP3 players. The functions controlled in portable products are battery charging, battery management, over voltage protection, low drop out regulation, LED backlight switching, Royer converter for LCD Backlights, vibrator, disc drives, and peripheral power, such as cameras and flash units. Other applications are low voltage servo motor controls in mass storage products such as disc drives and tape drives; controlling small motors as in electric shavers. In the automotive industry they can be used in air bag deployment, controlling mirrors and fuel pumps, and in circuits used for instrument clusters, steering, transmission, toll readers, LED lighting and power inverters. In industrial applications they are ideal for circuits providing control in smoke detectors, vending machines, In Focus projectors, gas meters, Telecommunication SLIC and RF access boxes. Where high currents need to be controlled at high frequencies the Low VCEsat BJT is the ideal driver for a highly efficient Trench MOSFET. The Linear Gain (Beta) of Low VCEsat BJT makes them ideal components in analog amplifiers. Technology The Low VCEsat BJT devices use a technology that was first developed over 20 years ago and was primarily used to achieve similar performance in a smaller die (die shrink). This technology is called a “Perforated Emitter” and today is being focused towards reducing the forward saturation voltage to achieve very low forward resistance. The perforated emitter is a method of extending the base electrical layer across the complete die to contact multiple perforations through the emitter. Each of these perforations creates miniature transistors within the device and thus allows the current to be distributed evenly and with greater efficiency. Photograph 1. Base Electrical layer fingers and perforations Collector is Substrate Base Contact Emitter Contact Photograph 1 Some of the new Low VCEsat BJT’s are now available with a saturation voltage at 1.0 Amps of well under 50 mV. This equates to a forward resistance of under 50 mΩ, and proves very competitive against a higher cost MOSFET. PMU with an external pass transistor The majority of portable products are moving towards an integrated Power Management Unit (PMU) circuit designed specifically to control the different functions within the product. The circuits, for the control of currents under 100 mA, are typically all imbedded within the PMU, including the final pass transistor. However, for the control of currents from 100 mA to 5.0 A an external pass transistor (MOSFET) is the typical design of choice. An alternative to the MOSFET is to use a lower cost Low VCEsat BJT. The new family of Low VCEsat BJTs offer potential savings of 5 to 20 cents compared to designs using MOSFETs. Low VCEsat BJTs perform the same function as a MOSFET at a lower cost, and as an added bonus, in many cases provide for improved power consumption resulting in improved battery life. In many designs the high current gain allows Low VCEsat BJT devices to be driven directly from the PMU’s control outputs. Design Considerations The Low VCEsat BJT is a current driven device, compared to the MOSFET which is a voltage driven device. For this reason the designer needs to understand the current limitations of the PMU control circuits being used, to determine the specific circuit requirements when designing with a Low VCEsat BJT. For example, if the Low VCEsat BJT is to control a current of 1.0 Amp and it has a worst case gain (hFE) of 100 then the base current will need to be a minimum of 10 mA (IB) to ensure the Low VCEsat BJT goes into saturation. The control pin of the PMU must be able to supply the 10 mA for the Low VCEsat BJT to be driven directly; otherwise an additional drive stage would be required. The designer also has to consider the power rating of the package for the Low VCEsat BJT. For Example; the On Semiconductor Low VCEsat BJT NSS12600CF8T1G is mounted on a FR4 printed circuit board 100 mm2 pads. The input voltage to be switched is 5.0 V and the maximum constant current is 6.0 A. Ambient temperature is 250 C. The Power rating (PD) with the specified pad is 1.0 W. The typical Vce-sat for the NSS12600CF8T1G at 1.0 A is 45 mV. This equates to a power dissipation of 45 mW. The Minimum Gain (hFE) at 1.0 A is 250. Thus the drive current (IB) would need to be a little over 4.0 mA. The maximum limit on Vce-sat at 1.0 A is 80 mV (from Data Sheet with beta 100), this equating to 80 mW, well below the 1.0 W rating for the package at 250 C. Derating the device for temperature. The Thermal Resistance for de-rating with minimum pads (RθJA) is 1250 C/W (From Data Sheet). The formula for de-rating is PD = (Tj-max – Tamb) / RθJA For an application where Tj-max = 750 C, The Maximum allowable Power dissipation would become PD = (75 – 25) / 125 = 400 mW. The maximum calculated power of 80 mW still falls below the adjusted power when the device is de-rated for a higher temperature of 750 C. Charging Circuit A review of charging circuit (Figure 1) in a portable product shows the pass transistor Q1 (Power MOSFET 2.0 Amp, 20 V, TSOP6 package) and the blocking Schottky Diode D1 can be replaced by a Low VCEsat BJT and a resistor. In this example the Low VCEsat BJT saved $0.10 from the typical MOSFET cost and 316 mW lower power loss. All the control for charging of the Lithium Ion battery is imbedded in a PMU. The PMU control pin changes to high to turn on the external pass transistor Q1 and the charging current is set at 1 Amp. The series Schottky Diode D1 is required to block any reverse current from the battery. The typical power dissipated through the pass transistor Q1 and the reverse blocking diode D1 was calculated as: Q1 Power = I2 x R, 1.0 Amp2 x RDS(ON) (60 mΩ) = 60 mW D1 Power = I x VF, 1.0 Amp x VF Schottky (360 mV) = 360 mW Total Power dissipated through Q1 and D1 = 420mW The typical high volume cost of the MOSFET and Schottky Diode is $0.175 MOSFET 5.0 Vin Q1 7 Vcc Schottky (1 Amp) R_sens 4.2 D1 OUT 8 V_BAT BOM ASP Q1 MOSFET D1 Schottky Diode Total: $0.100 $0.075 $0.175 ISNS 1 PMU CHG_Indicator 6 CFLG 3 COMP 4 GND VSNS 5 Power Dissipated through Pass Elements ISEL 2 CHG_Control Rcomp 60K Q1: 1.0 Amp, 60 mΩ D1: 1.0 Amp, 360 mV Power Dissipated Ccomp Figure 1 The charging circuit (Figure 2) can be configured using a Low VCEsat BJT to replace the MOSFET and the Schottky Diode. The Schottky Diode is not required because the Low VCEsat BJT has this function inherent to its design. The control pin on the PMU is able to = 60 mW = 360 mW = 420 mW provide a maximum of 20 mA. The PMU would initiate a fast charge with the battery voltage of 3.0V. With Q2 in saturation both the collector and emitter will be at approximately 3.0 V, thus the base would be 2.3 v. The base current required to drive the ON Semiconductor NSS12600CF8T1G Low VCEsat BJT, which has a minimum gain of 250, into saturation needs to be a little over 4.0 mA for a 1.0 Amp charging current. Selecting a standard resistor value of 200 Ω for the base resistor will ensure the Low VCEsat BJT is in saturation and that the limit for the drive pin is not exceeded. The typical power dissipated through the pass transistor Q2 and bias resistor R1 was calculated as: Q2 Power = I x V, 1.0 Amp x Vce-sat (1.0 Amp, Beta 100 = 80 mV) = 80 mW R1 Power = I2 x R, 0.011 Amp2 x 200 Ω = 24 mW Total Power dissipated through Q2 and R1 = 104 mW The typical high volume cost of the Low VCEsat BJT and Resistor is $0.10 5.0 NSS12600CF8T1G Vin R_sens V_BAT BOM ASP Q2 R1 200Ω 7 Vcc OUT 8 PMU CHG_Indicator 6 CFLG CHG_Control 3 COMP 4 GND Rcomp 4.2 NSS12600CF8T1G $0.090 R1 $0.010 Total: $0.100 ISNS 1 VSNS 5 Power Dissipated through Pass Element ISEL 2 Q2: 1.0 amp, Beta 100, VCEsat 135 mV = 80 mW R1: 11mA, 200 Ω = 24 mW Power Dissipated = 104 mW 60K Ccomp Figure 2 Charging Circuit Savings The savings resulting from exchanging the MOSFET bypass transistor and Schottky Diode with a Low VCEsat BJT and bias resistor were $0.075 per unit. The exchange also resulted in a power dissipation savings of 316 mW making the thermal considerations of the portable product much simpler. More Complex Circuits Integrated Circuits designed specifically with an external bypass MOSFET may not have the ability to supply the required current to drive the Low VCEsat BJT into saturation directly. In these circuits an extra digital transistor or small general purpose MOSFET (Q4) can be used as illustrated in Figure 3. 5.0 NSS12600CF8T1G R_sens V_BAT Q3 Vin 4.2 Q4 7 Vcc OUT 8 6 CFLG CHG_Control $0.090 $0.010 $0.020 $0.120 Power Dissipated through Pass Element VSNS 5 3 COMP 4 GND Q3 NSS12600CF8T1G R2 Q4 (General purpose MOSFET) Total: ISNS 1 PMU CHG_Indicator BOM ASP Q3: 1.0 amp, Beta 100, VCEsat 80 mV Q4: 11 mA, 100 mΩ R2: 11 mA, 200 Ω Power Dissipated ISEL 2 Rcomp 60K R2 200Ω = 80 mW = 0.1 mW = 24 mW = 104 mW Ccomp Figure 3 The results are not quite as significant as the charging example. The cost savings is still $0.055 per unit. Power saving is significant also – 316 mW less. Bi-Directional Current Control Figure 4 is an illustration of a battery management application with a dual MOSFET configuration. By connecting the MOSFETs with their drains together one eliminates the requirement for a blocking schottky diode and it also allows for the control of current in either direction. i.e. Charging current in to the battery, power out to support USB. The disadvantage of having the two MOSFETs in series is the doubling of the resistance through the pass elements and thus doubling the power loss. It is a better solution compared to the use of a blocking schottky diode, but it does cost significantly more. Q6 Q5 5.0 Vin R_sens V_BAT 4.2 7 Vcc OUT 8 BOM ASP Q5 Q6 Total: $0.160 $0.160 $0.320 ISNS 1 PMU CHG_Indicator 6 CFLG 3 COMP 4 GND VSNS 5 Power Dissipated through Pass Elements ISEL 2 CHG_Control Rcomp 60K Ccomp Full Charge Ireg = 1.0 A Rds(on) MOSFET Q5, 80 mΩ = 80 mW Rds(on) MOSFET Q6, 80 mΩ = 80 mW Power Dissipated = 160 mW Figure 4 Figure 5 is a similar battery management application using two Low VCEsat BJT. In this design the Low VCEsat BJTs are connected in parallel and only one is turned on at a time; Q7 for charging the battery, Q8 to allow power out to a peripheral. As only one device is turned on at a time we only have to consider the resistance and power loss through one device. There are also significant savings in the cost of two Low VCEsat BJT compared to two MOSFETs. External Connect Q8 Battery R_sens BOM ASP Q7 R3 Q7/8 NSS12600CF8T1G x 2 $0.180 R3 & R4 $0.020 Total: $0.200 R4 7 Vcc OUT 8 ISNS 1 PMU CHG_Indicator 6 CFLG CHG_Control 3 COMP 4 GND Rcomp Power Dissipated through Pass Element Q6 or Q7: 1.0 amp, Beta 100, VCEsat 80 mV = 80 mW R3 or R4: 11 mA, 200 Ω = 24 mW Power Dissipated = 104 mW VSNS 5 ISEL 2 60K Ccomp Figure 5 Load Switch - Vibrator Control in Cellular Phones A Low VCEsat BJTs is an ideal switch for controlling functions within a portable product that are only on for a short duration. The vibrator in a cellular phone is good example; Figure 6 is an illustration of the use of a Low VCEsat BJTs, being controlled with a Digital Transistor, to turn the vibrator on and off. A MOSFET approach may be more efficient and the power loss less but considering the short time the vibrator is on, the lower cost of the Low VCEsat BJTs is very attractive. VBAT Vibrator ON Vibrator NSS404000CF8 Figure 6 Load Switch – Back Light Control in a cellular phone Cellular phones often use multiple arrays of LED for illumination of keypads. Figure 7 is an illustration using a Low VCEsat BJT to control the LED backlights. VBA LED NSS30301L Backlight Figure 7 LED Driver Circuits Pass Element in Buck / Boost converter The MC34063 Step Up / Down / Inverting Switching Regulator can be configured to drive LEDs. The Low VCEsat BJT, NSS40600CF8T1G is an ideal pass element, Q1 as shown in Figure 8. Figure 8 Q1 NSS20600CF8T1G 5 2.7 6.8K Q2 NSS20200LT1G MMSZ5V 680 Over Voltage Protection NCP3063 Buck reference designs Load Additional Advantages of using a Low VCEsat BJT The Low VCEsat BJT is less susceptible to ESD damage compared to the MOSFET and thus a savings can be found in not having to provide extra ESD protection. The Low VCEsat BJT has a lower turn on voltage (0.7v typical) compared to a MOSFET (typically 4.0v – 10.0v) and is thus very attractive for low voltage circuits and for situations where a controlled power down is required as the battery voltage drops. The low turn on voltage would also eliminate the need for an oscillator and charge pump, normally needed for a MOSFET. The Low VCEsat BJT blocks voltage in both directions, eliminating the need for a blocking schottky diode which is sometimes required when using a MOSFET. The Low VCEsat BJT typically have a better temperature coefficient compared to a MOSFET which provides for higher efficiency when operating at high temperatures resulting in less temperature elevation in the portable product.. Feature BJT MOSFET Low Vce-sat / Rds-on Excellent in saturation Needs current drive Bi-directional Needs high voltage Gate Drive to get 100% enhancement Mono-directional, needs schottky diode Good per Si Density 2v to 10v depending on design Low High Blocking Capability Pulse Current Drive Voltage Drive Current Switching Speed ESD Sensitivity Cost Rce /mm2 High Current Switching High per Si Density Less than 1V Moderate Saturated – Low Linear - High Excellent Excellent Moderate High Voltage Switching Low Voltage Switching Excellent Excellent Sensitive Moderate Excellent Excellent Poor Application Feature Benefit Pulsed Mode Battery Charging Low Vce-sat hFE > 200 Low Rce /mm2 Small size – 4.0 mm2 Low profile – 0.75 mm PNP transistor High power dissipation / mm2 hFE > 200 Low Rce /mm2 Small size – 4.0 mm2 Low profile – 0.75 mm PNP transistor High Pulse Current High Frequency hFE > 200 Low Rce /mm2 Small size - 4.0 mm2 Low profile – 0.75 mm PNP / NPN transistor Low Vce-sat High Frequency hFE > 200 Low Rce /mm2 Small size - 4.0 mm2 Low profile – 0.75 mm PNP / NPN transistor Low Vce-sat High power dissipation / mm2 hFE > 200 Low Rce /mm2 Small size - 4.0 mm2 Low profile – 0.75 mm PNP / NPN transistor High efficiency High gain Low cost vs MOSFET Less board space More compact design High side control, Bi-directional voltage blocking Efficient charging time High gain Low cost vs MOSFET Less board space More compact design Bi-directional voltage blocking Fast switching time Fast switching time High current gain Low cost vs MOSFET Less board space More compact design High / Low switch High efficiency Fast switching time High current gain Low cost vs MOSFET Less board space More compact design Design flexibility, Bi-directional voltage blocking High efficiency High current control High gain Low cost vs MOSFET Less board space More compact design High or Low side control, Bi-directional voltage blocking High / Low Bridge, Bi-directional voltage blocking High efficiency Low switching Losses High current gain – lower control current Low cost vs MOSFET Less board space Design flexibility High / Low Bridge, Bi-directional voltage blocking High efficiency High current control Low switching Losses High current gain – lower control current Low cost vs MOSFET Less board space Design flexibility Linear Mode Battery Charging MosFET Gate Drive Royer Converter for LCD Backlight Low Drop Out (LDO) Regulator Servo Motor Drive Over voltage protection PNP / NPN transistor Low Vce-sat High Frequency hFE > 200 Low Rce /mm2 Small size - 4.0 mm2 Low profile – 0.75 mm PNP / NPN transistor Low Vce-sat High power dissipation / mm2 High Frequency hFE > 200 Low Rce /mm2 Small size - 4.0 mm2 Low profile – 0.75 mm