Supertex inc. AN-D14 Application Note Low Dropout 3.0 Volt Linear Regulator by Jimes Lei, Applications Engineering Manager Introduction to the open-loop gain of A2. The output of A2 regulates the gate of LP07 for a VOUT of 0.2V x [R1/(R1 + R2) x (R4/R3 + 1)]. The resistor values are chosen (explained in detail in the design considerations section of this application note) and R1 adjusted for an output voltage of 3.0V. C3 is in parallel with R4 to reject external noise. C1 and C2 are bypass capacitors. Low dropout regulators are becoming increasingly important as more and more equipment utilizes 3.0 and 5.0V analog and digital circuits. The main advantage of low dropout 3.0V linear regulators is full utilization of battery life which makes them desirable for battery-powered applications. The low dropout feature will allow for output regulation even when the input battery voltage is discharged close to its output regulated voltage. This will extend the operating input voltage range and allow circuits to operate at a lower battery voltage. Any small decrease in VOUT due to a load applied to the output is sensed by R3 and R4 which is fed back to the noninverting input of A2. The output of A2 will drive the gate of the LP07 to a lower potential thereby increasing the gate drive adequately to source current to the output load and maintain a constant output voltage. This application note discusses the advantages of using Supertex part number LP0701N3, which is a very low gate threshold voltage P-Channel MOSFET. This part has a guaranteed maximum threshold of -1.0V and a maximum RDS(ON) of 2.0Ω at -3.0V drive. This performance is essential for designing an ultralow dropout, low voltage linear regulator. Design Considerations The objective is to implement a 3.0V linear regulator with the lowest possible voltage drop from input to output. The output transistor for a linear regulator can be designed with N-Channel or P-Channel MOSFETs or bipolar NPN or PNP transistors. Figures 2A to 2D show the four possibilities. Circuit Description The low dropout 3.0V linear regulator shown on Figure 1 utilizes an LP07, an LM10, 4 resistors, and 3 capacitors. The LP07 is a 16.5V, 2.0Ω, P-Channel MOSFET with a maximum threshold of -1.0V. The LM10 is a dual op-amp with a 0.2V reference. R1 is a potentiometer. R2, R3, and R4 are 5%, 1/4 watt resistors. C1, C2, and C3 can be either ceramic or electrolytic capacitors. In Figure 2A, the dropout voltage using an N-Channel MOSFET is too large since it cannot be better than the threshold voltage of the MOSFET, which is 1.0 to 4.0V, depending on the type of device used. In figure 2b, the dropout voltage using an NPN is lower but still fairly large. The dropout voltage is typically 0.7V, which is the VBE rating of the transistor. A1 is configured as a unity gain buffer for the 0.2V reference. The output of A1 is attenuated by R1 and R2 and is connected to the inverting input of A2. A2 is configured as a noninverting amplifier with a closed-loop gain of (R4/R3 + 1). The LP07 is configured as a common source amplifier, which functions as a series pass transistor while contributing additional gain 8 + VREF + 0.2V 1 A1 1/2 LM10 R2 22kΩ 1/2 In Figure 2C, the dropout voltage using a PNP transistor is limited by the VCE(sat) rating of the transistor, which is typically -200mV at low collector current. This approach also requires the output of the op-amp to operate 0.7V below its most positive rail at all times. LM10 2 R1 2.0kΩ 3 + 7 A2 6 4 LP0701N3 R4 150k R3 500Ω C3 0.01µF C1 1.0µF VOUT = 3.0V C2 10µF Figure 1: Low Dropout 3.0V Linear Regulator Doc.# DSAN-AN-D14 A040213 Supertex inc. www.supertex.com AN-D14 + + - VOUT = 3.0V R4 NPN R4 Fig 2A: N-Channel MOSFET VOUT = 3.0V Typ Max Units Conditions VGS(th) -0.5 -0.7 -0.1 V VGS = VDS, ID = -1.0mA - 2.0 4.0 Ω - 1.7 2.0 Ω - 1.3 1.5 Ω The LP07 acts as an additional gain stage to the open-loop gain of A2. The increase in open-loop gain causes the loop gain to be greater than 1 at low closed-loop gain conditions, which causes oscillation. Oscillation can be eliminated by setting the loop-gain to be less than 1. This can be achieved by setting ß(negative feedback) < 1 / gain contributed by the LP07. The gain contributed by the LP07 is a function of the load and the transconductance, GFS, of the LP07. Figure 3 shows an equivalent circuit of the open-loop gain of the LP07. GFS = LP07 Vg VOUT δ Id δ Vg (R3 + R4)(RLOAD) R3 + R4 + RLOAD VOUT = Id R4 VOUT = GFS Vg RLOAD R3 (R3 + R4)(RLOAD) R3 + R4 + RLOAD Figure 3: LP07 Open-Loop VGS = -2.0V, ID = -50mA The GFS of the LP07 varies with ID, which is also the load current. Typical GFS versus ID for low and high currents of the LP07 is shown on figure 4a and 4b respectively. VGS = -3.0V, ID = -150mA VGS = -5.0V, ID = -300mA For the no load condition, ID = 3.0V/(R3 + R4). It is desirable have R3 + R4 large to minimize the amount of biasing current. The sum of R3 + R4 is chosen to be approximately 150K. From figure 4a, GFS is 0.62m for an ID of 20µA. VOUT/ VG is calculated as (0.62m )(150K) = 93. At -3.0V, the on-resistance is 1.7Ω typical and 2.0Ω maximum, which helps achieve a low drain-to-source voltage drop. Since the LM10 can swing very close to ground i.e., 0V, the dropout voltage can be estimated to be 2.0Ω x (ILOAD). For a 50mA load, the dropout voltage is 0.1V which means the battery voltage can be 3.1V with the output still regulated at 3.0V. Ω Ω For a load current of 100mA, RLOAD = 3.0V/100mA. Using figure 4b, VOUT/VG is calculated as (310m )(30Ω) = 9.3. The open-loop gain varies with load and is at its maximum during Ω Doc.# DSAN-AN-D14 A040213 Fig 2D: P-Channel MOSFET Preventing Unwanted Oscillation The Supertex LP07 has a guaranteed maximum threshold of -1.0V and guaranteed on-resistance at -2.0V, -3.0V, and -5.0V drives. The specifications are shown on the following table: Min VOUT = 3.0V R4 R3 Figure 2C: PNP Transistor Conventional P-Channel MOSFETs have guaranteed maximum thresholds of -4.0V, which would require the supply voltage to be greater than 4.0V for adequate turn on. A low threshold, low on-resistance P-Channel MOSFET is ideal for this approach. Parameter P-Channel + R3 Figure 2B: NPN Transistor - VOUT = 3.0V R4 In Figure 2D, the dropout voltage for the P-Channel MOSFET approach is determined by the on-resistance of the device times the load current. The device is driven by the battery voltage minus the minimum output voltage of the opamp. Similar to the PNP approach, the op-amp is required to operate one threshold below the battery voltage during the no load condition. When the battery voltage is discharged close to 3.0V, the MOSFET chosen should have a very low threshold and a very low on-resistance at low VGS ratings to achieve low dropout. RDS(ON) PNP + R3 R3 - ≈ N-Channel VBATT VBATT ≈ - VBATT ≈ ≈ VBATT 2 Supertex inc. www.supertex.com 300 1.00 250 GFS (m ) 1.20 0.80 Ω Ω GFS (m ) AN-D14 0.60 200 150 0.40 100 0.20 50 0 0 10 20 30 40 50 60 0 70 Figure 4a: GFS vs. ID at Low Currents To determine the range of R1, the range of VI needs to be determined under the worst case conditions. Using superposition, VOUT is calculated as: R3 R2 80 100 120 140 157.5k 30nA (2k) ( + 1) 475 3.0V = 3332.6Vi + 1.330V + 4.725mV + 19.95mV The offset voltage, VOS, input biasing current, Ib+ and Ib-, and tolerances of the external resistors will affect the output voltage. R1 is used to adjust VOUT to 3.0V. Figure 5 is an equivalent circuit showing VOS, Ib+ , and Ib-. (R1R2 ) 60 157.5k 3.0V = (Vi + 4mV) ( + 1) + 30nA(157.5k) + 475 Calculations + 1) + (Ib+) R4 + (Ib-) 40 The LM10 guarantees VOS = 4.0mV max and Ib = 30nA max. (R1 • R2) / (R1 + R2) is set at 2K. For minimum Vi: It is desirable to set ß <<1 / 93 since 1 / 93 is a typical value. R3 and R4 are chosen to be 500Ω and 150k respectively for a ß of 1/301, providing an adequate safety margin. R4 20 Figure 4b: GFS vs. ID at High Currents the no load condition. The negative feedback, ß, is R3 / (R3 + R4) and should be set less than or equal to 1/(VOUT/VG). VOUT = (VOS + Vi ) ( 0 ID (mA) ID (mA) ( Vi(min) = 4.947mV R4 +1) (R1 + R2 ) R3 VBATT Vi Ib- - + - VOS + R1 A2 R4 LP07 VOUT R3 Ib+ Figure 5: Offset Voltage and Input Biasing Current Doc.# DSAN-AN-D14 A040213 3 Supertex inc. www.supertex.com AN-D14 200 3.0 RLOAD = 50Ω VOUT (Volts) VOUT - VIN (mV) 150 VOUT = 3.0V 100 1.0 50 0 2.0 0 25 50 75 100 0 IL (mA) 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 VIN (Volts) Figure 6: Dropout Voltage Figure 7: VOUT vs VIN Measurements For maximum Vi: Actual measurements were recorded and are shown on figures 6 and 7. Figure 6 shows the dropout voltage at different load currents. Figure 7 shows the output voltage regulation versus the decrease in battery voltage with a fixed load. 142.5k 3.0V = (Vi - 4mV) ( + 1) - 30nA (142.5k) 525 142.5k 30nA (2k) ( + 1) 525 3.0V = 272.4Vi - 1.090V - 4.275mV - 16.35mV 5.0V Regulators The low dropout 3.0V regulator in figure 1 can be easily modified to a 5.0V or adjustable low dropout regulator by changing R1 to a 5.0k potentiometer. Using a voltage controlled resistor for R1 will allow for a programmable low dropout regulator. Vi(max) = 15.09mV The range for R1 is: R1 Vi = R1 = R1 + R2 R2 39.40 Conclusion (200mV) to Low dropout 3.0V linear voltage regulators are ideal for portable battery operated applications to help extend battery life. The low dropout voltage allows the battery powered equipment to operate at a lower battery voltage. In addition to the other advantages discussed, MOSFETs increase the efficiency of the circuit because of the current required to drive the gate is virtually zero as it is usually in the sub nanoampere area. Bipolars need base current and this is undesirable especially when battery energy is at a budget. LP07 is ideal for linear applications requiring high efficiency because of its low threshold voltage and low guaranteed onresistances at 2V, 3V and 5V drives. R2 12.25 Choosing R1 to be a 2k potentiometer, R2 = (2k)(12.25) = 24.5k. R2 should be less than 24.5k so under the worst case conditions, R1 would not operate at its maximum value of 2k. R2 is chosen to be 22k. The range of R1 is calculated as: R1 = 22k(0.95) / 39.4 to 22k(1.05) / 12.25 R1 = 531Ω to 1.89kΩ Supertex inc. does not recommend the use of its products in life support applications, and will not knowingly sell them for use in such applications unless it receives an adequate “product liability indemnification insurance agreement.” Supertex inc. does not assume responsibility for use of devices described, and limits its liability to the replacement of the devices determined defective due to workmanship. No responsibility is assumed for possible omissions and inaccuracies. Circuitry and specifications are subject to change without notice. For the latest product specifications refer to the Supertex inc. (website: http//www.supertex.com) Supertex inc. ©2013 Supertex inc. All rights reserved. Unauthorized use or reproduction is prohibited. Doc.# DSAN-AN-D14 A040213 4 1235 Bordeaux Drive, Sunnyvale, CA 94089 Tel: 408-222-8888 www.supertex.com