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1.5A Low Dropout Positive Regulators
April 1999
LM1086 1.5A Low Dropout Positive Regulators
General Description
The LM1086 is a series of low dropout positive voltage regulators with a maximum dropout of 1.5V at 1.5A of load current. It has the same pin-out as National Semiconductor's industry standard LM317. The LM1086 is available in an adjustable version, which can set the output voltage with only two external resistors. It is also available in three fixed voltages: 2.85V, 3.3V and 5.0V. The fixed versions integrate the adjust resistors. The LM1086 circuit includes a zener trimmed bandgap reference, current limiting and thermal shutdown. The LM1086 series is available in TO-220 and TO-263 packages. D-Pak is available upon special request; contact the National Semiconductor sales representative in your area.
Features
n n n n n Available in 2.85V, 3.3V, 5V and Adjustable Versions Current Limiting and Thermal Protection Output Current 1.5A Line Regulation 0.015% (typical) Load Regulation 0.1% (typical)
Applications
n n n n n n SCSI-2 Active Terminator High Efficiency Linear Regulators Battery Charger Post Regulation for Switching Supplies Constant Current Regulator Microprocessor Supply
Connection Diagrams
TO-220 TO-263
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Top View
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Top View
Note: D-Pak package also available. Contact National Semiconductor sales representative in your area.
Basic Functional Diagram, Adjustable Version
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(c) 1999 National Semiconductor Corporation
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Ordering Information
Package 3-lead TO-263 Temperature Range -40C to +125C Part Number LM1086IS-ADJ LM1086ISX-ADJ LM1086IS-2.85 LM1086ISX-2.85 LM1086IS-3.3 LM1086ISX-3.3 LM1086IS-5.0 LM1086ISX-5.0 0C to +125C LM1086CS-ADJ LM1086CSX-ADJ LM1086CS-2.85 LM1086CSX-2.85 LM1086CS-3.3 LM1086CSX-3.3 LM1086CS-5.0 LM1086CSX-5.0 3-lead TO-220 -40C to +125C LM1086IT-ADJ LM1086IT-2.85 LM1086IT-3.3 LM1086IT-5.0 0C to +125C LM1086CT-ADJ LM1086CT-2.85 LM1086CT-3.3 LM1086CT-5.0
Note: D-Pak package also available. Contact National Semiconductor sales representative in your area.
Transport Media Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel Rails Tape and Reel Rails Rails Rails Rails Rails Rails Rails Rails
NSC Drawing
TS3B
T03B
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Simplified Schematic
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Absolute Maximum Ratings (Note 1)
If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Maximum Input-to-Output Voltage Differential LM1086-ADJ LM1086-2.85 LM1086-3.3 LM1086-5.0 Power Dissipation (Note 2) Junction Temperature (TJ)(Note 3) Storage Temperature Range 29V 27V 27V 25V Internally Limited 150C -65C to 150C
Lead Temperature ESD Tolerance (Note 4)
260C, to 10 sec 2000V
Operating Ratings (Note 1)
Junction Temperature Range (TJ) (Note 3) C Grade Control Section Output Section I Grade Control Section Output Section -40C to 125C -40C to 150C 0C to 125C 0C to 150C
Electrical Characteristics
Typicals and limits appearing in normal type apply for TJ = 25C. Limits appearing in Boldface type apply over the entire junction temperature range for operation. Symbol VREF Parameter Reference Voltage Conditions LM1086-ADJ IOUT = 10mA, VIN-VOUT = 3V 10mA IOUT IFULL LOAD,1.5V VIN-VOUT 15V (Note 7) LM1086-2.85 IOUT = 0mA, VIN = 5V 0 IOUT IFULL LOAD, 4.35V VIN 18V LM1086-3.3 IOUT = 0mA, VIN = 5V 0 IOUT IFULL LOAD, 4.75V VIN18V LM1086-5.0 IOUT = 0mA, VIN = 8V 0 IOUT IFULL LOAD, 6.5V VIN20V VOUT Line Regulation (Note 8) LM1086-ADJ IOUT = 10mA, 1.5V (VIN-VOUT) 15V LM1086-2.85 IOUT = 0mA, 4.35V VIN 18V LM1086-3.3 IOUT = 0mA, 4.5V VIN 18V LM1086-5.0 I OUT = 0mA, 6.5V VIN 20V VOUT Load Regulation (Note 8) LM1086-ADJ (VIN-V OUT ) = 3V, 10mA IOUT IFULL LM1086-2.85 VIN = 5V, 0 IOUT IFULL LM1086-3.3 VIN = 5V, 0 IOUT IFULL LM1086-5.0 VIN = 8V, 0 IOUT IFULL Dropout Voltage (Note 9) LM1086-2.85/3.3/5/ADJ VREF = 1%, IOUT = 1.5A
LOAD LOAD
Min (Note 6) 1.238 1.225
Typ (Note 5) 1.250 1.250
Max (Note 6) 1.262 1.270
Units
V V
VOUT
Output Voltage (Note 7)
2.82 2.79 3.267 3.235 4.950 4.900
2.85 2.85 3.300 3.300 5.000 5.000 0.015 0.035 0.3 0.6 0.5 1.0 0.5 1.0 0.1 0.2 3 6 3 7 5 10 1.3
2.88 2.91 3.333 3.365 5.050 5.100 0.2 0.2 6 6 10 10 10 10 0.3 0.4 12 20 15 25 20 35 1.5
V V V V V V % % mV mV mV mV mV mV % % mV mV mV mV mV mV V
LOAD
LOAD
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Electrical Characteristics
(Continued)
Typicals and limits appearing in normal type apply for TJ = 25C. Limits appearing in Boldface type apply over the entire junction temperature range for operation. Symbol ILIMIT Parameter Current Limit LM1086-ADJ VIN-VOUT = 5V VIN-VOUT = 25V LM1086-2.85 VIN = 8V LM1086-3.3 VIN = 8V LM1086-5.0 VIN = 10V Minimum Load Current (Note 10) Quiescent Current LM1086-ADJ VIN -VOUT = 25V LM1086-2.85 VIN 18V LM1086-3.3 VIN 18V LM1086-5.0 VIN 20V TA = 25C, 30ms Pulse fRIPPLE = 120Hz, COUT = 25F Tantalum, IOUT = 1.5A LM1086-ADJ, CADJ = 25F, (VIN-VO) = 3V LM1086-2.85, VIN = 6V LM1086-3.3, VIN = 6.3V LM1086-5.0 VIN = 8V Adjust Pin Current Adjust Pin Current Change Temperature Stability Long Term Stability RMS Output Noise (% of VOUT) Thermal Resistance Junction-to-Case TA = 125C, 1000Hrs 0.3 10Hz f 10kHz 0.003 1.0 % % LM1086 10mA IOUT IFULL
LOAD,
Conditions
Min (Note 6) 1.50 0.05 1.5 1.5 1.5
Typ (Note 5) 2.00 0.15 2.00 2.00 2.00 5.0 5.0 5.0 5.0 0.008
Max (Note 6)
Units
A A A A A 10.0 10.0 10.0 10.0 0.04 mA mA mA mA %/W
Thermal Regulation Ripple Rejection
60 60 60 60
75 72 72 68 55 120
dB dB dB dB A
1.5V (VIN-VOUT) 15V 0.2 0.5 5 A %
3-Lead TO-263: Control Section/Output Section 3-Lead TO-220: Control Section/Output Section
1.5/4.0 1.5/4.0
C/W C/W
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications and the test conditions, see the Electrical Characteristics. Note 2: Power dissipation is kept in a safe range by current limiting circuitry. Refer to Overload Recovery in Application Notes. Note 3: The maximum power dissipation is a function of TJ(max) , JA, and TA. The maximum allowable power dissipation at any ambient temperature is PD = (TJ(max)-T A)/JA. All numbers apply for packages soldered directly into a PC board. Refer to Thermal Considerations in the Application Notes. Note 4: For testing purposes, ESD was applied using human body model, 1.5k in series with 100pF. Note 5: Typical Values represent the most likely parametric norm. Note 6: All limits are guaranteed by testing or statistical analysis. Note 7: IFULL LOAD is defined in the current limit curves. The IFULL LOAD Curve defines current limit as a function of input-to-output voltage. Note that 15W power dissipation for the LM1086 is only achievable over a limited range of input-to-output voltage. Note 8: Load and line regulation are measured at constant junction temperature, and are guaranteed up to the maximum power dissipation of 15W. Power dissipation is determined by the input/output differential and the output current. Guaranteed maximum power dissipation will not be available over the full input/output range. Note 9: Dropout voltage is specified over the full output current range of the device. Note 10: The minimum output current required to maintain regulation.
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Typical Performance Characteristics
Dropout Voltage vs Output Current Short-Circuit Current vs Input/Output Difference
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Load Regulation vs Temperature
Percent Change in Output Voltage vs Temperature
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Adjust Pin Current vs Temperature
Maximum Power Dissipation vs Temperature
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Typical Performance Characteristics
Ripple Rejection vs Frequency (LM1086-Adj.)
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Ripple Rejection vs Output Current (LM1086-Adj.)
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Ripple Rejection vs Frequency (LM1086-5)
Ripple Rejection vs Output Current (LM1086-5)
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Line Transient Response
Load Transient Response
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APPLICATION NOTE
General Figure 1 shows a basic functional diagram for the LM1086-Adj (excluding protection circuitry) . The topology is basically that of the LM317 except for the pass transistor. Instead of a Darlingtion NPN with its two diode voltage drop, the LM1086 uses a single NPN. This results in a lower dropout voltage. The structure of the pass transistor is also known as a quasi LDO. The advantage a quasi LDO over a PNP LDO is its inherently lower quiescent current. The LM1086 is guaranteed to provide a minimum dropout voltage 1.5V over temperature, at full load. crease phase margin and thus increase stability. The equivalent series resistance (ESR) of solid tantalum or aluminum electrolytic capacitors is used to provide the appropriate zero (approximately 500 kHz). The Aluminum electrolytic are less expensive than tantalums, but their ESR varies exponentially at cold temperatures; therefore requiring close examination when choosing the desired transient response over temperature. Tantalums are a convenient choice because their ESR varies less than 2:1 over temperature. The recommended load/decoupling capacitance is a 10uF tantalum or a 50uF aluminum. These values will assure stability for the majority of applications. The adjustable versions allows an additional capacitor to be used at the ADJ pin to increase ripple rejection. If this is done the output capacitor should be increased to 22uF for tantalums or to 150uF for aluminum. Capacitors other than tantalum or aluminum can be used at the adjust pin and the input pin. A 10uF capacitor is a reasonable value at the input. See Ripple Rejection section regarding the value for the adjust pin capacitor. It is desirable to have large output capacitance for applications that entail large changes in load current (microprocessors for example). The higher the capacitance, the larger the available charge per demand. It is also desirable to provide low ESR to reduce the change in output voltage: V = I x ESR It is common practice to use several tantalum and ceramic capacitors in parallel to reduce this change in the output voltage by reducing the overall ESR. Output capacitance can be increased indefinitely to improve transient response and stability. Ripple Rejection Ripple rejection is a function of the open loop gain within the feed-back loop (refer to Figure 1 and Figure 2). The LM1086 exhibits 75dB of ripple rejection (typ.). When adjusted for voltages higher than VREF, the ripple rejection decreases as function of adjustment gain: (1+R1/R2) or VO/VREF. Therefore a 5V adjustment decreases ripple rejection by a factor of four (-12dB); Output ripple increases as adjustment voltage increases. However, the adjustable version allows this degradation of ripple rejection to be compensated. The adjust terminal can be bypassed to ground with a capacitor (CADJ). The impedance of the CADJ should be equal to or less than R1 at the desired ripple frequency. This bypass capacitor prevents ripple from being amplified as the output voltage is increased. 1/(2*fRIPPLE*CADJ) R1 Load Regulation The LM1086 regulates the voltage that appears between its output and ground pins, or between its output and adjust pins. In some cases, line resistances can introduce errors to the voltage across the load. To obtain the best load regulation, a few precautions are needed.
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FIGURE 1. Basic Functional Diagram for the LM1086, excluding Protection circuitry Output Voltage The LM1086 adjustable version develops at 1.25V reference voltage, (VREF), between the output and the adjust terminal. As shown in figure 2, this voltage is applied across resistor R1 to generate a constant current I1. This constant current then flows through R2. The resulting voltage drop across R2 adds to the reference voltage to sets the desired output voltage. The current IADJ from the adjustment terminal introduces an output error . But since it is small (120uA max), it becomes negligible when R1 is in the 100 range. For fixed voltage devices, R1 and R2 are integrated inside the devices.
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FIGURE 2. Basic Adjustable Regulator Stability Consideration Stability consideration primarily concern the phase response of the feedback loop. In order for stable operation, the loop must maintain negative feedback. The LM1086 requires a certain amount series resistance with capacitive loads. This series resistance introduces a zero within the loop to inwww.national.com 8
Figure 3 shows a typical application using a fixed output regulator. Rt1 and Rt2 are the line resistances. VLOAD is less than the VOUT by the sum of the voltage drops along the line resistances. In this case, the load regulation seen at the RLOAD would be degraded from the data sheet specification.
APPLICATION NOTE
(Continued)
To improve this, the load should be tied directly to the output terminal on the positive side and directly tied to the ground terminal on the negative side.
withstand microsecond surge currents of 10A to 20A. With an extremely large output capacitor (1000 f), and with input instantaneously shorted to ground, the regulator could be damaged. In this case, an external diode is recommended between the output and input pins to protect the regulator, shown in Figure 5.
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FIGURE 3. Typical Application using Fixed Output Regulator When the adjustable regulator is used (Figure 4), the best performance is obtained with the positive side of the resistor R1 tied directly to the output terminal of the regulator rather than near the load. This eliminates line drops from appearing effectively in series with the reference and degrading regulation. For example, a 5V regulator with 0.05 resistance between the regulator and load will have a load regulation due to line resistance of 0.05 x IL. If R1 ( = 125) is connected near the load the effective line resistance will be 0.05 (1 + R2/R1) or in this case, it is 4 times worse. In addition, the ground side of the resistor R2 can be returned near the ground of the load to provide remote ground sensing and improve load regulation.
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FIGURE 5. Regulator with Protection Diode Overload Recovery Overload recovery refers to regulator's ability to recover from a short circuited output. A key factor in the recovery process is the current limiting used to protect the output from drawing too much power. The current limiting circuit reduces the output current as the input to output differential increases. Refer to short circuit curve in the curve section. During normal start-up, the input to output differential is small since the output follows the input. But, if the output is shorted, then the recovery involves a large input to output differential. Sometimes during this condition the current limiting circuit is slow in recovering. If the limited current is too low to develop a voltage at the output, the voltage will stabilize at a lower level. Under these conditions it may be necessary to recycle the power of the regulator in order to get the smaller differential voltage and thus adequate start up conditions. Refer to curve section for the short circuit current vs. input differential voltage. Thermal Considerations ICs heats up when in operation, and power consumption is one factor in how hot it gets. The other factor is how well the heat is dissipated. Heat dissipation is predictable by knowing the thermal resistance between the IC and ambient (JA). Thermal resistance has units of temperature per power (C/W). The higher the thermal resistance, the hotter the IC. The LM1086 specifies the thermal resistance for each package as junction to case (JC). In order to get the total resistance to ambient (JA), two other thermal resistance must be added, one for case to heat-sink (CH) and one for heatsink to ambient (HA). The junction temperature can be predicted as follows: TJ = TA + PD (JC + CH + HA) = TA + PD JA TJ is junction temperature, TA is ambient temperature, and PD is the power consumption of the device. Device power consumption is calculated as follows: IIN = IL + IG PD = (VIN-VOUT) IL + VINIG
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FIGURE 4. Best Load Regulation using Adjustable Output Regulator 3.0 Protection Diodes Under normal operation, the LM1086 regulator does not need any protection diode. With the adjustable device, the internal resistance between the adjustment and output terminals limits the current. No diode is needed to divert the current around the regulator even with a capacitor on the adjustment terminal. The adjust pin can take a transient signal of 25V with respect to the output voltage without damaging the device. When an output capacitor is connected to a regulator and the input is shorted, the output capacitor will discharge into the output of the regulator. The discharge current depends on the value of the capacitor, the output voltage of the regulator, and rate of decrease of VIN. In the LM1086 regulator, the internal diode between the output and input pins can
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APPLICATION NOTE
(Continued)
Figure 6 shows the voltages and currents which are present in the circuit.
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FIGURE 6. Power Dissipation Diagram Once the devices power is determined, the required JA is calculated as: JA (min) = TR(max)/PD = TJ(max) - TA(max)/PD The LM1086 specifies maximum junction temperature for two sections of the IC. One for the control section and one for the output section. The control section's maximum temperature is 125C for specified operation, while the maximum temperature for the output section is 150C before damage occurs. Both have different junction to case thermal resistances (See specification table). The maximum power dissipation curve in the curve section illustrates the differences between the control and output sections. The two negative slopes correspond to their different thermal resistances and their different maximum temperature intersects. The flat slope corresponds to the maximum power of the IC itself, regardless of package considerations. JA (min) should be calculated for each section. That is JA (min) should be calculated using 125C for TJ(max) and again using 150C for TJ(max). Each of these calculation should checked against their respective JC given in data table. If each calculation shows as less than JA(min) for each respective section, then no heatsink is required. If a heatsink is required, Its required thermal resistance can be calculated as follows: HA(min) = JA(min) - (JC + CH) Once the required thermal resistance for the heat sink is known, the size of the heat sink can be determined from Figure 7, which is based on PC board copper and no air flow.
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FIGURE 7. Heat sink thermal Resistance vs Area
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Typical Applications
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5V to 3.3V, 1.5A Regulator
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Adjustable @ 5V
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5V Regulator with Shutdown
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1.2V to 15V Adjustable Regulator
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Adjustable Fixed Regulator
Battery Charger
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Regulator with Reference
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High Current Lamp Driver Protection
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Typical Applications
(Continued)
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Battery Backup Regulated Supply
Ripple Rejection Enhancement
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Automatic Light control
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Remote Sensing
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Typical Applications
(Continued)
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SCSI-2 Active termination
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Physical Dimensions
inches (millimeters) unless otherwise noted
3-Lead TO-263 Package Order Number LM1086S-ADJ, LM1086S-3.3, LM1086S-5.0, or LM1086S-12 NSC Package Number TS3B
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1.5A Low Dropout Positive Regulators
Physical Dimensions
inches (millimeters) unless otherwise noted (Continued)
3-Lead TO-220 Package Order Number LM1086T-ADJ, LM1086T-3.3, LM1086T-5.0, or LM1086T-12 NSC Package Number T03B
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