National Instruments Thermostat LM1085 Series User Manual

June 2005  
LM1085  
3A Low Dropout Positive Regulators  
General Description  
Features  
n Available in 3.3V, 5.0V, 12V and Adjustable Versions  
n Current Limiting and Thermal Protection  
n Output Current  
The LM1085 is a series of low dropout positive voltage  
regulators with a maximum dropout of 1.5V at 3A of load  
current. It has the same pin-out as National Semiconductor’s  
industry standard LM317.  
3A  
n Line Regulation  
n Load Regulation  
0.015% (typical)  
0.1% (typical)  
The LM1085 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: 3.3V, 5.0V and 12.0V.  
The fixed versions integrate the adjust resistors.  
Applications  
n High Efficiency Linear Regulators  
n Battery Charger  
The LM1085 circuit includes a zener trimmed bandgap ref-  
erence, current limiting and thermal shutdown.  
n Post Regulation for Switching Supplies  
n Constant Current Regulator  
n Microprocessor Supply  
The LM1085 series is available in TO-220 and TO-263 pack-  
ages. Refer to the LM1084 for the 5A version, and the  
LM1086 for the 1.5A version.  
Connection Diagrams  
Application Circuit  
TO-220  
10094702  
Top View  
TO-263  
10094752  
1.2V to 15V Adjustable Regulator  
10094704  
Top View  
Basic Functional Diagram,  
Adjustable Version  
10094765  
© 2005 National Semiconductor Corporation  
DS100947  
 
Absolute Maximum Ratings (Note 1)  
If Military/Aerospace specified devices are required,  
please contact the National Semiconductor Sales Office/  
Distributors for availability and specifications.  
Junction Temperature (TJ)(Note 3)  
Storage Temperature Range  
Lead Temperature  
150˚C  
-65˚C to 150˚C  
260˚C, to 10 sec  
2000V  
ESD Tolerance (Note 4)  
Maximum Input to Output Voltage Differential  
LM1085-ADJ  
LM1085-12  
29V  
Operating Ratings (Note 1)  
Junction Temperature Range (TJ) (Note 3)  
18V  
27V  
LM1085-3.3  
Control Section  
−40˚C to 125˚C  
−40˚C to 150˚C  
LM1085-5.0  
25V  
Output Section  
Power Dissipation (Note 2)  
Internally Limited  
Electrical Characteristics  
Typicals and limits appearing in normal type apply for TJ = 25˚C. Limits appearing in Boldface type apply over the entire junc-  
tion temperature range for operation.  
Min  
(Note 6)  
Typ  
(Note 5)  
Max  
(Note 6)  
Symbol  
Parameter  
Conditions  
Units  
VREF  
Reference Voltage  
LM1085-ADJ  
IOUT = 10mA, VIN−VOUT = 3V  
10mA IOUT IFULL LOAD,1.5V (VIN−VOUT) 15V  
(Note 7)  
1.238  
1.250  
1.262  
V
V
1.225  
1.250  
1.270  
VOUT  
Output Voltage  
(Note 7)  
LM1085-3.3  
3.270  
3.300  
3.330  
V
V
IOUT = 0mA, VIN = 5V  
3.235  
3.300  
3.365  
0 IOUT IFULL LOAD, 4.8VVIN 15V  
LM1085-5.0  
4.950  
5.000  
5.050  
V
V
IOUT = 0mA, VIN = 8V  
4.900  
5.000  
5.100  
0 IOUT IFULL LOAD, 6.5V VIN 20V  
LM1085-12  
11.880  
12.000  
12.120  
V
V
IOUT = 0mA, VIN = 15V  
0 IOUT IFULL LOAD, 13.5V VIN 25V  
LM1085-ADJ  
11.760  
12.000  
12.240  
VOUT  
Line Regulation  
(Note 8)  
0.015  
0.035  
0.5  
1.0  
0.5  
1.0  
1.0  
2.0  
0.1  
0.2  
3
0.2  
0.2  
6
%
IOUT =10mA, 1.5V(VIN-VOUT) 15V  
LM1085-3.3  
%
mV  
mV  
mV  
mV  
mV  
mV  
%
IOUT = 0mA, 4.8V VIN 15V  
LM1085-5.0  
6
10  
10  
25  
25  
0.3  
0.4  
15  
20  
20  
35  
36  
72  
IOUT = 0mA, 6.5V VIN 20V  
LM1085-12  
I
=0mA, 13.5V VIN 25V  
OUT  
VOUT  
Load Regulation  
(Note 8)  
LM1085-ADJ  
(VIN-V OUT) = 3V, 10mA IOUT IFULL LOAD  
LM1085-3.3  
%
mV  
mV  
mV  
mV  
mV  
mV  
VIN = 5V, 0 IOUT IFULL LOAD  
LM1085-5.0  
7
5
VIN = 8V, 0 IOUT IFULL LOAD  
LM1085-12  
10  
12  
VIN = 15V, 0 IOUT IFULL LOAD  
LM1085-ADJ, 3.3, 5, 12  
VREF, VOUT = 1%, IOUT = 3A  
24  
Dropout Voltage  
(Note 9)  
1.3  
1.5  
V
3
 
Electrical Characteristics (Continued)  
Typicals and limits appearing in normal type apply for TJ = 25˚C. Limits appearing in Boldface type apply over the entire junc-  
tion temperature range for operation.  
Min  
Typ  
Max  
Symbol  
Parameter  
Current Limit  
Conditions  
Units  
(Note 6) (Note 5) (Note 6)  
ILIMIT  
LM1085-ADJ  
VIN−VOUT = 5V  
VIN−VOUT = 25V  
LM1085-3.3  
VIN = 8V  
3.2  
0.2  
5.5  
0.5  
A
A
3.2  
3.2  
3.2  
5.5  
5.5  
5.5  
5.0  
5.0  
5.0  
A
A
LM1085-5.0  
VIN = 10V  
LM1085-12  
VIN = 17V  
A
Minimum Load  
LM1085-ADJ  
VIN −VOUT = 25V  
LM1085-3.3  
VIN 18V  
Current (Note 10)  
Quiescent Current  
10.0  
10.0  
10.0  
mA  
mA  
mA  
LM1085-5.0  
VIN 20V  
LM1085-12  
VIN 25V  
5.0  
10.0  
mA  
Thermal Regulation TA = 25˚C, 30ms Pulse  
.004  
0.02  
%/W  
Ripple Rejection  
fRIPPLE = 120Hz, COUT = 25µF Tantalum, IOUT = 3A  
LM1085-ADJ, CADJ = 25µF, (VIN−VO) = 3V  
LM1085-3.3, VIN = 6.3V  
60  
60  
60  
54  
75  
72  
68  
60  
55  
dB  
dB  
dB  
dB  
µA  
LM1085-5.0, VIN = 8V  
LM1085-12 VIN = 15V  
Adjust Pin Current  
Adjust Pin Current  
Change  
LM1085  
120  
5
10mA IOUT IFULL LOAD, 1.5V VIN−VOUT 25V  
0.2  
0.5  
µA  
%
Temperature  
Stability  
Long Term Stability TA=125˚C, 1000Hrs  
0.3  
1.0  
%
%
RMS Output Noise 10Hz f10kHz  
0.003  
(% of VOUT  
Thermal Resistance 3-Lead TO-263: Control Section/Output Section  
Junction-to-Case 3-Lead TO-220: Control Section/Output Section  
)
0.7/3.0  
0.7/3.0  
˚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 T  
, θ , and T . The maximum allowable power dissipation at any ambient temperature is  
JA  
J(max)  
A
P
= (T  
–T )/θ . All numbers apply for packages soldered directly into a PC board. Refer to Thermal Considerations in the Application Notes.  
D
J(max) JA  
A
Note 4: For testing purposes, ESD was applied using human body model, 1.5kin 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: I  
is defined in the current limit curves. The I  
Curve defines the current limit as a function of input-to-output voltage. Note that 30W power  
FULL LOAD  
FULL LOAD  
dissipation for the LM1085 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 30W. 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.  
4
 
Typical Performance Characteristics  
Dropout Voltage vs. Output Current  
Short-Circuit Current vs. Input/Output Difference  
10094768  
10094763  
Percent Change in Output Voltage vs. Temperature  
Adjust Pin Current vs. Temperature  
10094799  
10094798  
Maximum Power Dissipation vs. Temperature  
Ripple Rejection vs. Frequency (LM1085-Adj.)  
10094743  
10094770  
5
 
Typical Performance Characteristics (Continued)  
Ripple Rejection vs. Output Current (LM1085-Adj.)  
Line Transient Response  
10094744  
10094772  
Load Transient Response  
10094771  
6
 
STABILITY CONSIDERATION  
Application Note  
Stability consideration primarily concern the phase response  
of the feedback loop. In order for stable operation, the loop  
must maintain negative feedback. The LM1085 requires a  
certain amount series resistance with capacitive loads. This  
series resistance introduces a zero within the loop to in-  
crease phase margin and thus increase stability. The equiva-  
lent series resistance (ESR) of solid tantalum or aluminum  
electrolytic capacitors is used to provide the appropriate zero  
(approximately 500 kHz).  
GENERAL  
Figure 1 shows a basic functional diagram for the LM1085-  
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 LM1085  
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 LM1085 is guaran-  
teed to provide a minimum dropout voltage 1.5V over tem-  
perature, at full load.  
The Aluminum electrolytic are less expensive than tantal-  
ums, but their ESR varies exponentially at cold tempera-  
tures; 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 tantal-  
ums 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.  
10094765  
It is desirable to have large output capacitance for applica-  
tions that entail large changes in load current (microproces-  
sors 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:  
FIGURE 1. Basic Functional Diagram for the LM1085,  
excluding Protection circuitry  
OUTPUT VOLTAGE  
V = I x ESR  
The LM1085 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.  
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  
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 100range.  
For fixed voltage devices, R1 and R2 are integrated inside  
the devices.  
Ripple rejection is a function of the open loop gain within the  
feed-back loop (refer to Figure 1 and Figure 2). The LM1085  
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. There-  
fore 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 imped-  
ance 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 in-  
creased.  
1/(2π*fRIPPLE*CADJ) R1  
LOAD REGULATION  
10094717  
The LM1085 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 regula-  
tion, a few precautions are needed.  
FIGURE 2. Basic Adjustable Regulator  
7
 
adjustment terminal. The adjust pin can take a transient  
signal of 25V with respect to the output voltage without  
damaging the device.  
Application Note (Continued)  
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.  
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.  
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 LM1085 regu-  
lator, the internal diode between the output and input pins  
can withstand microsecond surge currents of 10A to 20A.  
With an extremely large output capacitor (1000 µf), and  
with input instantaneously shorted to ground, the regulator  
could be damaged. In this case, an external diode is recom-  
mended between the output and input pins to protect the  
regulator, shown in Figure 5.  
10094718  
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 regu-  
lation. For example, a 5V regulator with 0.05resistance  
between the regulator and load will have a load regulation  
due to line resistance of 0.05x 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 sens-  
ing and improve load regulation.  
10094715  
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 lim-  
iting 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.  
10094719  
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 (θJA).  
Thermal resistance has units of temperature per power (C/  
W). The higher the thermal resistance, the hotter the IC.  
FIGURE 4. Best Load Regulation using Adjustable  
Output Regulator  
PROTECTION DIODES  
Under normal operation, the LM1085 regulator does not  
need any protection diode. With the adjustable device, the  
internal resistance between the adjustment and output ter-  
minals limits the current. No diode is needed to divert the  
current around the regulator even with a capacitor on the  
The LM1085 specifies the thermal resistance for each pack-  
age as junction to case (θJC). In order to get the total  
resistance to ambient (θJA), two other thermal resistance  
8
 
IIN = IL + IG  
Application Note (Continued)  
must be added, one for case to heat-sink (θCH) and one for  
heatsink to ambient (θHA). The junction temperature can be  
predicted as follows:  
PD = (VIN−VOUT) IL + VIN G  
I
Figure 6 shows the voltages and currents which are present  
in the circuit.  
TJ = TA + PD (θJC + θCH + θHA) = TA + PD θJA  
TJ is junction temperature, TA is ambient temperature, and  
PD is the power consumption of the device. Device power  
consumption is calculated as follows:  
10094716  
FIGURE 6. Power Dissipation Diagram  
Once the devices power is determined, the maximum allow-  
able (θJA(max)) is calculated as:  
θJA(max) = TR(max)/PD = TJ(max − TA(max))/PD  
The LM1085 has different temperature specifications for two  
different sections of the IC: the control section and the output  
section. The Electrical Characteristics table shows the junc-  
tion to case thermal resistances for each of these sections,  
while the maximum junction temperatures (TJ(max)) for each  
section is listed in the Absolute Maximum section of the  
datasheet. TJ(max) is 125˚C for the control section, while  
TJ(max) is 150˚C for the output section.  
θHA(max) = θJA(max) − (θJC + θCH)  
θHA(max) should also be calculated twice as follows:  
θHA(max) = θJA (max, CONTROL SECTION) - (θJC (CON-  
TROL SECTION) + θCH  
HA(max)=θJA(max, OUTPUT SECTION)  
SECTION) + θCH  
)
θ
-
(θJC(OUTPUT  
)
If thermal compound is used, θCH can be estimated at 0.2  
C/W. If the case is soldered to the heat sink, then a θCH can  
be estimated as 0 C/W.  
After, θHA(max) is calculated for each section, choose the  
lower of the two θHA(max) values to determine the appropriate  
heat sink.  
θJA(max) should be calculated separately for each section as  
follows:  
If PC board copper is going to be used as a heat sink, then  
Figure 7 can be used to determine the appropriate area  
(size) of copper foil required.  
θJA (max, CONTROL SECTION) = (125˚C - TA(max))/PD  
θ
JA(max, OUTPUT SECTION) = (150˚C - TA(max))/PD  
The required heat sink is determined by calculating its re-  
quired thermal resistance (θHA(max)).  
10094764  
FIGURE 7. Heat sink thermal Resistance vs Area  
9
 
Typical Applications  
10094767  
5V to 3.3V, 1.5A Regulator  
10094754  
Battery Charger  
10094750  
@
Adjustable 5V  
10094755  
Adjustable Fixed Regulator  
10094756  
Regulator with Reference  
10094752  
1.2V to 15V Adjustable Regulator  
10094757  
High Current Lamp Driver Protection  
10094753  
5V Regulator with Shutdown  
10  
 
Typical Applications (Continued)  
10094761  
Automatic Light control  
10094759  
Battery Backup Regulated Supply  
10094762  
Generating Negative Supply voltage  
10094760  
Ripple Rejection Enhancement  
10094758  
Remote Sensing  
11  
 
Physical Dimensions inches (millimeters) unless otherwise noted  
3-Lead TO-263  
NS Package Number TS3B  
3-Lead TO-220  
NS Package Number T03B  
12  
 
Notes  
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves  
the right at any time without notice to change said circuitry and specifications.  
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WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR  
CORPORATION. As used herein:  
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which, (a) are intended for surgical implant into the body, or  
(b) support or sustain life, and whose failure to perform when  
properly used in accordance with instructions for use  
provided in the labeling, can be reasonably expected to result  
in a significant injury to the user.  
2. A critical component is any component of a life support  
device or system whose failure to perform can be reasonably  
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