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150mA ADJUSTABLE VOLTAGE REGULATOR
March 17, 1998
LP2951A
TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com
DESCRIPTION
The LP2951A is an improved version of the LP2951 and features tighter tolerance on the output voltage and reference voltage specifications. Both the LP2951 and LP2951A feature 150mA output current capability. The LP2951 series of low power voltage regulators have low quiescent current and low dropout voltage. The quiescent current increases minimally during dropout conditions thereby extending battery life. Available in the eight lead SOIC package, the LP2951 series includes features such as shutdown and low output voltage detect (typically due to low battery conditions) . This function may also be used as a power on reset function when triggered by CMOS or TTL inputs. The circuit can be used as a fixed voltage 5 volt regulator or adjusted between 1.24 volts and 29 volts using external resistor pairs.
FEATURES *= Guaranteed 150mA current *= Adjustable output voltage - 1.24V to 29V *= Accurate 5V output @150mA *= Low dropout voltage - 450mV @ 150mA *= Regulator or reference functions *= Direct replacement for LP2951AC, MIC2951-02,
AS2951AC
APPLICATIONS *= Microcontroller supplies *= Linear regulators *= Adjustable supplies *= Switching power supplies - post-regulation *= Portable modems *= Battery powered systems *= Cellular telephones *= Voltage references ORDERING INFORMATION
DEVICE LP2951ACM
(1)
BLOCK DIAGRAM
VOUTVOLTS ADJ
PACKAGE SO-8
Note: (1) Add suffix `TR' for tape and reel.
ABSOLUTE MAXIMUM RATINGS
Parameter Supply Voltage Shutdown Input Voltage Symbol VIN Maximum -0.3 to 30 -0.3 to 30 -0.3 to 30 PD TJ TSTG TLEAD Internally Limited -40 to 125 -65 to 150 260 Units V V V W C C C
PIN CONFIGURATION
Error Comp. Output Voltage Power Dissipation Operating Junction Temperature Range Storage Temperature Range Lead Temperature (Soldering) 5 Sec
1 (c) 1998 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
150mA ADJUSTABLE VOLTAGE REGULATOR
March 17, 1998
LP2951A
ELECTRICAL CHARACTERISTICS
Unless specified, limits are over operating temperature range (TJ = TA), VIN = VOUT(NOM) + 1V, IL = 100A, CL = 1F Parameter Output Voltage Temp Coefficient Line Regulation Load Regulation Dropout Voltage
(1)
Symbol VOUT TC REG(LINE) REG(LOAD) VD
Conditions TJ=25C, IL = 100A 0C TJ =70C 6V VIN==30V 100A IL==150mA IL===100A IL===150mA IL===100A IL===150mA VIN = 4.5V, IL===100A VOUT = 0 VREF VOUT (VIN - 1V), TJ ==25C, 100A IL==100mA
Min 4.975
Typ 5.000 20 0.1 0.1 80 380 120 8 110 200
Max 5.025 120 0.5 0.4 150 600 160 14 250 250 1.25 60
Units V ppm/C % % mV
Ground Current
IGND
A mA A mA V nA
Dropout Ground Current Current Limit Reference Voltage Feedback Bias Current Error Comparator Output High Leakage Current Output Low Voltage Threshold Voltage
IGND(D) ICL VREF IFB
1.22
1.235 20
VOH = 30V VIN = 4.5V, IOL = 400A Upper Lower 25 150 60 75 15
2 400
A mV mV
140 mV
Hysteresis Shutdown Input Input Logic Voltage VSD ISD IO(SD) Low High Input Current VSHUTDOWN = 2.4V VSHUTDOWN = 30V Regulator Shutdown Output Current VSHUTDOWN 2V, VIN 30V, VOUT = 0, Feedback pin to Tap 5V 2.0
0.6
V V
100 750 20
A
A
NOTES: (1) Temperature coefficient is defined as the worst case voltage change divided by total temperature range.
2 (c) 1998 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
150mA ADJUSTABLE VOLTAGE REGULATOR
March 17, 1998
LP2951A
PRINCIPLES OF OPERATION (LP2951 AND LP2951A)
Setting the Output Voltage The LP2951(A) can be set to deliver any output voltage from 1.24V to 30V by using an external voltage divider. In addition, an internal voltage divider is provided if a 5V output is desired. To use the internal voltage divider, simply connect the sense pin to the output and the tap pin to the feedback pin (see block diagram). When using an external divider the sense and tap pins are left open, and the divider is installed from the output to ground, with its center connected to the feedback pin (see Adjustable Regulator figure below). When using an external voltage divider, resistances can be calculated from the following formula: VOUT = eae 1 .235 + 20 x10 - 9 o x R1u + 1 .235 V / ec
ee R 2 o
are connected to the output, but this has not been observed in practice. It is also important that the capacitance be mounted close (1cm or less) to the output pin of the regulator.
An upper limit of values for R2 occurs at ~1.2M if the regulator is to be operated when completely unloaded, as this allows the feedback divider to provide the 1A minimum load recommended for the LP2951(A). If the regulator always has a load of 1A or more connected externally, higher resistor values can be used, but attention must be paid to the -20nA (typical) bias current required by the feedback pin. Using a 1.2M resistor for R2, this bias current will already cause a 2% shift in output voltage between full load and no load. Larger values of R2 exacerbate the problem. Using a 120K resistor for R2 reduces the error caused by feedback bias current to 0.2% while still only requiring 10A to feed the divider string. Output Filtering An output filter capacitor is always necessary with the LP2951(A) in order to assure output stability. The size eae 1 .235 u o of this capacitor varies- 9with1output voltage (smaller at + 20 x10 /xR + 1 .235 V ec higher output voltages) and output current (smaller at e R2 o e lower output currents). For 5V operation 1F is sufficient. For regulator operation at a minimum output voltage, (1.24V) and output currents of 100mA, the required filter increases to 3.3F. Any type of capacitor may be used, although if aluminum electrolytics are chosen, the equivalent series resistance (ESR) should be held to 5 or less. For small load currents the capacitance can be reduced. 0.33F will be satisfactory for output currents of 10mA or less, and 0.1F will work if output current is below 1mA. Theoretically, it is also possible for the regulator to become unstable if very large capacitances (>10,000F)
Adjustable Regulator If the lead inductance between the input of the LP2951(A) and its power source exceeds ~500nH (approximately 10"/25cm of 0.031"/0.78mm trace) it may also be necessary to add a filter capacitor between the input terminal and ground. A 1F tantalum or aluminum capacitor is usually sufficient. Lower values can be used if load currents are small. Noise injection into the feedback terminal of the LP2951(A) from nearby noise sources can also upset the output. Generally this can be cured by the addition of 100pF or so from the feedback terminal to the output.
3 (c) 1998 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
150mA ADJUSTABLE VOLTAGE REGULATOR
March 17, 1998
LP2951A
PRINCIPLES OF OPERATION (LP2951 and LP2951A) (cont.)
Reducing Output Noise In ultra-quiet systems, or when the LP2951(A) is being used as a reference, it may be desirable to perform additional output filtering to reduce noise. While this can be done simply using larger capacitors on the output, that solution tends to be bulky and expensive, and eventually, with huge capacitors (>1,000F) may cause instability in the regulator. Generally, it is more costeffective to let the regulator regulate output noise away. This can be done by bypassing the upper resistor in the feedback divider with a small capacitor to provide a more direct path for AC feedback. The size of this capacitor can be calculated from the formula: CBYPASS =
1 2 R 1fcorner
where R1 is the upper resistor of the feedback divider and fcorner is the frequency above which the increased AC feedback is to become active. Because the gain of the error amplifier in the LP2951(A) begins to roll off at about 300 Hz, this is generally an optimum choice for corner frequency. The reduction of the output noise will be proportional to the ratio of the two resistors in the feedback divider,
R1 R1 + R 2
and will increase 20 dB per decade at frequencies above the corner frequency chosen, up to the frequency where the error amplifier's gain has rolled off to 1 (100KHz). In order to maintain regulator stability when using a noise-reducing bypass capacitor, it will also be necessary to increase the size of the output filter capacitor by the ratio
R1 R1 + R 2
4 (c) 1998 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320
150mA ADJUSTABLE VOLTAGE REGULATOR
March 17, 1998
LP2951A
OUTLINE DRAWING SO-8
LAND PATTERN SO-8
5 (c) 1998 SEMTECH CORP. 652 MITCHELL ROAD NEWBURY PARK CA 91320

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