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PDF LP4951CM Data sheet ( Hoja de datos )
| Número de pieza | LP4951CM | |
| Descripción | Adjustable Micropower Voltage Regulators | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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September 2002
LP4950C-5V and LP4951C
Adjustable Micropower Voltage Regulators
General Description
The LP4950C and LP4951C are micropower voltage regu-
lators with very low quiescent current (75µA typ.) and very
low dropout voltage (typ. 40mV at light loads and 380mV at
100mA). They are ideally suited for use in battery-powered
systems. Furthermore, the quiescent current of the
LP4950C/LP4951C increases only slightly in dropout, pro-
longing battery life.
The LP4950C in the popular 3-pin TO-92 package is pin
compatible with older 5V regulators. The 8-lead LP4951C is
available in a plastic surface mount package and offers
additional system functions.
One such feature is an error flag output which warns of a low
output voltage, often due to falling batteries on the input. It
may be used for a power-on reset. A second feature is the
logic-compatible shutdown input which enables the regulator
to be switched on and off. Also, the part may be pin-strapped
for a 5V output or programmed from 1.24V to 29V with an
external pair of resistors.
Careful design of the LP4950C/LP4951C has minimized all
contributions to the error budget. This includes a tight initial
tolerance (.5% typ.), extremely good load and line regulation
(.05% typ.) and a very low output voltage temperature coef-
ficient, making the part useful as a low-power voltage refer-
ence.
Features
n High accuracy 5V guaranteed 100mA output
n Extremely low quiescent current
n Low dropout voltage
n Extremely tight load and line regulation
n Very low temperature coefficient
n Use as Regulator or Reference
n Needs only 1µF for stability
n Current and Thermal Limiting
LP4951C versions only
n Error flag warns of output dropout
n Logic-controlled electronic shutdown
n Output programmable from 1.24 to 29V
Block Diagram and Typical
Applications
LP4950C
LP4951C
20052825
20052801
© 2002 National Semiconductor Corporation DS200528
www.national.com
1 page  
Typical Performance Characteristics
Quiescent Current
Dropout Characteristics
Input Current
20052827
Input Current
20052828
20052829
Output Voltage vs. Temperature of 3
Representative Units
Quiescent Current
20052830
20052831
5
20052832
www.national.com
5 Page 
Application Hints (Continued)
20052820
*When VIN ≤ 1.3V, the error flag pin becomes a high impedance, and the
error flag voltage rises to its pull-up voltage. Using VOUT as the pull-up
voltage (see Figure 2), rather than an external 5V source, will keep the
error flag voltage under 1.2V (typ.) in this condition. The user may wish to
divide down the error flag voltage using equal-value resistors (10 kΩ
suggested), to ensure a low-level logic signal during any fault condition,
while still allowing a valid high logic level during normal operation.
FIGURE 1. ERROR Output Timing
PROGRAMMING THE OUTPUT VOLTAGE (LP4951C)
The LP4951C may be pin-strapped for 5V using its internal
voltage divider by tying the pin 1 (output) to pin 2 (sense)
pins together, and also tying the pin 7 (feedback) and pin 6
(VTAP) pins together. Alternatively, it may be programmed for
any output voltage between its 1.235V reference and its 30V
maximum rating. As seen in Figure 2, an external pair of
resistors is required.
The complete equation for the output voltage is
where VREF is the nominal 1.235 reference voltage and IFB is
the feedback pin bias current, nominally −20 nA. The mini-
mum recommended load current of 1µA forces an upper limit
of 1.2 MΩ on the value of R2, if the regulator must work with
no load (a condition often found in CMOS in standby). IFB will
produce a 2% typical error in VOUT which may be eliminated
at room temperature by trimming R1. For better accuracy,
choosing R2 = 100k reduces this error to 0.17% while in-
creasing the resistor program current to 12µA. Since the
LP4951C typically draws 60µA at no load with Pin 2 open-
circuited, this is a small price to pay.
*See Application Hints
20052807
**Drive with TTL-high to shut down. Ground or leave open if shutdown
feature is not to be used.
Note: Pins 2 and 6 are left open.
FIGURE 2. Adjustable Regulator (LP4951C)
REDUCING OUTPUT NOISE
In reference applications it may be advantageous to reduce
the AC noise present at the output. One method is to reduce
the regulator bandwidth by increasing the size of the output
capacitor. This is the only way noise can be reduced on the
3 lead LP4950C but is relatively inefficient, as increasing the
capacitor from 1µF to 220µF only decreases the noise from
430µV to 160µV rms for a 100kHz bandwidth at 5V output.
Noise can be reduced fourfold by a bypass capacitor across
R1, since it reduces the high frequency gain from 4 to unity.
Pick
or about 0.01µF. When doing this, the output capacitor must
be increased to 3.3µF to maintain stability. These changes
reduce the output noise from 430µV to 100µV rms for a
100kHz bandwidth at 5V output. With the bypass capacitor
added, noise no longer scales with output voltage so that
improvements are more dramatic at higher output voltages.
11 www.national.com
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| PDF Descargar | [ Datasheet LP4951CM.PDF ] | |
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