LP2954ISX/NOPB National Semiconductor, LP2954ISX/NOPB Datasheet  Page 9
LP2954ISX/NOPB
Manufacturer Part Number
LP2954ISX/NOPB
Description
IC REG LDO MICROPOWER TO263
Manufacturer
National Semiconductor
Datasheet
1.LP2954ISNOPB.pdf
(15 pages)
Specifications of LP2954ISX/NOPB
Regulator Topology
Positive Fixed
Voltage  Output
5V
Voltage  Input
6 ~ 30 V
Voltage  Dropout (typical)
0.47V @ 250mA
Number Of Regulators
1
Current  Output
250mA
Operating Temperature
40°C ~ 125°C
Mounting Type
Surface Mount
Package / Case
TO2633, D²Pak (3 leads + Tab), TO263AA
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Current  Limit (min)

Other names
*LP2954ISX
*LP2954ISX/NOPB
LP2954ISX
*LP2954ISX/NOPB
LP2954ISX
Application Hints
(Continued)
DROPOUT VOLTAGE
The dropout voltage of the regulator is defined as the mini
mum inputtooutput voltage differential required for the out
put voltage to stay within 100 mV of the output voltage
measured with a 1V differential. The dropout voltages for
various values of load current are listed under Electrical
Characteristics.
If the regulator is powered from a rectified AC source with a
capacitive filter, the minimum AC line voltage and maximum
load current must be used to calculate the minimum voltage
at the input of the regulator. The minimum input voltage,
including AC ripple on the filter capacitor , must not drop
below the voltage required to keep the LP2954 in regulation.
It is also advisable to verify operating at minimum operating
ambient temperature, since the increasing ESR of the filter
capacitor makes this a worstcase test for dropout voltage
due to increased ripple amplitude.
HEATSINK REQUIREMENTS
A heatsink may be required with the LP2954 depending on
the maximum power dissipation and maximum ambient tem
perature of the application. Under all possible operating
conditions, the junction temperature must be within the
range specified under Absolute Maximum Ratings.
To determine if a heatsink is required, the maximum power
dissipated by the regulator, P(max), must be calculated. It is
important to remember that if the regulator is powered from
a transformer connected to the AC line, the maximum
specified AC input voltage must be used (since this pro
duces the maximum DC input voltage to the regulator).
Figure 1 shows the voltages and currents which are present
in the circuit. The formula for calculating the power dissi
pated in the regulator is also shown in Figure 1.
*See External Capacitors
P
= (V
−5) I
+ (V
) I
Total
IN
L
IN
G
FIGURE 1. Basic 5V Regulator Circuit
The next parameter which must be calculated is the maxi
mum allowable temperature rise, T
(max). This is calculated
R
by using the formula:
T
(max) = T
(max) − T
(max)
R
J
A
where: T
(max) is the maximum allowable junction
J
temperature
T
(max) is the maximum ambient temperature
A
Using the calculated values for T
(max) and P(max), the
R
required value for junctiontoambient thermal resistance,
θ
, can now be found:
(JA)
θ
= T
(max)/P(max)
(JA)
R
If the calculated value is 60˚ C/W or higher , the regulator
may be operated without an external heatsink. If the calcu
lated value is below 60˚ C/W, an external heatsink is re
quired. The required thermal resistance for this heatsink can
be calculated using the formula:
θ
= θ
− θ
− θ
(HA)
(JA)
(JC)
(CH)
where:
θ
is the junctiontocase thermal resistance, which is
(JC)
specified as 3˚ C/W maximum for the LP2954.
θ
is the casetoheatsink thermal resistance, which is
(CH)
dependent on the interfacing material (if used). For details
and typical values, refer to (Note 2) listed at the end of the
ELECTRICAL CHARACTERISTICS section.
θ
is the heatsinktoambient thermal resistance. It is this
(HA)
specification (listed on the heatsink manufacturers data
sheet) which defines the effectiveness of the heatsink. The
heatsink selected must have a thermal resistance which is
equal to or lower than the value of θ
above listed formula.
PROGRAMMING THE OUTPUT VOLTAGE
The regulator may be pinstrapped for 5V operation using its
internal resistive divider by tying the Output and Sense pins
together and also tying the Feedback and 5V Tap pins
together.
Alternatively, it may be programmed for any voltage between
the 1.23V reference and the 30V maximum rating using an
external pair of resistors (see Figure 2). The complete equa
tion for the output voltage is:
where V
is the 1.23V reference and I
REF
pin bias current (−20 nA typical). The minimum recom
mended load current of 1 µA sets an upper limit of 1.2 MΩ on
the value of R2 in cases where the regulator must work with
no load (see MINIMUM LOAD ). I
error in V
which can be eliminated at room temperature
OUT
by trimming R1. For better accuracy, choosing R2 = 100 kΩ
will reduce this error to 0.17% while increasing the resistor
program current to 12 µA. Since the typical quiescent current
is 120 µA, this added current is negligible.
01112805
* See Application Hints
** Drive with TTLlow to shut down
FIGURE 2. Adjustable Regulator
9
calculated from the
(HA)
is the Feedback
FB
will produce a typical 2%
FB
01112836
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