LP2954IMX National Semiconductor, LP2954IMX Datasheet - Page 9

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LP2954IMX

Manufacturer Part Number
LP2954IMX
Description
Manufacturer
National Semiconductor
Datasheet

Specifications of LP2954IMX

Voltage Regulator Type
Linear
Topology
LDO
Regulator Output Type
Adjustable
Polarity Type
Positive
Number Of Outputs
Single
Input Voltage (min)
-20V
Input Voltage (max)
30V
Output Voltage
1.23 to 29V
Package Type
SOIC N
Output Current
250mA
Load Regulation
0.2%
Line Regulation
0.2%
Operating Temp Range
-40C to 125C
Operating Temperature Classification
Automotive
Pin Count
8
Mounting
Surface Mount
Lead Free Status / Rohs Status
Not Compliant

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Application Hints
DROPOUT VOLTAGE
The dropout voltage of the regulator is defined as the mini-
mum input-to-output 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 worst-case 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
The next parameter which must be calculated is the maxi-
mum allowable temperature rise, T
by using the formula:
T
where: T
T
Using the calculated values for T
required value for junction-to-ambient thermal resistance,
θ
θ
If the calculated value is 60˚ C/W or higher , the regulator
may be operated without an external heatsink. If the calcu-
Total
(J-A)
(J-A)
R
A
temperature
(max) = T
(max) is the maximum ambient temperature
= (V
, can now be found:
= T
IN
J
R
FIGURE 1. Basic 5V Regulator Circuit
(max) is the maximum allowable junction
−5) I
(max)/P(max)
J
(max) − T
L
+ (V
IN
) I
G
A
(max)
(Continued)
R
R
(max). This is calculated
(max) and P(max), the
01112805
9
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
where:
θ
specified as 3˚ C/W maximum for the LP2954.
θ
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
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 pin-strapped 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
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
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.
* See Application Hints
** Drive with TTL-low to shut down
(H-A)
(J-C)
(C-H)
(H-A)
is the heatsink-to-ambient thermal resistance. It is this
= θ
is the junction-to-case thermal resistance, which is
is the case-to-heatsink thermal resistance, which is
REF
(J-A)
OUT
is the 1.23V reference and I
FIGURE 2. Adjustable Regulator
− θ
which can be eliminated at room temperature
(J-C)
− θ
(C-H)
FB
will produce a typical 2%
(H-A)
FB
calculated from the
is the Feedback
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