LP2975AIMMX-12 National Semiconductor, LP2975AIMMX-12 Datasheet - Page 13

• Manufacturer Part Number: LP2975AIMMX-12
• Description: IC DRIVER/CONTR MOSFET LDO 8MSOP
• Manufacturer: National Semiconductor
• Type: Positive Fixedr

Specifications of LP2975AIMMX-12

• Number Of Outputs: 1
• Voltage - Output: 12V
• Current - Supply: 180µA
• Voltage - Input: 1.8 ~ 24 V
• Operating Temperature: -40°C ~ 125°C
• Package / Case: 8-MSOP, Micro8™, 8-uMAX, 8-uSOP,
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: *LP2975AIMMX-12

Application Hints
of 0.3–0.5˚C/W. The best source of information for this is
heatsink catalogs (Wakefield, AAVID, Thermalloy) since they
also sell mounting hardware.
θ
defines how well a heatsink transfers heat into the air. Once
this is determined, a heatsink must be selected which has a
value which is less than or equal to the computed value. The
value of θ
sheet for a heatsink, but the information is sometimes given
in a graph of temperature rise vs. dissipated power.
DESIGN EXAMPLE: A design is to be done which takes
3.3V in and provides 2.5V out at a load current of 7A. The
power dissipation will be calculated for both normal opera-
tion and short circuit conditions.
For normal operation:
If the output is shorted to ground:
(Assuming that a sense resistor is selected to set the value
of I
θ
a maximum T
For normal operation:
For designs which must operate with the output shorted to
ground:
The value of 14.3˚C/W can be easily met using a TO-220
device. Calculating the value of θ
value of θ
Any heatsink may be used with a thermal resistance ≤
10.3˚C/W
data sheet curves). Examples of suitable heatsinks are Ther-
malloy #6100B and IERC #LATO127B5CB.
However, if the design must survive a sustained short on the
output, the calculated θ
possibility of using a TO-220 package device.
Assuming a TO-3 device is selected with a θ
1.5˚C/W and θ
value of θ
A θ
sink, or possibly some kind of forced airflow for cooling.
SHORT-CIRCUIT CURRENT LIMITING
Short-circuit current limiting is easiliy implemented using a
single external resistor (R
calculated from:
Where:
I
V
The value of V
listed in the Electrical Characteristics section. When doing a
SC
S-A
J-A
CL
SC
S-A
is the desired short circuit current.
will be calculated assuming a maximum T
is the current limit sense voltage.
is the heatsink-to-ambient thermal resistance, which
10% above the nominal 7A).
value ≤1.3˚C/W would require a relatively large heat-
P
J-C
S-A
@
S-A
D
(SC) = V
θ
θ
5.6W power dissipation (refer to manufacturer’s
P
θ
:
θ
= 3˚C/W and θ
J-A
J-A
J
S-A
D
S-A
C-S
is usually listed in the manufacturer’s data
CL
of 150˚C:
= (V
θ
θ
= (150 − 70) / 5.6 = 14.3˚C/W
= (150 − 70) / 25.4 = 3.2˚C/W
θ
= 3.2 − (1.5 + 0.4) = 1.3˚C/W
S-A
= 14.3 − (3 + 1) = 10.3˚C/W
S-A
J-A
is 57 mV (typical), with guaranteed limits
= 0.4˚C/W, we can calculate the required
IN
IN
= (T
= θ
= θ
R
− V
SC
J-A
x I
J-A
J-A
J
OUT
SC
= V
SC
− T
value of 3.2˚C/W eliminates the
C-S
− (θ
− (θ
= 3.3 x 7.7 = 25.4W
). The value of R
) x I
CL
A
(Continued)
)/P
= 1˚C/W):
J-C
J-C
/ I
LOAD
S-A
D
SC
+ θ
+ θ
(MAX)
required (assuming a
C-S
C-S
= 5.6W
)
)
A
J-C
of 70˚C and
SC
value of
can be
13
worst-case calculation for power dissipation in the FET, it is
important to consider both the tolerance of V
tolerance (and temperature drift) of R
For maximum accuracy, the INPUT and CURRENT LIMIT
pins must be Kelvin connected to R
caused by voltage drops along the traces carrying the cur-
rent from the input supply to the Source pin of the FET.
EXTERNAL CAPACITORS
The best capacitors for use in a specific design will depend
on voltage and load current (examples of tested circuits for
several different output voltages and currents are provided in
a previous section.)
Information in the next sections is provided to aid the de-
signer in the selection of the external capacitors.
Input Capacitor
Although not always required, an input capacitor is recom-
mended. Good bypassing on the input assures that the
regulator is working from a source with a low impedance,
which improves stability. A good input capacitor can also
improve transient response by providing a reservoir of stored
energy that the regulator can utilize in cases where the load
current demand suddenly increases. The value used for C
may be increased without limit. Refer to the Reference De-
signs section for examples of input capacitors.
Output Capacitor
The output capacitor is required for loop stability (compen-
sation) as well as transient response. During sudden
changes in load current demand, the output capacitor must
source or sink current during the time it takes the control loop
of the LP2975 to adjust the gate drive to the pass FET. As a
general rule, a larger output capacitor will improve both
transient response and phase margin (stability). The value of
C
OUTPUT CAPACITOR AND COMPENSATION: Loop com-
pensation for the LP2975 is derived from C
cases, the feed-forward capacitor C
C
load resistance which causes the loop gain to roll off (de-
crease) at an additional −20 dB/decade. The frequency of
the pole is:
Where:
R
C
ESR is the equivalent series resistance of C
As a general guideline, the frequency of f
Hz. It should be noted that higher load currents correspond
to lower values of R
to keep f
DESIGN EXAMPLE: Select the minimum required output
capacitance for a design whose output specifications are 5V
Re-written:
Values used for the calculation:
f
Solving for C
would be 180 µF).
@
p
OUT
OUT
L
OUT
= 200 Hz, R
1A:
is the load resistance.
may be increased without limit.
is the value of the output capacitor.
forms a pole (referred to as f
p
at a given frequency.
f
f
C
OUT
p
p
L
OUT
= 0.16 / [ (R
= 0.16 / [ (R
= 5Ω, ESR = 0.1Ω (assumed).
, we get 157 µF (nearest standard size
L
= 0.16 / [f
, which requires that C
L
L
p
+ ESR) x C
+ ESR) x C
x (R
L
p
F
) in conjuction with the
+ ESR) ]
SC
(see next section).
SC
.
OUT
p
OUT
, to avoid errors
OUT
OUT
should be ≤ 200
]
]
and, in some
be increased
OUT
CL
www.national.com
.
and the
IN