LP2975AIMM-12 National Semiconductor, LP2975AIMM-12 Datasheet - Page 13

• Manufacturer Part Number: LP2975AIMM-12
• Description: MOSFET LDO Driver/Controller
• Manufacturer: National Semiconductor

Application Hints
It follows that the output voltage will be:
Some important considerations for an adjustable design:
The tolerance of the internal 24 k
Also, its temperature coefficient is almost certainly different
than the TC of the external resistor that is used for R2.
For these reasons, it is recommended that R2 be set at a
value that is not greater than 1.2k. In this way, the value of
R2 will dominate R
nal 24k resistor will have a negligible effect on output voltage
accuracy.
To determine the value for R1:
External Capacitors (Adjustable Application)
All information in the previous section EXTERNAL CAPACI-
TORS applies to the adjustable application with the excep-
tion of how to select the value of the feed-forward capacitor.
The feed-forward capacitor C
(see Typical Application Circuit) performs exactly the same
function as described in the previous section FEEDFOR-
WARD CAPACITOR. However, because R1 is user-
selected, a different formula must be used to determine the
value of C
As stated previously, the optimal frequency at which to place
the zero f
OPTIMIZING DESIGN STABILITY
Because the LP2975 can be used with a variety of different
applications, there is no single set of components that are
best suited to every design. This section provides informa-
tion which will enable the designer to select components that
optimize stability (phase margin) for a specific application.
Gate Capacitance
An important consideration of a design is to identify the fre-
quency of the pole which results from the capacitance of the
Gate of the FET (this pole will be referred to as f
gets closer to the loop crossover frequency, the phase mar-
gin is reduced. Information will now be provided to allow the
total Gate capacitance to be calculated so that f
proximated.
The first step in calculating fp is to determine how much ef-
fective Gate capacitance (C
calculating C
Where:
C
from the values (refer to FET data sheet for values of C
and C
G
G
sheet specifies forward transconductance (G
value of drain current (defined as I
sired value of load current (defined as I
GS
GD
m
is the transconductance of the FET. The FET data
is the Gate-to-Drain capacitance, which is equal to:
is the Gate-to-Source capacitance, which is found
RSS
C
zf
):
C
EFF
:
is usually between 5 kHz and 50 kHz.
EFF
R
= C
V
R1 = R
EQ
OUT
is:
C
GS
EQ
G
= (R2 x 24k) / (R2 + 24k)
C
C
= 1.24 [ (R1 / R
m
, and the tolerance and TC of the inter-
= 1 / (2
+ C
GS
EQ
= G
C
= C
GD
GD
[ (V
fs
[1 + G
ISS
= C
EFF
x (I
OUT
C
(Continued)
in the adjustable application
x R1 x f
− C
RSS
L
) is present. The formula for
/ 1.24) − 1]
/ I
D
m
RSS
D
resistor is about
). To find Gm at the de-
EQ
(R
)
1/2
) + 1]
zf
L
L
), use the formula:
)
/ / ESR) ]
pg
fs
) at some
can be ap-
pg
). As f
±
20%.
ISS
pg
13
Where:
R
ESR is the equivalent series resistance of the output ca-
pacitor.
The term R
It can be seen from these equations that C
To get the worst-case (maximum) value for C
maximum value of load current, which also means the mini-
mum value of load resistance R
most cases, the ESR is the dominant term which determines
the value of R
Gate Capacitance Pole Frequency (f
The pole frequency resulting from the Gate capacitance
C
Where:
R
drives the Gate of the FET. It is important to note that R
a function of input supply voltage (see graph GATE PIN
OUTPUT IMPEDANCE). As shown, the minimum value of
R
@
Using the equation for f
showing how f
(see graph f
L
EFF
O
O
V
is the load resistance.
is the output impedance of the LP2975 Gate pin which
is about 550
IN
is defined as f
= 3V.
L
pg
/ / ESR is defined as:
L
pg
Gate Pin Output Impedance
vs. C
/ / ESR.
(R
varies with C
f
pg
@
L
pg
EFF
V
x ESR) / (R
IN
and can be approximated from:
0.16 / (R
):
f
pg
pg
= 24V, increasing to about 1.55 k
, a family of curves are provided
vs. C
EFF
L
O
EFF
L
. It should be noted that in
x C
+ ESR)
for several values of R
EFF
pg
)
)
EFF
DS100034-20
DS100034-21
varies with R
EFF
www.national.com
, use the
O
L
is
O
.