Datasheet

LTC4081
20
4081f
NTC Trip Point Error
When a 1% resistor is used for R
HOT
, the major error in the 40°C
trip point is determined by the tolerance of the NTC thermistor.
A typical 100k NTC thermistor has ±10% tolerance. By look-
ing up the temperature coeffi cient of the thermistor at 40°C,
the tolerance error can be calculated in degrees centigrade.
Consider the Vishay NTHS0603N01N1003J thermistor, which
has a temperature coeffi cient of –4%/°C at 40°C. Dividing
the tolerance by the temperature coeffi cient, ±5%/(4%/°C) =
±1.25°C, gives the temperature error of the hot trip point.
The cold trip point error depends on the tolerance of the NTC
thermistor and the degree to which the ratio of its value at
0°C and its value at 40°C varies from 6.14 to 1. Therefore,
the cold trip point error can be calculated using the tolerance,
TOL, the temperature coeffi cient of the thermistor at 0°C, TC
(in %/°C), the value of the thermistor at 0°C, R
COLD
, and the
value of the thermistor at 40°C, R
HOT
. The formula is:
Temperature Error C
TOL
R
R
COLD
HOT
()
.
•
°=
+1
614
−−
⎛
⎝
⎜
⎞
⎠
⎟
1 100•
TC
For example, the Vishay NTHS0603N01N1003J thermistor
with a tolerance of ±5%, TC of –5%/°C and R
COLD
/R
HOT
of
6.13, has a cold trip point error of:
Temperature Error C()
.
.
•.
°=
+
−
⎛
⎝
1005
614
613 1
⎜⎜
⎞
⎠
⎟
−
• 100
5
.,.=− ° °095 105CC
SWITCHING REGULATOR
Setting the Buck Converter Output Voltage
The LTC4081 regulator compares the FB pin voltage with
an internal 0.8V reference to generate an error signal at the
output of the error amplifi er. A voltage divider from V
OUT
to ground (as shown in the Block Diagram) programs the
output voltage via FB using the formula:
VV
R
R
OUT
=+
⎡
⎣
⎢
⎤
⎦
⎥
08 1
7
8
.•
Keeping the current low (<5μA) in these resistors maxi-
mizes effi ciency, but making them too low may allow stray
capacitance to cause noise problems and reduce the phase
margin of the error amp loop. To improve the frequency
response, a phase-lead capacitor (C
PL
) of approximately
10pF can be used. Great care should be taken to route the
FB line away from noise sources, such as the inductor or
the SW line.
Inductor Selection
The value of the inductor primarily determines the cur-
rent ripple in the inductor. The inductor ripple
current ΔI
L
decreases with higher inductance and
increases with higher V
IN
or V
OUT
:
ΔI
V
fL
V
V
L
OUT
OSC
OUT
IN
=−
⎛
⎝
⎜
⎞
⎠
⎟
•
•1
APPLICATIO S I FOR ATIO
WUU
U