Application Information: continued
and a 45˚C temperature rise above ambient, the recom-
mended cross section is 275 mil2.
VTH(TYP)
RSENSE(NOM)
86mV
3.3mΩ
ICL(NOM)
=
=
= 26A.
W × t = 275 mil2,
Maximum Current Limit Setpoint
From the overcurrent detection data in the electrical char-
acteristics table:
where
W = droop resistor width;
t = droop resistor thickness.
VTH(MAX) = 101mV,
For 1oz. copper, t= 1.37 mils, therefore W = 201 mils =
0.201 in.
VTH(MAX)
RSENSE(MIN)
VTH(MAX)
SENSE(NOM) × 0.79
ICL(MAX)
=
=
R
L
W × t
R = ρ ×
,
101mV
3.3mΩ × 0.79
=
= 38.7A.
where
R = droop resistor value;
Therefore, the range of load currents that will cause the
internal current sense comparator to detect an overload
condition through a 3.2mΩ embedded PCB trace is: 19.3A
< ICL < 38.7A, with 26A being the nominal overload condi-
tion.
ρ = 0.71786mΩ-mil (1 oz. copper);
L = droop resistor length;
W = droop resistor width.
RDROOP = 3.3mΩ.
Design Rules for Using a Droop Resistor
The basic equation for laying an embedded resistor is:
L
3.3mΩ = 0.71786mΩ-mil ×
.
201 mils × 1.37 mils
L
L
RAR = ρ ×
or R = ρ ×
,
A
(W × t)
Hence, L = 1265 mils = 1.265 in.
where
In layouts where it is impractical to lay out a droop resistor
in a straight line 1265 mils long, the embedded PCB trace
can be “snaked” to fit within the available space.
A= W × t = cross-sectional area;
ρ= the copper resistivity (µΩ-mil);
L= length (mils);
W = width (mils);
t = thickness (mils).
Thermal Management
Thermal Considerations for Power MOSFETs
For most PCBs the copper thickness, t, is 35µm (1.37 mils)
for one ounce copper; ρ = 717.86µΩ-mil.
In order to maintain good reliability, the junction tempera-
ture of the semiconductor components should be kept to a
maximum of 150°C or lower. The thermal impedance
(junction to ambient) required to meet this requirement can
be calculated as follows:
For a CPU load of 16A the resistance needed to create a
50mV drop at full load is:
50mV
IOUT
50mV
16A
RDROOP
=
=
= 3.1mΩ.
TJ(MAX) - TA
Thermal Impedance =
Power
The resistivity of the copper will drift with the temperature
according to the following guidelines:
A heatsink may be added to TO-220 components to reduce
their thermal impedance. A number of PC board layout
techniques such as thermal vias and additional copper foil
area can be used to improve the power handling capability
of surface mount components.
∆R = 12% @ TA = +50˚C;
∆R = 34% @TA = +100˚C.
Droop Resistor Length, Width, and Thickness
The minimum width and thickness of the droop resistor
should primarily be determined on the basis of the current-
carrying capacity required, and the maximum permissible
droop resistor temperature rise. PCB manufacturer design
charts can be used in determining current- carrying capaci-
ty and sizes of etched copper conductors for various tem-
perature rises above ambient.
For single conductor applications, such as the use of the
droop resistor, PCB design charts show that for a droop
resistor with a required current-carrying capacity of 16A,
EMI Management
As a consequence of large currents being turned on and off
at high frequency, switching regulators generate noise as a
consequence of their normal operation. When designing
for compliance with EMI/EMC regulations, additional
components may be added to reduce noise emissions.
These components are not required for regulator operation
and experimental results may allow them to be eliminated.
17
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
