CS51311GDR14 CHERRY [Cherry Semiconductor Corporation], CS51311GDR14 Datasheet - Page 17

• Manufacturer Part Number: CS51311GDR14
• Description: Synchronous CPU Buck Controller for 12V and 5V Applications
• Manufacturer: CHERRY [Cherry Semiconductor Corporation]

Maximum Current Limit Setpoint
From the overcurrent detection data in the electrical char-
acteristics table:
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
< I
tion.
Design Rules for Using a Droop Resistor
The basic equation for laying an embedded resistor is:
where
For most PCBs the copper thickness, t, is 35µm (1.37 mils)
for one ounce copper; ρ = 717.86µΩ-mil.
For a CPU load of 16A the resistance needed to create a
50mV drop at full load is:
The resistivity of the copper will drift with the temperature
according to the following guidelines:
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,
CL
A= W × t = cross-sectional area;
ρ= the copper resistivity (µΩ-mil);
L= length (mils);
W = width (mils);
t = thickness (mils).
< 38.7A, with 26A being the nominal overload condi-
I
CL(MAX)
I
CL(NOM)
R
DROOP
R
AR
=
=
= ρ ×
=
R
3.3mΩ × 0.79
∆R = 34% @T
∆R = 12% @ T
=
V
SENSE(MIN)
TH(MAX)
R
V
101mV
50mV
SENSE(NOM)
V
TH(MAX)
I
OUT
A
L
TH(TYP)
or R = ρ ×
=
=
= 101mV,
A
R
= 38.7A.
A
SENSE(NOM)
= +100˚C.
50mV
= +50˚C;
16A
=
V
3.3mΩ
86mV
TH(MAX)
(W × t)
= 3.1mΩ.
L
Application Information: continued
× 0.79
= 26A.
,
17
and a 45˚C temperature rise above ambient, the recom-
mended cross section is 275 mil
where
For 1oz. copper, t= 1.37 mils, therefore W = 201 mils =
0.201 in.
where
Hence, L = 1265 mils = 1.265 in.
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.
Thermal Considerations for Power MOSFETs
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:
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.
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.
W = droop resistor width;
t = droop resistor thickness.
R = droop resistor value;
ρ = 0.71786mΩ-mil (1 oz. copper);
L = droop resistor length;
W = droop resistor width.
3.3mΩ = 0.71786mΩ-mil ×
Thermal Impedance =
Thermal Management
EMI Management
R
R = ρ ×
W × t = 275 mil
DROOP
= 3.3mΩ.
W × t
2
L
.
201 mils × 1.37 mils
T
2
J(MAX)
,
,
Power
L
- T
A
.