Application Information: continued
Maximum Current Limit Setpoint
Therefore,
L= length (mils)
ICL(MAX) = VTRIP(MAX) / RSENSE(MAX)
W = width (mils)
t = thickness (mils)
For most PCBs the copper thickness, t, is 35µm (1.37 mils)
for one ounce copper. ρ = 717.86µΩ-mil
ICL(MAX) = 135mV / 3.18mΩ × 0.71 = 60A
For a Pentium®II load of 14.2A the resistance needed to
create a 43mV drop at full load is:
Therefore, the range of load currents that will cause the
internal current sense comparator to detect an overload
condition through a 3mΩ embedded PCB trace is: 14.2A <
ICL 60A, with 28.6A being the nominal overload condition.
43mV
IOUT
43mV
14.2A
RDROOP
=
=
= 3.0mΩ
Design Rules for Using a Droop Resistor
The resistivity of the copper will drift with the temperature
according to the following guidelines:
The basic equation for laying an embedded resistor is:
L
A
L
∆R = 12% @ TA = +50˚C
∆R = 34% @TA = +100˚C
RAR = ρ ×
or R = ρ ×
(W × t)
Droop Resistor Width Calculations
where:
The droop resistor must have the ability to handle the load
current and therefore requires a minimum width which is
calculated as follows (assume one ounce copper thickness):
A= W × t = cross-sectional area
ρ= the copper resistivity (µΩ - mil)
5V
1200µF/10V x 3
12V
1µF
IRL3103S
GATE(H)
V
CC
1.2µH
3.0mΩ
C
OFF
2.8V/30A
Power
1200µF
330pF
510
SS
I
SENSE
10V x 5
Supply
CS5166
COMP
0.1µF
0.1µF
0.1µF
IRL3103S
V
ID0
GATE(L)
V
ID1
PWRGD
PGnd
LGnd
V
ID2
V
ID3
V
ID4
3.3K
V
FB
V
PWRGD
ID4
V
V
V
ID3
ID2
ID1
1000pF
V
ID0
5V
12V
1200µF/10V x 3
1µF
IRL3103S
1.2µH
GATE(H)
V
CC
C
OFF
3.0mΩ
330pF
SS
510
I
SENSE
0.1µF
CS5166
COMP
0.1µF
IRL3103S
V
V
ID0
ID1
GATE(L)
PGnd
LGnd
V
ID2
V
ID3
3.3K
V
ID4
V
FB
1000pF
Figure 23: Current sharing of a 2.8V/30A power supply using two CS5166 synchronous buck regulators.
16
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
