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RC5036
PRODUCT SPECIFICATION
12
Short Circuit Considerations
For the Switch-Mode Regulator
The RC5036 uses a current sensing scheme to limit the load
current if an output fault condition occurs. The current sense
resistor carries the peak current of the inductor, which is
greater than the maximum load current due to ripple currents
flowing in the inductor. The RC5036 will begin to limit the
output current to the load by turning off the top-side FET
driver when the voltage across the current-sense resistor
exceeds the short circuit comparator threshold voltage (V
th
).
When this happens the output voltage will temporarily go
out of regulation. As the voltage across the sense resistor
becomes larger, the top-side MOSFET will continue to turn
off until the current limit value is reached. At this point, the
RC5036 will continuously deliver the limit current at a
reduced output voltage level. The short circuit comparator
threshold voltage is typically 90mV, with a variability of
±10mV. The ripple current flowing through the inductor is
typically 0.5A. Refer to Application Note AM-53 for
detailed discussions. The sense resistor value can be approx-
imated as follows:
where TF = Tolerance Factor for the sense resistor and 0.5A
accounts for the inductor current ripple.
Since the value of the sense resistor is often less than 10m
,
care should be taken in the layout of the PCB. Trace resis-
tance can contribute significant errors. The traces to the
IFBH and IFBL pins of the RC5036 should be Kelvin con-
nected to the pads of the current-sense resistor. To minimize
the influence of noise, the two traces should be run next to
each other.
For the Linear Regulator
The analysis for short circuit protection of the linear regula-
tor is much simpler than that of the switching regulator. The
formula for the inception point of short-circuit protection for
the linear regulator is:
V
th
= 45mV ± 8mV and I
LOAD,MAX
= 5A,
Schottky Diode
In Figure 1, MOSFET Q1 and flyback diode D1 are used as
complementary switches in order to maintain a constant cur-
rent through the output inductor L1. As a result, D1 will have
to carry the full current of the output load when the power
MOSFET is turned off. The power in the diode is a direct
function of the forward voltage at the rated load current dur-
ing the off time of the FET. The following equation can be
used to estimate the diode power:
where I
D
is the forward current of the diode, V
D
is the for-
ward voltage of the diode, and DutyCycle is defined the
same as
For the Motorola MBRB1545CT Power Rectifier used in
Figure 1,
It is recommended that the diode T0-220 package be
attached to a heatsink.
Board Design Considerations
RC5036 Placement
Preferably the PC layer directly underneath the RC5036
should be the ground layer. This serves as extra isolation
from noisy power planes.
MOSFET Placement
Placement of the power MOSFET is critical in the design of
the switch-mode regulator. The FET should be placed in
such a way as to minimize the length of the gate drive path
from the RC5036 SDRV pin. This trace should be kept under
0.5" for optimal performance. Excessive lead length on this
trace causes high frequency noise resulting from the parasitic
inductance and capacitance of the trace. Since this voltage
can transition nearly 12V in around 100nsec, the resultant
ringing and noise will be very difficult to suppress. This trace
should be routed on one layer only and kept well away from
the “quiet” analog pins of the device: VREF, CEXT, FBSW,
IFBH, IFBL, and VFBL. Refer to Figure 3.
Inductor and Schottky Diode Placement
The inductor and fly-back Schottky diode must be placed
close to the source of the power MOSFET. The node con-
necting the inductor and the diode swing between the drain
voltage of the FET and the forward voltage of the Schottky
diode. It is recommended that this node be converted to a
plane if possible. This node is part of the high current path in
the design, and is best treated as a plane to minimize the par-
asitic resistance and inductance on that node.
R
SENSE
V
PK
----I
1
TF
–
(
)
×
V
LOAD,MAX
0.5A
1
TF
–
(
)
×
=
=
R
SENSE
V
LOAD,MAX
I
1
TF
–
(
)
×
=
R
SENSE
5A
37mV
1
29%
–
(
)
×
=
= 5.3m
for using an
embedded PC trace
resistor
R
SENSE
5A
37mV
1
5%
–
(
)
×
=
=
7.0m
for using a
discrete resistor
P
DIODE
I
D
V
D
1
DutyCycle
–
(
)
×
×
=
Duty Cycle
Vin
=
P
DIODE
10A
0.65
1
73.1%
–
(
)
×
×
1.75W
=
=