Fairchild SEMICONDUCTOR RC5051 User Manual
Page 13
APPLICATION NOTE
AN50
13
tion. The embedded sense resistor allows the user to choose a
plus or a minus delta resistance tap to offset any large sheet
resistivity change. In this design, the center tap yields 6m
Ω
,
the left tap yields 6.7m
Ω
, and the right tap yields 5.3m
Ω
.
RC5050 and RC5051 Short Circuit Current
Characteristics
The RC5050 and RC5051 short circuit current characteristic
includes a hysteresis function that prevents the DC-DC con-
verter from oscillating in the event of a short circuit. Figure
12 shows the typical characteristic of the DC-DC converter
circuit with a 6m
Ω
sense resistor. The converter exhibits a
normal load regulation characteristic until the voltage across
the resistor exceeds the internal short circuit threshold of
120mV. At this point, the internal comparator trips and
signals the controller to turn off the gate drive to the power
MOSFET. This causes a drastic reduction in output voltage
as the load regulation collapses into the short circuit control
mode. The output voltage does not return to its nominal
value the output current is reduced to a value within the safe
range for the DC-DC converter.
Figure 12. RC5050 Short Circuit Characteristic
Power Dissipation Consideration During a
Short Circuit Condition
The RC5050 and RC5051 controllers respond to an output
short circuit by drastically changing the duty cycle of the
gate drive signal to the power MOSFET. In doing this, the
power MOSFET is protected from stress and from eventual
failure. Figure 13A shows the gate drive signal of a typical
RC5050 operating in continuous mode with a load current of
10A. The duty cycle is set by the ratio of the input voltage to
the output voltage. If the input voltage is 5V, and the output
voltage is 3.1V, the ratio of Vout/ Vin is 62%. Figure 13B
shows the result of a RC5050 going into its short circuit
mode with a duty cycle approximately of 20%. Calculating
the power in the MOSFET at each condition on the graph
(Figure 12) shows how the protection works. The power dis-
sipated in the MOSFET at normal operation for a load cur-
rent of 14.5A, is given by:
for each MOSFET.
The power dissipated in the MOSFET at short circuit
condition for a peak short current of 20A, is given by:
for each MOSFET.
These calculations show that the MOSFET is not being
over-stressed during a short circuit condition.
1.0
0.5
0
1.5
2.0
2.5
3.0
3.5
0
5
10
15
20
25
Output Current
Output Voltage
P
D
I
2
R
ON
×
DutyCycle
14.5
2
----------
2
.037
˙
×
.62
1.2W
=
Ч
=
Ч
=
P
D
20
2
------
2
.037
Ч
.2
Ч
0.74W
=
=
Figure 11. Short Circuit Sense Resistor Design Using a PC Trace Resistor and an Optional Discrete Sense Resistor
Embedded Sense Resistor
IFBH
IFBL
MnCu Discrete
Resistor
R21
R22
Output Power
Plane (Vout)
R-
∆
r
R
R+
∆
r