Fairchild RC5050 and RC5051 Short Circuit Current Characteristics, For each Mosfet

APPLICATION NOTE AN50

IFBH

IFBL

MnCu Discrete Resistor

	Embedded Sense Resistor
R21	R22
	Output Power
	Plane (Vout)
	R-Δr
	R
	R+Δr

Figure 11. Short Circuit Sense Resistor Design Using a PC Trace Resistor and an Optional Discrete Sense Resistor

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 6mΩ, the left tap yields 6.7mΩ, and the right tap yields 5.3mΩ.

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 6mΩ 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.

	3.5
Voltage	3.0
	2.5
Output	2.0
	1.5
	1.0

0.5

0

0 5 10 15 20 25

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:

P		= I	2	⋅ R		⋅ DutyCycle =		14.5 2	˙	⋅ .62 = 1.2W
	D				ON			---------	⋅ .037
								2 

for each MOSFET.

The power dissipated in the MOSFET at short circuit condition for a peak short current of 20A, is given by:

P		=	 20	2	⋅ .037	⋅ .2 = 0.74W
	D		-----
			 2 

for each MOSFET.

These calculations show that the MOSFET is not being over-stressed during a short circuit condition.

Output Current

Figure 12. RC5050 Short Circuit Characteristic

13