Fairchild RC5040 and RC5042 Short Circuit Current Characteristics, For each Mosfet

APPLICATION NOTE AN42

IFBH

IFBL

MnCu Discrete Resistor

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

Figure 12. Short Circuit Sense Resistor Design Using PC Trace Resistor and Optional Discrete Sense Resistor

currents to change. Therefore, combining an embedded resistor with a discrete resistor may be a desirable option. This section discusses a design that provides flexibility and addresses wide tolerances. Refer to Figure 12.

In this design, the user has the option to choose either an embedded or a discrete MnCu sense resistor. To use the dis- crete sense resistor, populate R21 with a shorting bar (zero Ohm resistor) for a proper Kelvin connection and add the MnCu sense resistor. To use the embedded sense resistor, populate R22 with a shorting bar for a Kelvin connection. The embedded sense resistor allows you 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, and the left or the right tap yield 6.7 or 5.3 mΩ, respectively.

RC5040 and RC5042 Short Circuit Current Characteristics

The RC5040 and RC5042 have a short circuit current char- acteristic that includes a hysteresis function. This function prevents the DC-DC converter from oscillating in the event of a short circuit. Figure 13 shows the typical characteristic of the DC-DC converter using a 6.5 mΩ sense resistor.

	3.5
Voltage	3.0
	2.5
Output	2.0
	1.5
	1.0

0.5

0

0 5 10 15 20 25

Output Current

Figure 13. RC5040/RC5042 Short Circuit Characteristic

The converter exhibits at normal load regulation until the voltage across the resistor reaches 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 the output voltage as the load regulation col- lapses into the short circuit control mode. The output voltage does not return to its nominal value until the output short cir- cuit current is reduced to within the safe range for the DC- DC converter.

Power Dissipation Consideration During a Short Circuit Condition

The RC5040 and RC5042 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 over-stress and eventual destruction. Figure 14A shows the gate drive signal of a typ- ical RC5040 operating in continuous mode with a load cur- rent of 10A. The duty cycle is then 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 14B shows the result of the RC5040 going into its short circuit mode when the duty cycle is around 20%. Cal- culating the power on the MOSFET at each condition on the graph in Figure 13 shows how the protection scheme works. The power dissipated in the MOSFET at normal operation for a load current of 14.5A, is given by:

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

Thus, the MOSFET is not being over-stressed during a short circuit condition.

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