Fairchild RC5042, RC5040 Selecting the Inductor, Implementing Short Circuit Protection

AN42		APPLICATION NOTE
PDLOSS = 2.19W + 1.0W + 0.65W + 0.045W + 1.35W + 0.010W + 0.37W + 0.2W = 5.815W
∴ Efficiency =	3.3 ⋅ 10	≈ 85%
	3.3---------------------------------------⋅10 + 5.815

Selecting the Inductor

Selecting the right inductor component is critical in the DC-DC converter application. The inductor’s critical param- eters to consider are inductance (L), maximum DC current (IO), and coil resistance (Rl).

The inductor core material is crucial in determining the amount of current it can withstand. As with all engineering designs, tradeoffs exist between various types of core mate- rials. In general, Ferrites are popular due to their low cost, low EMI properties, and high frequency (>500KHz) charac- teristics. Molypermalloy powder (MPP) materials exhibit good saturation characteristics, low EMI, and low hysteresis losses; however, they tend to be expensive and more effec- tively utilized at operating frequencies below 400KHz.

Another critical parameter is the DC winding resistance of the inductor. This value should typically be as low as possi- ble because the power loss in DC resistance degrades the efficiency of the converter by PLOSS = IO2 x Rl. The value of the inductor is a function of the oscillator duty cycle (TON) and the maximum inductor current (IPK). IPK can be calculated from the relationship:

IPK	= IMIN	VIN – VSW – VD
		+ 	----------------------------------------L-	 TON

Where TON is the maximum duty cycle and VD is the forward voltage of diode DS1.

The inductor value can be calculated using the following relationship:

VIN – VSW – VO
L = 	----------------------------------------IPK	– IMIN	- TON

Table 6. RC5040 and RC5042 Short Circuit Comparator Threshold Voltage

	Short Circuit Comparator
	Vthreshold (mV)
Typical	120
Minimum	100
Maximum	140

When designing the external current sense circuitry, pay careful attention to the output limitations during normal operation and during a fault condition. If the short circuit protection threshold current is set too low, the converter may not be able to continuously deliver the maximum CPU load current. If the threshold level is too high, the output driver may not be disabled at a safe limit and the resulting power dissipation within the MOSFET(s) may rise to destructive levels.

The design equation used to set the short circuit threshold limit is as follows:

RSENSE	Vth	, where: ISC = output short circuit current
	= -------
	ISC
ISC ≥ Iinductor		= ILoad, max +	(Ipk – Imin )
			----------------------------2

where Ipk and Imin are peak ripple currents and Iload, max is the maximum output load current.

You must also take into account the current (Ipk –Imin), or the ripple current flowing through the inductor under normal operation. Figure 9 illustrates the inductor current waveform for the RC5040 and RC5042 DC-DC converters at maxi- mum load.

Where VSW (RDS,ON x IO) is the drain-to-source voltage of M1 when it is turned on.

Implementing Short Circuit Protection

Intel currently requires all power supply manufacturers to provide continuous protection against short circuit conditions that may damage the CPU. To address this requirement, Raytheon Electronics has implemented a cur- rent sense methodology on the RC5040 and RC5042 con- trollers. This methodology limits the power delivered to the load during an overcurrent condition. The voltage drop cre- ated by the output current flowing across a sense resistor is presented to one terminal of an internal comparator with hysterisis. The other comparator terminal has a threshold voltage, nominally 120mV. Table 6 states the limits for the comparator threshold of the switching regulator:

		Ipk
I	(Ipk – Imin )/2
	Imin		ILOAD, MAX
	TON	TOFF	t
		T = 1/fs

Figure 9. Typical DC-DC Converter

Inductor Current Waveform

The calculation of this ripple current is as follows:

(----------------------------Ipk– Imin )	=	(-----------------------------------------------VIN–VSW– VOUT )-	⋅	--------------------------------------------( VOUT + VD )	T
2		L		(VIN – VSW + VD )

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