Fairchild RC5050, RC5051 Selecting the Inductor, Implementing Short Circuit Protection

AN50 APPLICATION NOTE

Selecting the Inductor

The inductor is one of the most critical components to be selected for a DC-DC converter application. The critical parameters are inductance (L), maximum DC current (IO), and DC coil resistance (Rl). The inductor core material is a crucial factor in determining the amount of current the inductor is able to withstand. As with all engineering designs, tradeoffs exist between various types of core materi- als. In general, Ferrites are popular due to low cost, low EMI properties, and high frequency (>500KHz) characteristics. Molypermalloy powder (MPP) materials exhibit good satu- ration characteristics, low EMI, and low hysteresis losses, but tend to be expensive and more effectively utilized at operating frequencies below 400KHz. Another critical parameter is the DC winding resistance of the inductor. This value should typically be reduced as much as possible, as the power loss in the DC resistance degrades the efficiency of the converter by the relationship: 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.

Then the inductor value can be calculated using the relationship:

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

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

Implementing Short Circuit Protection

Intel currently requires all power supply manufacturers to provide continuous protection against short circuit condi- tions that may damage the CPU. To address this requirement, Raytheon Electronics has implemented a current sense meth- odology to limit the power delivered to the load in the event of overcurrent. The voltage drop created by the output cur- rent across a sense resistor is presented to one terminal of an internal comparator with hysterisis. The other comparator terminal has the threshold voltage, nominally of 120mV. Table 6 states the limits for the comparator threshold of the Switching Regulator.

Table 6. RC5050 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 DC-DC con- verter may not be able to continuously deliver the maximum CPU load current. If the threshold level is too high, the out- put driver may not be disabled at a safe limit and the result- ing power dissipation within the MOSFET(s) may rise to destructive levels.

The following is the design equation used to set the short cir- cuit threshold limit:

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

Where Ipk and Imin are peak ripple current and Iload, max = 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 8 illustrates the inductor current waveform for the RC5050 DC-DC converter at maximum load.

		Ipk
I	(Ipk-Imin)/2
	Imin		ILOAD, MAX
	TON	TOFF	t
		T=1/f s

Figure 8. Typical DC-DC Converter

Inductor Current Waveform

The calculation of this ripple current is as follows:

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

where:

VIN = input voltage to converter,

VSW = voltage across switcher MOSFET = ILOAD x RDS,ON, VD = Forward Voltage of the Schottky diode,

T = the switching period of the converter = 1/fS, and fS = switching frequency.

For an input voltage of 5V, output voltage of 3.3V, L equals 1.3∝H and a switching frequency of 285KHz (using CEXT = 100pF), the inductor current can be calculated at approximately 1A:

(Ipk	– Imin )	(5.0 – 14.5 × 0.037 – 3.3)				×
--------------------------	- =	------------------------------------------------------------	1.3 × 10–6		-
	2
	(3.3 + 0.5)			-----------------------1	3 = 2A
---------------------------------------------------------5.0 – 14.5 × 0.037 + 0.5 ×				285 × 10

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