Fairchild RC5050, RC5051 Selecting the Inductor, Implementing Short Circuit Protection, Table 6. RC5050 Short Circuit Comparator Threshold Voltage

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:

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:

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

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:

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.

Figure 8. Typical DC-DC Converter

Inductor Current Waveform

The calculation of this ripple current is as follows:

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

1.3µH and a switching frequency of 285KHz (using

CEXT = 100pF), the inductor current can be calculated at

approximately 1A:

Short Circuit Comparator

Vthreshold (mV)

Typical 120

Minimum 100

Maximum 140

IPK IMIN

VIN VSW

– VD

–

-----------------------------------------

 

  TON

LVIN VSW

– VO

–

IPK IMIN

–

-----------------------------------------

 

  TON

RSENSE

Vth

ISC

--------, where: I SC = Output short circuit current=

ISC Iinductor

≥ILoad, max

Ipk Imin

–( )

----------------------------+=

T=1/fs

TON TOFF

ILOAD, MAX

(Ipk-Imin)/2

Ipk

Imin

Ipk Imin

–( )

---------------------------- VIN VSW

– VOUT

–( )

----------------------------------------------------- VOUT VD

+( )

VIN VSW

– VD

+( )

-----------------------------------------------T×=

Ipk Imin

–( )

---------------------------- 5.0 14.5 0.037×– 3.3–( )

1.3 10 6–

-------------------------------------------------------------- ×=

3.3 0.5+( )

5.0 14.5 0.037×– 0.5+

--------------------------------------------------------- 1

285 103

-----------------------

×2A=