7

Figure 7. Forward Current vs. Forward Voltage at 25°C.

Becausetheautomatic,pick-and-placeequipmentused

toassembletheseproductsselectsdicefromadjacent

sitesonthewafer,thetwodiodeswhichgointotheHBAT-

540orHBAT-540C(seriespair)arecloselymatched—

withouttheaddedexpenseoftestingandbinning.

Current Handling in Clipping/Clamping Circuits

Thepurposeofaclipping/clampingdiodeistohandle

high currents, protecting delicate circuits downstream

of the diode. Current handling capacity is determined

bytwosetsofcharacteristics,thoseofthechipordevice

itselfandthoseofthepackageintowhichitismounted.

Maximum reliability is obtained in a Schottky diode

when the steady state junction temperature is main-

tainedatorbelow150°C,althoughbriefexcursionsto

higherjunctiontemperaturescanbetoleratedwithno

significant impact upon mean-time-to-failure, MTTF. In

ordertocomputethejunctiontemperature,Equations

(1)and(3)belowmustbesimultaneouslysolved.

Figure 7. Forward Current vs.

Forward Voltage at 25

°C.

.01

10

1

.1

300

100

0

0.1

0.3

0.2

0.5

0.4

0.6

HSMS-270x

HBAT-540x

current

limiting

pull-down

(or pull-up)

long cross-site cable

noisy data-spikes

Vs

0V

voltage limited to

Vs + Vd

0V – Vd

Figure 8. Two Schottky Diodes Are Used for Clipping/Clamping in a Circuit.

Consider the circuit shown in Figure 8, in which two

Schottkydiodesareusedtoprotectacircuitfromnoise

spikesonastreamofdigitaldata.Theabilityofthediodes

tolimitthevoltagespikesisrelatedtotheirabilitytosink

theassociatedcurrentspikes.Theimportanceofcurrent

handlingcapacityisshowninFigure9,wheretheforward

voltagegeneratedbyaforwardcurrentiscomparedin

twodiodes.ThefirstisaconventionalSchottkydiodeof

ThesecondisaSchottkydiodeofidenticalcharacteris-

diode’sterminalstoriseascurrentincreases.Thepower

climb,givingrisetoarunawaythermalcondition.Inthe

placeandthevoltageacrossthediodeterminalsismain-

tainedatalowlimitevenathighvaluesofcurrent.

Figure 9. Comparison of Two Diodes.

0

0.1

0.2

0.3

0.5

0.4

0

3

2

1

6

4

5

–1

nTJ

(1)

e

298

2

n

1

TJ

1

298

–4060

–

(2)

(3)

where:

n=diodeidealityfactor

(diodelead)

Equation(1)describestheforwardV-IcurveofaSchottky

diode.Equation()providesthevalueforthediode’ssat-

urationcurrent,whichvalueispluggedinto(1).Equation

(3)givesthevalueofjunctiontemperatureasafunction

of power dissipated in the diode and ambient (lead)

temperature.

Figure 9. Comparison of Two Diodes.