

Schottky Diode Application Selection Guide
SMT Clamping, Clipping and Analog Switching Diodes


Features:
fast response time
appropriate threshold (clipping level)
low series resistance for high current handling
low capacitance
Key Parameters:
 Speed :
 10s of psec ( vs. 10's of nsec for Au doped PN) for all Agilent Technologies Schottky diodes
 I_{f max} (maximum forward current) :
 750 mA for HSMS270x
 250 mA for HBAT540x
 160 mA for HSMS282x
 70 mA for HSMS280x
[values assume use of thermally superior SOT323 package]
 R_{s} (series resistance) :
 0.65 for HSMS270x
 2.4 for HBAT540x
 7 for HSMS282x
 30 for HSMS280x
 Barrier height :
 0.34  0.41 V for standard Schottky diodes
 0.19 V for zero bias Schottky diodes
 0.7 V for GaAs Schottky diodes
 Breakdown voltage :
 15 V for HSMS270x
 16 V for HSMS282x
 30 V for HBAT540x
 70 V for HSMS280x
 Typical Capacitance :
 1.0 pF for HSMS282x
 2.0 pF for HSMS280x
 3.0 pF for HBAT540x
 6.7 pF for HSMS270x
Markets:
Computers (spike elimination, reduction of "ringing", elimination of reflections of high speed data signals, digital circuit switch)
Analog and digital designs requiring circuit protection or waveform cleanup with high switching speed.
Clamping Diode Terms
Speed
 What:
 how fast the diode can respond to a signal (alternately, how fast a signal can be clipped or clamped).
 Goodness:
 faster is better.
Forward Current of a Schottky Diode (I_{f})
 What:
 The maximum forward current of a Schottky diode is measure of how much current the diode can sink. The forward current I_{f} is inversely proportional to R_{s}, V_{f}, and _{jc}, and can be calculated from:
I_{f} = [T_{j}  T_{a}]/[_{jc} R_{s}]
or
I_{f} = [T_{j}  T_{a}]/[_{jc} V_{f}]
Plots of I_{f} vs. V_{f}, and of T_{j} and P_{diss} vs. I_{f} can be calculated using the Mathcad worksheet SCHOTTKY.MCD
 Since I_{f} is inversely proportional to _{jc}, it is very importat that a thermally efficient package be used. All four clamping diode families are available both SOT23 and SOT323 (SC70) packages. The following table shows why the SOT323 package is preferred for clamping applications.
package 
lead material 
thermal resistance 
SOT23 
iron alloy 
460 C/W 
SOT323 
copper 
100 C/W 
 Goodness:
 higher is better.
Series Resistance of a Schottky Diode (R_{s})
 What:
 Schottky diode resistance may be expressed as a series resistance R_{s} or as a dynamic resistance R_{D}. These two terms are related by the equation R_{D} = R_{s} + R_{j}, where R_{j} is the resistance of the junction. Junction resistance of a diode with DC bias is quite accurately calculated by R_{j} = 26/I_{b} where I_{b} is the bias current in milliamperes. Either resistance is a measure of the "loss" in the diode.
 Goodness:
 lower is better.
Barrier height
 What:
 The term "Barrier height" is related to the voltage required for a given current in a Schottky barrier diode. Low voltage corresponds to low barrier height. Barrier height also sets V_{f}, the voltage at which the diode starts to clip.
 Goodness:
 depends on the application  the V_{f} must match the desired clipping voltage.
Breakdown voltage (Vbr)
 What:
 the voltage a Schottky barrier diode can tolerate before being damaged.
 Goodness:
 In clamping applications, breakdown voltage only matters if an antiparallel pair isn't used (most people use a pair), then a higher breakdown voltage is (usually) better, at the cost of increased R_{s} (current carrying capacity).

