HBAT-5400 Datasheet, PDF (7/8 Pages) Agilent(Hewlett-Packard) – High Performance Schottky Diode for Transient Suppression

7
300
HSMS-270x
100
HBAT-540x
10
1
.1
.01
0 0.1 0.2 0.3 0.4 0.5 0.6
V_F – FORWARD VOLTAGE (V)
Figure 7. Forward Current vs.
Forward Voltage at 25°C.
Because the automatic, pick-and-
place equipment used to assemble
these products selects dice from
adjacent sites on the wafer, the
two diodes which go into the
HBAT-5402 or HBAT-540C (series
pair) are closely matched —
without the added expense of
testing and binning.
Current Handling in Clipping/
Clamping Circuits
The purpose of a clipping/clamp-
ing diode is to handle high cur-
rents, protecting delicate circuits
downstream of the diode. Current
handling capacity is determined
by two sets of characteristics,
those of the chip or device itself
and those of the package into
which it is mounted.
noisy data-spikes
current
limiting
Vs
long cross-site cable
pull-down
(or pull-up)
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 Schottky
diodes are used to protect a
circuit from noise spikes on a
stream of digital data. The ability
of the diodes to limit the voltage
spikes is related to their ability to
sink the associated current spikes.
The importance of current
handling capacity is shown in
Figure 9, where the forward
voltage generated by a forward
current is compared in two
diodes. The first is a conventional
Schottky diode of the type gener-
ally used in RF circuits, with an R_S
of 7.7 Ω. The second is a Schottky
diode of identical characteristics,
save the R_S of 1.0 Ω. For the
conventional diode, the relatively
high value of R_S causes the
voltage across the diode’s termi-
nals to rise as current increases.
The power dissipated in the diode
heats the junction, causing R_S to
climb, giving rise to a runaway
thermal condition. In the second
diode with low R_S , such heating
does not take place and the
voltage across the diode terminals
is maintained at a low limit even
at high values of current.
Maximum reliability is obtained in
a Schottky diode when the steady
state junction temperature is
maintained at or below 150°C,
although brief excursions to
higher junction temperatures can
be tolerated with no significant
impact upon mean-time-to-failure,
MTTF. In order to compute the
junction temperature, Equations
(1) and (3) below must be simulta-
neously solved.
11600 ( V _F – I _FR _S)
I_F = I_S e
nTJ
–1 ⁽¹⁾
I_S = I₀ ^{T J}
2
n –4060
e
1_– 1
TJ 298
(2)
298
T_J = V _F I _F θ _JC + T_A
(3)
where:
I_F = forward current
I_S = saturation current
V_F = forward voltage
R_S = series resistance
T_J = junction temperature
I_O = saturation current at 25°C
n = diode ideality factor
θ_JC = thermal resistance from
junction to case (diode lead)
= θ_package + θ_chip
T_A = ambient (diode lead)
temperature
Equation (1) describes the for-
ward V-I curve of a Schottky
diode. Equation (2) provides the
value for the diode’s saturation
current, which value is plugged
into (1). Equation (3) gives the
value of junction temperature as a
function of power dissipated in
the diode and ambient (lead)
temperature.
6
5
4
R_s = 7.7 Ω
3
2
1
R_s = 1.0 Ω
0
0
0.1 0.2 0.3 0.4 0.5
I_F – FORWARD CURRENT (mA)
Figure 9. Comparison of Two
Diodes.