300
100
I
F
– FORWARD CURRENT (mA)
HSMS-270x
HBAT-540x
10
1
Maximum reliability is obtained in a Schottky diode
when the steady state junction temperature is main-
tained 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 simultaneously solved.
11600 (V
F
– I
F
R
S
)
.1
.01
0
0.1
0.2
0.3
0.4
0.5
0.6
I
F
= I
S
e
nT J
–1
(1)
V
F
– FORWARD VOLTAGE (V)
Figure 7. Forward Current vs.
Forward Voltage at 25°C.
Figure 7. Forward Current vs. Forward Voltage at 25°C.
I
S
= I
0
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-
540 or HBAT-540C (series pair) are closely matched —
without the added expense of testing and binning.
T
J
298
2
1
1
n –4060
T
–
J
298
e
(2)
(3)
T
J
= V
F
I
F
θ
JC
+ T
A
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 5°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 forward V-I curve of a Schottky
diode. Equation () provides the value for the diode’s sat-
uration 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
Current Handling in Clipping/Clamping Circuits
The purpose of a clipping/clamping diode is to handle
high currents, 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
h
andling 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 generally used in RF circuits, with an R
S
of 7.7Ω.
The second is a Schottky diode of identical characteris-
tics, 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 terminals 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 main-
tained at a low limit even at high values of current.
7
V
F
– FORWARD VOLTAGE (V)
5
4
3
2
1
0
R
s
= 1.0
Ω
R
s
= 7.7
Ω
0
0.1
0.2
0.3
0.4
0.5
I
F
– FORWARD CURRENT (mA)
Figure 9. Comparison of Two Diodes.
Figure 9. Comparison of Two Diodes.
AVAGO [ AVAGO TECHNOLOGIES LIMITED ]
