hsms-2700 Avago Technologies, hsms-2700 Datasheet - Page 7

• Manufacturer Part Number: hsms-2700
• Description: High Performance Schottky Diode For Transient Suppression
• Manufacturer: Avago Technologies

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
HSMS-2702 or HSMS-270C (series pair) are closely
matched —without the added expense of testing and
binning.
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.
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.
Figure 9. Comparison of Two Diodes.
The first is a conventional Schottky diode of the type
generally used in RF circuits, with an R
second is a Schottky diode of identical characteristics,
save the R
relatively high value of R
diode’s terminals to rise as current increases. The power
dissipated in the diode heats the junction, causing R
climb, giving rise to a runaway thermal condition. In the
second diode with low R
place and the voltage across the diode terminals is main-
7
noisy data-spikes
long cross-site cable
Figure 9. Comparison of Two Diodes.
6
5
4
3
2
1
0
pull-down
(or pull-up)
0
S
I
F
of 1.0 Ω. For the conventional diode, the
– FORWARD CURRENT (mA)
0.1
current
limiting
0.2
R
R
s
s
= 7.7 Ω
= 1.0 Ω
0V
S
0.3
causes the voltage across the
S
voltage limited to
Vs + Vd
0V – Vd
, such heating does not take
Vs
0.4
0.5
S
of 7.7 Ω. The
S
to
tained 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 simultaneously solved.
where:
I
I
V
R
T
I
n = diode ideality factor
θ
lead)
= θ
T
Equation (1) describes the forward V-I curve of a Schottky
diode. Equation (2) provides the value for the diode’s satu-
ration 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.
The key factors in these equations are: R
tance of the diode where heat is generated under high
current conditions; θ
the Schottky die; and θ
r esistance.
R
and is the lowest of any Schottky diode available from
Avago. Chip thermal resistance is typically 40°C/W; the
thermal resistance of the iron-alloy-leadframe, SOT-23
package is typically 460°C/W; and the thermal resistance
of the copper-leadframe, SOT-323 package is typically
110°C/W. The impact of package thermal resistance on
the current handling capability of these diodes can be
seen in Figures 3 and 4. Here the computed values of
junction temperature vs. forward current are shown
F
S
O
J
F
S
A
S
= forward current
= saturation current
JC
= junction temperature
= saturation current at 25°C
= forward voltage
= series resistance
for the HSMS-270x family of diodes is typically 0.7 Ω
= ambient (diode lead) temperature
I
T
= thermal resistance from junction to case (diode
I
S
F
J
= I
= I
= V
package
0
S
F
I
F
e
298
11600 (V
θ
T
+ θ
JC
J
+ T
chip
n
2
A
nT J
e
F
–4060
– I
chip
F
R
, the chip thermal resistance of
package
S
–1
)
T
1
J
–
, or the package thermal
298
1
(1)
(2)
(3)
S
, the series resis-