Rainbow Electronics MAX14541Е - Manual (Page 5)

3-Channel, Low-Leakage ESD Protector
MAX14541E
_______________________________________________________________________________________ 5
________________Detailed Description
The MAX14541E low-leakage, low-capacitance, Q15kV
ESD-protection diode arrays are suitable for high-speed
and general-signal ESD protection. Low input capaci-
tance makes this device ideal for ESD protection of
high-speed signals. Each channel consists of a pair of
diodes that steer ESD current pulses to V
CC or GND. The
MAX14541E is a 3-channel device (see the Functional
Diagram).
The MAX14541E is designed to work in conjunction with
a device’s intrinsic ESD protection. The MAX14541E
limits the excursion of the ESD event to below Q25V
peak voltage when subjected to the Human Body Model
waveform. When subjected to the IEC 61000-4-2 Contact
Discharge waveform, the peak voltage is limited to
Q
60V. The peak voltage is limited to Q100V when sub-
jected to Air-Gap Discharge. The device protected by
the MAX14541E must be able to withstand these peak
voltages, plus any additional voltage generated by the
parasitic of the board.
___________Applications Information
Design Considerations
Maximum protection against ESD damage results from
proper board layout (see the Layout Recommendations
section). A good layout reduces the parasitic series
inductance on the ground line, supply line, and protect-
ed signal lines. The MAX14541E ESD diodes clamp the
voltage on the protected lines during an ESD event and
shunt the current to GND or V
CC . In an ideal circuit, the
clamping voltage (V C ) is defined as the forward voltage
drop (V F ) of the protection diode, plus any supply volt-
age present on the cathode.
For positive ESD pulses:
V C = V CC + V F
For negative ESD pulses:
V C = -V F
The effect of the parasitic series inductance on the lines
must also be considered (Figure 1).
For positive ESD pulses:
For negative ESD pulses:
where I ESD is the ESD current pulse.
During an ESD event, the current pulse rises from zero
to peak value in nanoseconds (Figure 2). For example,
in a +15kV IEC 61000-4-7 Air-Gap Discharge ESD event,
the pulse current rises to approximately 45A in 1ns
(di/dt = 45 x 10
9 ). An inductance of only 10nH adds an
additional 450V to the clamp voltage, and represents
approximately 0.5in of board trace. Regardless of the
device’s specified diode clamp voltage, a poor layout
with parasitic inductance significantly increases the
effective clamp voltage at the protected signal line.
Minimize the effects of parasitic inductance by placing
the MAX14541E as close as possible to the connector
(or ESD contact point).
A low-ESR 0.1FF capacitor is required between V
CC and
GND to get the maximum ESD protection possible. This
bypass capacitor absorbs the charge transferred by a
positive ESD event. Ideally, the supply rail (V
CC ) would
absorb the charge caused by a positive ESD strike
without changing its regulated value. All power supplies
have an effective output impedance on their positive
rails. If a power supply’s effective output impedance is
1I, then by using V = I x R, the clamping voltage of V
C
increases by the equation V
C
= I
ESD
x R
OUT
. A +8kV IEC
61000-4-2 ESD event generates a current spike of 24A.
The clamping voltage increases by V
C = 24A x 1I, or
V C = 24V. Again, a poor layout without proper bypassing
increases the clamping voltage. A ceramic chip capaci-
tor mounted as close as possible to the MAX14541E
V
CC pin is the best choice for this application. A bypass
capacitor should also be placed as close as possible to
the protected device.
( )
ESD ESD
C CC F(D1)
d I d(I )
V V V L1 L2
dt dt
   
= + + Ч + Ч    
   
ESD ESD
C F(D2)
d(I ) d(I )
V V L1 L3
dt dt
     
= − + Ч + Ч
     
     
Figure 1. Parasitic Series Inductance
L1
PROTECTED
LINE
L3
D2
GROUND RAIL
POSITIVE SUPPLY RAIL
I/O_
D1
L2