Si8751/52 Data Sheet
Isolated FET Driver with Pin Control or Diode Emulator Inputs
KEY FEATURES
The Si875x enables new pathways to the creation of custom Solid State Relay (SSR) con-
figurations. The Si875x integrates robust isolation technology with an SSR FET driver. A
floating secondary side dc power supply is unnecessary as the product generates its own
self-contained gate drive output voltage. When combined with a customer-selected external
FET, a complete Solid State Relay is formed, allowing customers to optimize their system
for cost, PCB area, power, On-Resistance, and thermal performance.
Customers have a choice of digital input control (Si8751) or diode emulation control
(Si8752) to best suit their application. The Si875x integrates versatile outputs that support
driving AC or DC load configurations.
The Si875x eliminates the need for bulky mechanical relays which can be difficult to assem-
ble onto PCBs and add switching noise to the system.
Traditional SSRs integrate optocoupler-style LED inputs, which limit the operating tempera-
ture range of the solution. The Si875x experiences no such limitation and can support full
industrial and automotive temperature ranges with increased stability and longer life.
The Si875x drives FET gates with a nominal 10 V using as little as 1 mA input current. In-
creasing the input current to 10 mA enables turn-on times as fast as 94 μs. Input side vol-
tages on the Si8751 are flexible from 2.25 V to 5.5 V supporting seamless connection to
low-power controllers. The Si875x devices provide an Active Miller Clamp to prevent the un-
intended turn-on of the external FET when a high dV/dt is present on the FET’s drain.
The Si875x is qualified to the AEC-Q100 standard, making it suitable for automotive applica-
tions. Further, its 2.5 KVrms isolation rating forms the basis for full certification to UL, CSA,
VDE, and CQC.
Applications include mechanical relay, photo switch, or SSR replacement in motor control,
valve control, HVAC relay, automotive, charging, battery monitoring, ac mains line switch-
ing, and more.
The Si8751 and Si8752 come in ROHS-compliant SOIC-8 packaging, providing a compact,
industry-standard footprint and generous margin to creepage and clearance requirements.
• Drives user-selected external FETs
• Choice of digital input control (Si8751)
or diode emulation control (Si8752)
• Internally generated secondary side
power supply
• 10 V output with 1 mA input current
• As fast as 82 μs turn-on time and 46
us turn-off time
• Active Miller Clamp to prevent
unintended turn-on and reduce
inductive chatter
• Supports AC or DC load switching
• 2.5 KVrms isolation rating
• UL, CSA, VDE, and CQC certifications
• AEC-Q100 qualified
• Industrial –40 to 105 °C or Automotive
–40 to 125 °C temperature ranges
• ROHS-compliant SOIC-8 Package
APPLICATIONS
• Motor Controls
• Valve Controls
• HVAC Relays
• HEV/EV Automotive Charging
• Battery Monitoring
• AC Mains Line Switching
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Rev. 1.0
Si8751/52 Data Sheet
Ordering Guide
1. Ordering Guide
Table 1.1. Si8751/2 Ordering Guide
Ordering Part Number
1,
2
Input Support
Digital CMOS
Digital CMOS
Diode Emulation
Diode Emulation
Package
SOIC-8
SOIC-8
SOIC-8
SOIC-8
Temperature Range (Ambient)
–40 to 105 °C Industrial
–40 to 125 °C Automotive
–40 to 105 °C Industrial
–40 to 125 °C Automotive
Isolation Rating (kVrms)
2.5 kV
2.5 kV
2.5 kV
2.5 kV
Si8751AB-IS
Si8751AB-AS
Si8752AB-IS
Si8752AB-AS
Note:
1. "Si" and "SI" are used interchangeably.
2. Add an “R” at the end of the device to denote tape and reel option
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Rev. 1.0 | 2
Si8751/52 Data Sheet
System Overview
2. System Overview
Signal & Power Transmitter
VDD
MCAP1
CMOS Isolation
TT
Receiver
IN
GATE
SOURCE
GND
MCAP2
Figure 2.1. Si8751 Block Diagram
ANODE
MCAP1
Signal & Power Transmitter
CMOS Isolation
e
Receiver
GATE
SOURCE
MCAP2
CATHODE
Figure 2.2. Si8752 Block Diagram
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Si8751/52 Data Sheet
System Overview
The operation of an Si875x channel is analogous to that of an optocoupler and gate driver, except an RF carrier is modulated instead of
light. This simple architecture provides a robust isolated data path and requires no special considerations or initialization at start-up. A
simplified block diagram for a single Si875x channel is shown in the figure below.
Transmitter
A
MODULATOR
RF OSCILLATOR
Semiconductor-
Based Isolation
Barrier
Receiver
DEMODULATOR
B
Figure 2.3. Simplified Channel Diagram
A channel consists of an RF Transmitter and RF Receiver separated by a semiconductor-based isolation barrier. Referring to the
Transmitter, input A modulates the carrier provided by an RF oscillator using on/off keying. The Receiver contains a demodulator that
decodes the input state according to its RF energy content and applies the result to output B via the output driver. This RF on/off keying
scheme is superior to pulse code schemes as it provides best-in-class noise immunity, low power consumption, and better immunity to
magnetic fields. See figure below for more details.
Input Signal
Modulation Signal
Output Signal
Figure 2.4. Modulation Scheme
2.1 Device Behavior
The following are truth tables for the Si875x family.
Table 2.1. Si8751 Truth Table
VDD
Powered
Powered
Unpowered
IN
H
L
X
Table 2.2. Si8752 Truth Table
Input Current
> I
f(TH)
< I
f(TH)
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Gate
H
L
L
Gate
H
L
Rev. 1.0 | 4
Si8751/52 Data Sheet
System Overview
2.2 Power Supply Connections (Si8751 Only)
The Si8751 requires a 0.1 µF bypass capacitor between VDD and GND. The capacitor should be placed as close as possible to the
package. To enhance the robustness of a design, the user may also include a 1 µf capacitor for bulk decoupling as well as a resistor
(50–300 Ω) in series with the input if the system is excessively noisy.
2.3 TT Pin Description (Si8751 Only)
The Si8751 provides a pin to control how much current is consumed by the supply when the input pin is logic high. The more current
consumed by the input supply, the faster the output can turn on the external FET. This allows the application designer to optimize the
tradeoff between power consumption and switching time.
Typically, this pin is connected to the supply ground through a resistor. The greater the value of the resistor, the less current is con-
sumed by the input supply. Values can range from 0 Ω (shorted to ground) to open (TT not connected).
In addition to a resistor, a capacitor, typically 0.1 µf, can be placed in parallel to the resistor. This allows the device to draw more current
to switch the external FET on quickly yet draw less supply current in the steady state. Total power over time is reduced while maintain-
ing fast switching of the FET.
Signal & Power Transmitter
VDD
MCAP1
Max Drive
Current
TT
CMOS Isolation
Receiver
IN
IN
GATE
Static Drive
Current
~1/C
t
SOURCE
GND
MCAP2
Figure 2.5. Si8751 TT Example
Figure 2.6. Drive Current vs. Time Using TT with Capacitor
2.4 LED Emulator Input (Si8752 Only)
Figure 2.8. Diode Emulator Model and I-V Curve
The Si8752 uses input current to achieve the development of power across the isolation barrier. Therefore, the more current provided to
the input, the more power is developed on the isolated side of the device. This translates into a faster turn on time of the external FET.
This benefit is limited to an input current of about 15 mA. Beyond that, increasing the input current has little effect on the switching time
of the external FET.
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Rev. 1.0 | 5
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