SI9976(1999) View Datasheet(PDF) - Vishay Semiconductors
Part Name
Description
Manufacturer
SI9976 Datasheet PDF : 5 Pages
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Si9976DY
Vishay Siliconix
DETAILED DESCRIPTION
Power On Conditioning
Bootstrap-type floating supplies require that the bootstrap
capacitor be charged at power on. In the case of the
Si9976DY, this is accomplished by pulsing the IN line low with
the EN line held high, thus turning on the low-side MOSFET
and providing the charging path for the capacitor.
Operating Voltage: 20 to 40 V
The Si9976DY is intended to be powered by a single power
supply within the range of 20 to 40 V and is designed to drive
a totem pole pair of NMOS power transistors such as those
within the Si9955. The power transistors must be powered by
the same power supply as this driver. In addition to the
high-voltage power supply (20 to 40 V), the Si9976DY must
have a power supply connected to the VCC terminal, if a fault
output signal is desired. This power supply provides
operating voltage for the fault output and allows the high
output voltage level to be compatible with system logic that
monitors the fault condition. The value of this power supply
must be within the range of 4.5 to 16.5 V to ensure
functionality of the output. Internal fault circuitry, which is
used for shorted-load protection, is not affected by this power
supply.
Cross-Conduction Protection
The high-side power transistor can only be turned on after a
fixed time delay following the return to ground of the low-side
power transistor’s gate. The low-side transistor can only be
turned on after a fixed time delay following the high-side
transistor turn-off signal.
Undervoltage Lockout
During power up, both power transistors are held off until the
internal regulated power supply, VDD, is approximately one
Vbe from the final value, nominally 16 V. After power up, the
undervoltage lockout circuitry continues to monitor VDD. If an
undervoltage condition occurs, both the high-side and
low-side transistors will be turned off and the fault output will
be set high. When the undervoltage condition no longer
exists, normal function will resume automatically. Separate
voltage sensing of the bootstrap capacitor voltage allows a
turn-on signal to be sent to the high-side drive circuit if either
the bootstrap capacitor has full voltage, or the load voltage is
high (driven high by an inductive load or shorted high). The
voltage sensing circuit will allow the high-side power transistor
to turn on if an on signal is present and the voltage on the
bootstrap capacitor rises from undervoltage to operating
voltage.
Short Circuit Protection
This device is intended to be used only in a half-bridge which
drives inductive loads. A shorted load is presumed if the load
voltage does not make the intended transition within an
allotted time. Separate timing is provided for the two
transitions. A longer time is allowed for the high-side to turn
on (300 ns vs. 200 ns) since the propagation delays are
longer. Excessive capacitive loading can be interpreted as a
short. The value of capacitance that is needed to produce the
indication of a short depends on the load driving capability of
the power transistors.
ESD Protection
Electrostatic discharge protection devices are between VDD
and GND, VCC and GND, and from terminals IN, EN, G2, and
FAULT to both VDD and GND. V+, CAP, S1, and G1 are not
ESD protected.
Fault Feedback
Detection of a shorted load sets a latch which turns off both
the high-side and the low-side power transistors. If VCC is
present, a one level will be present on the FAULT output. To
reset the system, the enable input, EN, must be lowered to a
logic zero and then raised to a logic one. The logic level of the
input, IN, will determine which power transistor will be turned
on first after reset. An undervoltage condition on VDD is not
latched, but causes a one level on the FAULT output, if VCC is
present.
Static (dc) Operation
All components of a charge pump, except the holding
(bootstrap) capacitor, are included in the circuit. This charge
pump will provide current that is sufficient to overcome any
leakage currents which would reduce the enhancement
voltage of the high-side power transistor while it is on. This
allows the high-side power transistor to be on continuously.
When the low-side power transistor is turned on, additional
charge is restored to the bootstrap capacitor, if needed. The
maximum switching speed of the system at 50% duty cycle is
limited by the on time of the low-side power transistor. During
this time, the bootstrap capacitor charge must be restored.
However, if the duty cycle is skewed so that the on time of the
high-side power transistor is long enough for the charge pump
to completely restore the charge lost during switching, then
the on time of the low-side power transistor is not restricted.
FaxBack 408-970-5600, request 70016
www.siliconix.com
S-60752-Rev. E, 05-Apr-99
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