Si9976

Vishay Siliconix

Document Number: 70016

S-40757—Rev. F, 19-Apr-04

www.vishay.com

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DETAILED DESCRIPTION

Power On Conditioning

Bootstrap-type floating supplies require that the bootstrap

capacitor be charged at power on. In the case of the Si9976,

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 Si9976 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 Si9976 must have

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

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

ESD protected.

Fault Feedback

Detection of a shorted load sets a latch which turns off both the

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

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.