IR21571 datasheet(12/17 Pages) IRF

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IR21571(S)

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Lamp Protection & Automatic Restart Circuitry Operation

Three leads on the IR21571 are used for protection, as shown in Figure 10 below. These are VDC (dc bus

monitor), SD (unlatched shutdown), CS (latched shutdown) and OC (CS+ threshold programming).

Figure 10: Lamp protection & automatic restart circuitry block diagram with external component connection.

Sensing the DC Bus Voltage

The first of these protection leads senses the voltage

on the DC bus by means of an external resistor

divider and an internal comparator with hysteresis.

When power is first supplied to the IC at system

startup, 3 conditions are required before oscillation

is initiated: 1.) the voltage on the VCC lead must exceed

the rising undervoltage lockout threshold (11.5V),

2.) the voltage at the VDC lead must exceed 5.1V, and

3.) the voltage on the SD lead must be below

approximately 1.85V. If a low dc bus condition occurs

during normal operation, or if power to the ballast is

shut off, the dc bus will collapse prior to the VCC of the

chip (assuming the VCC is derived from a charge

pump off of the output of the half-bridge). In this

case, the voltage on the VDC lead will shut the oscillator

off, thereby protecting the power transistors from

potentially hazardous hard switching. Approximately

2V of hysteresis has been designed into the internal

comparator sensing the VDC lead, in order to account

for variations in the dc bus voltage under varying

load conditions. When the dc bus recovers, the chip

restarts from the beginning of the control sequence,

as shown in timing diagram Figure 11.

OVER-

TEMP

DETECT

4.0V

5.1V

7.6V

3.0V

5.1V

1.0uA

CPH

VDC

2.0V

0.2V

CLK

UNDER-

VOLTAGE

DETECT

from oscillator

section

BUS

from lower

lamp cathode

50uA

7.6V

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