IR2157 データシートの表示（PDF） - International Rectifier

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IR2157

FULLY INTEGRATED BALLAST CONTROL IC

International Rectifier 

ADVANCED INFORMATION

IR2157

Lamp Protection & Automatic Restart Circuitry Operation

Three pins on the IR2157 are used for protection, as shown in Figure 10 below. These are VDC (dc bus

monitor), SD (unlatched shutdown), and CS (latched shutdown).

+ V BUS

R2

R1

R4

from lamp

lower cathode

R5

C2

VDC

3.0V

1

5.1V

C1

CPH

2

7.6V

1.0uA

5.1V

4.0V

DT

7

SD

8

2.0V

SQ

RQ

from oscillator

section

TQ

RQ

QS

QR

QD

CLK

QR

UNDER-

VOLTAGE

DETECT

OVER-

TEMP

DETECT

Q2

0.2V

CS

9

R3

RCS

1.0V

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

Sensing the DC Bus Voltage

The first of these protection pins 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 pin must

exceed the rising undervoltage lockout threshold

(11.5V), 2.) the voltage at the VDC pin must exceed

5.1V, and 3.) the voltage on the SD pin 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 pin 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 pin, 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 11 below.

13

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