IR2156 datasheet(16/24 Pages) IRF

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IR2156(S)&(PbF)

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Functional Description

Under-voltage Lock-Out Mode (UVLO)

The under-voltage lock-out mode (UVLO) is

defined as the state the IC is in when VCC is

below the turn-on threshold of the IC. To identify

the different modes of the IC, refer to the State

Diagram shown on page 6 of this document. The

IR2156 undervoltage lock-out is designed to

maintain an ultra low supply current of less than

200uA, and to guarantee the IC is fully functional

before the high and low side output drivers are

activated. Figure 1 shows an efficient supply

voltage using the start-up current of the IR2156

together with a charge pump from the ballast

output stage (RSUPPLY, CVCC, DCP1 and DCP2).

IR2156

COM

Half-Bridge

Output

SUPPLY

CP1

CP2

SNUB

BUS(+)

BOOT

BUS(-)

VCC

Figure 1, Start-up and supply circuitry.

The start-up capacitor (CVCC) is charged by

current through supply resistor (RSUPPLY) minus

the start-up current drawn by the IC. This resistor

is chosen to provide 2X the maximum start-up

current to guarantee ballast start-up at low line

input voltage. Once the capacitor voltage on VCC

reaches the start-up threshold, and the SD pin is

below 4.5 volts, the IC turns on and HO and LO

begin to oscillate. The capacitor begins to

discharge due to the increase in IC operating

current (Figure 2).

DISCHARGE

TIME

INTERNAL VCC

ZENER CLAMP VOLTAGE

VHYST

UVLO+

UVLO-

CHARGE PUMP

OUTPUT

SUPPLY & CVCC

TIME

CONSTANT

VCC

DISCHARGE

Figure 2, Supply capacitor (CVCC) voltage.

During the discharge cycle, the rectified current

from the charge pump charges the capacitor above

the IC turn-off threshold. The charge pump and

the internal 15.6V zener clamp of the IC take over

as the supply voltage. The start-up capacitor and

snubber capacitor must be selected such that

enough supply current is available over all ballast

operating conditions. A bootstrap diode (DBOOT)

and supply capacitor (CBOOT) comprise the

supply voltage for the high side driver circuitry.

To guarantee that the high-side supply is charged

up before the first pulse on pin HO, the first pulse

from the output drivers comes from the LO pin.

During undervoltage lock-out mode, the high- and

low-side driver outputs HO and LO are both low,

pin CT is connected internally to COM to disable

the oscillator, and pin CPH is connected internally

to COM for resetting the preheat time.

Preheat Mode (PH)

The preheat mode is defined as the state the IC

is in when the lamp filaments are being heated to

their correct emission temperature. This is

necessary for maximizing lamp life and reducing

the required ignition voltage. The IR2156 enters

preheat mode when VCC exceeds the UVLO

positive-going threshold. HO and LO begin to

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