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IR2156STRPBF Datasheet(PDF) 17 Page - International Rectifier

Part # IR2156STRPBF
Description  BALLAST CONTROL IC
Download  21 Pages
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Manufacturer  IRF [International Rectifier]
Direct Link  http://www.irf.com
Logo IRF - International Rectifier

IR2156STRPBF Datasheet(HTML) 17 Page - International Rectifier

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IR2156(S)PbF
17
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 2 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 IRS2156 together with a charge pump from
the ballast output stage (RSUPPLY , CVCC, DCP1 and DCP2).
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
VCCUV+, and the SD pin is below VSDTH-, 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
VCCHYS
V
ccuv+
V
ccuv-
CHARGE PUMP
OUTPUT
t
V
C1
R
SUPPLY & CVCC
TIME
CONSTANT
C
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.
An
external bootstrap diode (DBOOT) and the 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 VCCUV+ positive-going threshold.
HO and LO begin to oscillate at the preheat frequency with 50%
duty cycle and with a dead-time which is set by the value of the
external timing capacitor, CT, and internal deadtime resistor,
RDT. Pin CPH is disconnected from COM and an internal 4uA
current source (Figure 3) charges the external preheat timing
capacitor on CPH linearly. The over-current protection on pin CS
is disabled during preheat.
5
4
5uA
6
7
CPH
CT
RPH
RT
11
8
COM
LO
M2
RCS
OSC.
13
HO
M1
12
VS
Half
Bridge
Output
I
LOAD
V
BUS (+)
V
BUS (-)
Load
Return
Half-
Bridge
Driver
S4
CPH
CT
RPH
RT
Figure 3, Preheat circuitry
The preheat frequency is determined by the parallel
combination of resistors RT and RPH, together with timing
capacitor CT. CT charges and discharges between 1/3 and 3/5 of
VCC (see Timing Diagram, page 9). CT is charged exponentially
through the parallel combination of RT and RPH connected
internally to VCC through MOSFET S1. The charge time of CT
from 1/3 to 3/5 VCC is the on-time of the respective output gate
driver, HO or LO. Once CT exceeds 3/5 VCC, MOSFET S1 is
turned off, disconnecting RT and RPH from VCC. CT is then
discharged exponentially through an internal resistor, RDT,
through MOSFET S3 to COM. The discharge time of CT from 3/5
to 1/3 VCC is the dead-time (both off) of the output gate drivers,
HO and LO. The selected value of CT together with RDT
therefore program the desired dead-time (see Design Equations,
page 12, Equations 1 and 2). Once CT discharges below 1/3
VCC, MOSFET S3 is turned off, disconnecting RDT from COM,
and MOSFET S1 is turned on, connecting RT and RPH again to
VCC. The frequency remains at the preheat frequency until the
voltage on pin CPH exceeds 13V and the IC enters Ignition Mode.
During the preheat mode, both the over-current protection and the
14
13
12
11
8
LO
COM
VB
VS
HO
Half-Bridge
Output
RSUPPLY
DCP1
DCP2
M2
M1
CSNUB
VBUS(+)
CBOOT
RCS
VBUS(-)
2
VCC
CVCC
DBOOT
RLIM


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