IRS2168D(S)PbF
www.irf.com Page 11
I. Ballast Section
Functional Description
Undervoltage Lockout Mode (UVLO)
The undervoltage lockout mode (UVLO) is defined as the
state the IC is in when V
CC
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 3 of this document. The
IRS2168D undervoltage lockout is designed to maintain
an ultra low supply current of 250 µA (I
QCCUV
), 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 micro-power start-up
current of the IRS2168D together with a snubber charge
pump from the half-bridge output (R
VCC
, C
VCC1
, C
VCC2
,
C
SNUB
,
D
CP1
and D
CP2
).
IRS2168D
LO
COM
VB
VS
HO
V
BUS
(+)
V
BUS
(-)
VCC
C
BS
16
MHS
C
VCC2
R
VCC
D
CP1
D
CP2
To Load
R
CS
C
SNUB
MLS
15
14
13
12
11
V
RECT
(+)
C
VCC1
CS
10
R
3
C
CS
IC
COM
Load
Return
R
1
R
2
R
LO
R
HO
BSFET
CONTROL
BSFET
Figure 1: Start-up and supply circuitry
The V
CC
capacitors (C
VCC1
and C
VCC2
) are charged by the
current through supply resistor (R
VCC
) minus the start-up
current drawn by the IC. This resistor is chosen to set the
desired AC line input voltage turn-on threshold for the
ballast. When the voltage at V
CC
exceeds the IC start-up
threshold (V
CCUV+
) and the SD pin is below 3.0 V (V
SDTH
-),
the IC turns on and LO begins to oscillate. The capacitors
at V
CC
begin to discharge due to the increase in IC
operating current (Fig. 2). The high-side supply voltage,
V
B-
V
S
, begins to increase as capacitor C
BS
is charged
through the internal bootstrap MOSFET during the LO on-
time of each LO switching cycle. When the V
B-
V
S
voltage
exceeds the high-side start-up threshold (V
BSUV+
), HO then
begins to oscillate. This may take several cycles of LO to
charge V
B
-V
S
above V
BSUV+
due to R
DSon
of the internal
bootstrap MOSFET.
DISCHARGE
TIME
INTERNAL VCC
ZENER CLAMP VOLTAGE
VHYST
V
UVLO+
V
UVLO-
CHARGE PUMP
OUTPUT
t
V
C1
R
VCC
& C
VCC1,2
TIME
CONSTANT
C
VCC
DISCHARGE
Figure 2: V
CC
supply voltage
When LO and HO are both oscillating, the external
MOSFETs (MHS and MLS) are turned on and off with a
50% duty cycle and a non-overlapping deadtime of 1.6
µs (t
d
). The half-bridge output (pin V
S
) begins to switch
between the DC bus voltage and COM. During the
deadtime between the turn-off of LO and the turn-on of
HO, the half-bridge output voltage transitions from COM
to the DC bus voltage at a dV/dt rate determined by the
snubber capacitor (C
SNUB
). As the snubber capacitor
charges, current will flow through the charge pump diode
(D
CP2
) to V
CC
. After several switching cycles of the half-
bridge output, the charge pump and the internal 15.6 V
Zener clamp of the IC take over as the supply voltage.
Capacitor C
VCC2
supplies the IC current during the V
CC
discharge time and should be large enough such that
V
CC
does not decrease below UVLO- before the charge
pump takes over. Capacitor C
VCC1
is required for noise
filtering and must be placed as close as possible and
directly between V
CC
and COM, and should not be lower
than 0.1 µF Resistors R
1
and R
2
are recommended for
limiting high currents that can flow to V
CC
from the charge
pump during hard-switching of the half-bridge or during
lamp ignition. The internal bootstrap MOSFET and
supply capacitor (C
BS
) comprise the supply voltage for
the high side driver circuitry. During UVLO mode, the
high- and low-side driver outputs HO and LO are both
low, the internal oscillator is disabled, and pin CPH is
connected internally to COM for resetting the preheat
time.
