9
IRS2153(1)D
Functional Description
Under-voltage Lock-Out Mode (UVLO)
The under-voltage lockout mode (UVLO) is defined as the state
the IC is in when V
CC
is below the turn-on threshold of the IC. The
IRS2153(1)D under voltage lock-out is designed to maintain an
ultra low supply current of less than 170
µA, and to guarantee the
IC is fully functional before the high and low side output drivers
are activated. During under voltage lock-out mode, the high and
low-side driver outputs HO and LO are both low.
Supply voltage
RT
CT
COM
VCC
1
2
3
4
7
6
5
8
IRS2153(1)D
LO
VS
HO
VB
CBOOT
MHS
MLS
L
RL
RVCC
RT
CT
CVCC
+ AC Rectified Line
- AC Rectified Line
Fig. 1 Typical Connection Diagram
Fig. 1 shows an example of supply voltage. The start-up capacitor
(C
VCC
) is charged by current through supply resistor (R
VCC
) minus
the start-up current drawn by the IC. This resistor is chosen to
provide sufficient current to supply the IRS2153(1)D from the DC
bus. C
VCC
should be large enough to hold the voltage at Vcc
above the UVLO threshold for one half cycle of the line voltage as
it will only be charged at the peak, typically 0.1 uF. It will be
necessary for R
VCC
to dissipate around 1 W.
The use of a two diode charge pump made of DC1, DC2 and
CVS (Fig. 2) from the half bridge (V
S
) is also possible however
the above approach is simplest and the dissipation in R
VCC
should
not be unacceptably high.
RT
CT
COM
VCC
1
2
3
4
7
6
5
8
IRS2153(1)D
LO
VS
HO
VB
CBOOT
MHS
MLS
L
RL
RVCC
RT
CT
CVCC
+ AC Rectified Line
- AC Rectified Line
DC1
DC2
CVS
Fig. 2 Charge pump circuit
The supply resistor (R
VCC
) must be selected such that enough
supply current is available over all operating conditions.
Once the capacitor voltage on V
CC
reaches the start-up threshold
V
CCUV+
, the IC turns on and HO and LO begin to oscillate.
Bootstrap MOSFET
The internal bootstrap FET and supply capacitor (C
BOOT
) comprise
the supply voltage for the high side driver circuitry. The internal
boostrap FET only turns on when LO is high. 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.
Normal operating mode
Once the V
CCUV+
threshold is passed, the MOSFET M1 opens, RT
increases to approximately V
CC
(V
CC
-V
RT+)
and the external CT
capacitor starts charging. Once the CT voltage reaches V
CT
-
(about 1/3 of V
CC
), established by an internal resistor ladder, LO
turns on with a delay equivalent to the deadtime (t
d
). Once the CT
voltage reaches V
CT+
(approximately 2/3 of V
CC
), LO goes low, RT
goes down to approximately ground (V
RT-
), the CT capacitor
discharges and the deadtime circuit is activated. At the end of the
deadtime, HO goes high. Once the CT voltage reaches V
CT-,
HO
goes low, RT goes high again, the deadtime is activated. At the
end of the deadtime, LO goes high and the cycle starts over
again.
The following equation provides the oscillator frequency:
CTRT
f
××453.1
1
~
This equation can vary slightly from actual measurements due to
internal comparator over- and under-shoot delays. For a more
accurate determination of the output frequency, the frequency
characteristic curves should be used (RT vs. Frequency, page 3).
Shut-down
If CT is pulled down below
V
CTSD
(approximately 1/6 of V
CC
) by
an external circuit, CT doesn’t charge up and oscillation stops.
LO is held low and the bootstrap FET is off. Oscillation will
resume once CT is able to charge up again to V
CT-
.
