FAN7389_13 Datasheet, PDF (14/17 Pages) Fairchild Semiconductor – 3-Phase Half-Bridge Gate-Drive IC

2.4 Fault-Out ( _FO ) and Over-Current Protection
FAN7389 provides an integrated fault output ( _FO ) and
an adjustable fault-clear timer (t_FLTCLR). There are two
situations that cause the gate driver to report a fault via
the FO pin. The first is an under-voltage condition of
low-side gate driver supply voltage (V_DD) and the
second is when the current-sense pin (CS) recognizes a
fault. Once the fault condition occurs, the _FO pin is
internally pulled to COM, the fault-clear timer is
activated, and all outputs (HO1,2,3 and LO1,2,3) of the
gate driver are turned off. The fault output stays LOW
until the fault condition has been removed and the fault-
clear timer expires. Once the fault-clear timer expires,
the voltage on the FO pin returns to pull-up voltage.
The fault-clear time (t_FLTCLR) is determined by an internal
current source (I_RCIN=5 μA) and an external C_RCIN at the
RCIN pin, as shown in this equation:
t _FLTCLR
=
C_RCIN × V_RCIN,TH
I _RCIN
[s]
(1)
The R_DSRCIN of the MOSFET is a characteristic
discharge curve with respect to the external capacitor
C_RCIN. The time constant is defined by the external
capacitor C_RCIN and the R_DSRCIN of the MOSFET.
The output of current-sense comparator (CS_COMP)
passes a noise filter, which inhibits an over-current
shutdown caused by parasitic voltage spikes of V_CS.
This corresponds to a voltage level at the comparator of
V_CSTH+ - V_CSHYS= 500 mV - 60 mV =440 mV,
where V_CSHYS=60 mV is the hysteresis of the current
comparator (CS_COMP) as shown in Figure 41.
Figure 41.Over-Current Protection
Figure 42 shows the waveform definitions of RCIN, FO
and the low-side driver, which uses a soft turn-off
method when an under-voltage condition of the low-side
gate driver supply voltage (V_DD) or the current-sense pin
(CS) recognizes a fault. Once a fault condition occurs,
the _FO pin is internally pulled to COM and all outputs
(HO1,2,3 and LO1,2,3) of the gate driver are turned off.
Low-side outputs decline linearly by the internal sink
current source (I_SOFT=40 mA) for soft turn-off, as shown
in Figure 42.
Figure 42.R_CIN and Fault-Clear Waveform Definition
2.5 Recommended R_CIN
Figure 43 shows timing of t_CSOFF and t_CSFO versus C_RCIN.
It is strongly recommended that the capacitor on R_CIN
pin should be less than 5 nF in order to properly protect
power devices in over-current situations.
1200
1100
1000
900
800
700
600
500
400
tCSOFF
tCSFO
300
200
0 1 2 3 4 5 6 7 8 9 10 11 12
CRCIN [nF]
Figure 43. Timing of t_CSOFF and t_CSFO vs. C_RCIN
3. Noise Filter
3.1 Input Noise Filter
Figure 44 shows the input noise filter method, which has
symmetry duration between the input signal (t_INPUT) and
the output signal (t_OUTPUT) and helps to reject noise
spikes and short pulses. This input filter is applied to the
HINx, LINx, and EN inputs. The upper pair of waveforms
(Example A) shows an input signal duration (t_INPUT)
much longer than input filter time (t_FLTIN); it is
approximately the same duration between the input
signal time (t_INPUT) and the output signal time (t_OUTPUT).
The lower pair of waveforms (Example B) shows an
input signal time (t_INPUT) slightly longer than input filter
time (t_FLTIN); it is approximately the same duration
between input signal time (t_INPUT) and the output signal
time (t_OUTPUT).
© 2010 Fairchild Semiconductor Corporation
FAN7389 • Rev. 1.0.3
14
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