IR3846M_15 Datasheet, PDF (21/54 Pages) International Rectifier – Enhanced line/load regulation with Feed-Forward

IR3846
above their UVLO thresholds and generate the Power
On Ready (POR) signal. The internal soft-start
(Intl_SS) signal linearly rises with the rate of 0.4mV/µs
from 0V to 1.5V. Figure 10 shows the waveforms
during soft start. The normal Vout startup time is fixed,
and is equal to:
Tstart = ^{(0.75V − 0.15V )} = 1.5mS (1)
0.4mV / µS
During the soft start the over-current protection (OCP)
and over-voltage protection (OVP) is enabled to
protect the device for any short circuit or over voltage
condition.
POR
3.0V
1.5V
0.75V
0.15V
Intl_SS
Vout
t1 t2 t3
Figure 10: Theoretical operation waveforms during
soft-start (non tracking / non sequencing)
OPERATING FREQUENCY
The switching frequency can be programmed between
300kHz – 1500kHz by connecting an external resistor
from R_t pin to LGnd. Table 1 tabulates the oscillator
frequency versus R_t.
Table 1: Switching Frequency(Fs) vs. External
Resistor(Rt)
Rt (KΩ)
80.6
60.4
48.7
39.2
34
29.4
26.1
23.2
21
19.1
17.4
16.2
15
Freq
(KHz)
300
400
500
600
700
800
900
1000
1100
1200
1300
1400
1500
SHUTDOWN
IR3846 can be shutdown by pulling the Enable pin
below its 1.0V threshold. During shutdown the high
side and the low side drivers are turned off.
OVER CURRENT PROTECTION
The Over Current (OC) protection is performed by
sensing the inductor current through the R_DS(on) of the
Synchronous MOSFET. This method enhances the
converter’s efficiency, reduces cost by eliminating a
current sense resistor and any layout related noise
issues. The Over Current (OC) limit can be set to one
of three possible settings by floating the OCset pin, by
pulling up the OCset pin to VCC, or pulling down the
OCset pin to PGnd. The current limit scheme in the
IR3846 uses an internal temperature compensated
current source to achieve an almost constant OC limit
over temperature.
Over Current Protection circuit senses the inductor
current flowing through the Synchronous MOSFET.
To help minimize false tripping due to noise and
transients, inductor current is sampled for about 30 nS
on the downward inductor current slope approximately
12.5% of the switching period before the inductor
current valley. However, if the Synchronous MOSFET
is on for less than 12.5% of the switching period, the
current is sampled approximately 40nS after the start
of the downward slope of the inductor current. When
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