IR3891
19 www.irf.com © 2014 International Rectifier Submit Datasheet Feedback May 29, 2014
Each channel of the IR3891 can be shutdown
separately by pulling the channel EN pin below its low
threshold. Each EN pin controls only one channel to
allow the user to operate each independently.
OVER CURRENT PROTECT ION (CURRENT LIMIT
AND HICCU P MODE)
The over-current protection is performed by sensing
current through the RDS(on) of the Sync FET. This
method enhances the converter’s efficiency and
reduces cost by eliminating a current sense resistor.
The current limit is pre-set internally and compensated
to maintain an alm ost constant limit over temperature .
IR3891 determines over-current events when the
Synchronous FET is on. OCP circuit samples this
current for 40 nsec typically after the rising edge of the
PWM set pulse which has a width of 12.5% of the
switching period. The PWM pulse starts at the falling
edge of the PWM set pulse. This makes valley
current sense more robust as current is sensed close
to the bottom of the inductor downward slope where
transient and switching noise are lower and helps to
prevent false tripping due to noise and transient. An
OC condition is detected if the load current exceeds
the threshold, the converter enters into hiccup mode.
PGood will go low and the internal soft start signal will
be pulled low.
(2)
IOCP = DC current limit hiccup point
ILIMIT = Current Limit V al l ey Point
Δi = Inductor ripple current
Hiccup mode is when the converter stops and waits
before restarting. The channel waits for Tblk_Hiccup,
2.48 ms typical, before the OC signal resets and
restarts. In normal application, the converter restarts
with a pre-bias sequence and soft-start. Figure 11
shows the timing diagram of the above OC protection.
If another OC event is detected, the part repeats
hiccup mode.
0
IL
0
HDrv
Current Limit
0
LDrv
...
...
0
PGood
Hiccup
Tblk_Hiccup
20.48 mS*
*typical filter delay
Figure 11: Timing diagram for pulse-by-pulse current
limit and Hiccup mode
THERMAL SHUTDOWN
IR3891 provides thermal protection. A thermal fault is
detected, when the temperature of the part reaches
the Thermal Shutdown Threshold, 145°C typical. A
thermal fault results in both channels turning off. The
power MOSFETs are disabled during thermal
shutdown. IR3891 automatically restarts when the
temperature of the part drops back below the lower
thermal limit, typically 20°C below the Thermal
Shutdown Threshold.
FEED-FORWARD
Feed-Forward is an important feature which helps with
stability and preserves load transient performance
during PVin changes. In IR3891, Feed-Forward (F.F.)
function is enabled when Vin pin is connected to PVin
pin and Vin>5.0V. The PWM ramp amplitude (Vramp)
is proportionally changed with respect to Vin to
maintain PVin/Vramp ratio. The ratio is almost
constant throughout the Vin range (as shown in Figure
12). By maintaining a constant PVin/Vramp, the
control loop bandwidth and phase margin are more
constant. F.F. function also helps minimize the effect
of PVin changes on the output voltage.
Feed-Forward is based on the Vin voltage and needs
to be accounted for when calculating IR3891
compensation. The PVin/Vramp ratio is not
maintained when Vin and PVin are not equal. This is
the case when an external bias voltage for VCC.
When using an external VCC voltage, Vin pin should
be connected to the VCC pin instead of the PVin pin.
Compensation for the configuration should reflect the
separation.