IR21771STRPBF International Rectifier, IR21771STRPBF Datasheet - Page 14

• Manufacturer Part Number: IR21771STRPBF
• Description: IC CURRENT SENSE 16SOIC
• Manufacturer: International Rectifier

Specifications of IR21771STRPBF

• Function: Current Sense
• Voltage - Input: 8 ~ 20 V
• Operating Temperature: -40°C ~ 125°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (0.300", 7.5mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output: -
• Accuracy: -
• Sensing Method: -

2 Sizing tips
2.1 Bootstrap supply
The V
side drivers circuitry of the IR22771S/IR21771S.
V
must be floating.
The bootstrap method to generate V
be used with IR22771S/IR21771S current sensors.
The bootstrap supply is formed by a diode and a
capacitor connected as in Figure 16.
This method has the advantage of being simple and
low cost but may force some limitations on duty-
cycle and on-time since they are limited by the
requirement to refresh the charge in the bootstrap
capacitor.
Proper capacitor choice can reduce drastically
these limitations.
Bootstrap capacitor sizing
Given the maximum admitted voltage drop for V
namely ∆V
V
I
capacitor and can be ignored if other types of
capacitors are used. It is strongly recommend using
at least one low ESR ceramic capacitor (paralleling
electrolytic and low ESR ceramic may result in an
efficient solution).
14
LK_CAP
BS
BS
− Floating section quiescent current (I
− Floating section leakage current (I
− Bootstrap diode leakage current (I
− Charge required by the internal level shifters
− Bootstrap capacitor leakage current (I
− High side on time (T
decrease are:
supply sit on top of the V
Figure 16: bootstrap supply schematic
(Q
BS1,2,3
is only relevant when using an electrolytic
LS
); typical 20nC
BS
voltage provides the supply to the high
, the influencing factors contributing to
HON
).
S
voltage and so it
BS
LK
LK_DIODE
supply can
QBS
)
LK_CAP
);
);
);
BS
,
Then we have:
The minimum size of bootstrap capacitor is then:
Some important considerations
a) Voltage ripple
There are three different cases making the
bootstrap circuit get conductive (see Figure 16)
b) Bootstrap Resistor
A resistor (R
diode (see Figure 16) so to limit the current when
the bootstrap capacitor is initially charged. We
suggest not exceeding some Ohms (typically 5,
maximum 10 Ohm) to avoid increasing the V
constant. The minimum on time for charging the
bootstrap capacitor or for refreshing its charge must
be verified against this time-constant.
Q
IGBT displaying relevant V
on and V
freewheeling diode
TOT
In this case we have the lowest value for V
This represents the worst case for the bootstrap
capacitor sizing. When the IGBT is turned off the
Vs node is pushed up by the load current until the
high side freewheeling diode get forwarded
biased
In this case we have the highest value for V
Turning on the high side IGBT, I
and V
=
I
I
I
LOAD
LOAD
LOAD
IR22771S/IR21771S(PbF)
Q
S
LS
is pulled up.
CE
< 0; the load current flows in the low side
> 0; the load current flows through the
= 0; the IGBT is not loaded while being
+
boot
can be neglected
(
V
I
V
QBS
BS
) is placed in series with bootstrap
BS
C
V
=
BOOT
+
=
BS
V
+
V
CC
I
=
CC
min
LK
V
www.irf.com
−
CC
−
+
=
V
V
I
CEon
F
−
LK
Q
∆
F
−
V
V
TOT
_
+
DIODE
V
BS
F
V
CEon
FP
LOAD
+
I
flows into it
LK
_
CAP
BS
)
time-
⋅
T
BS
BS
HON
.
.