
IRS26310DJPbF
Figure 21: Difference between the input pulse and the output pulse
Integrated Bootstrap Functionality
The new IRS26310D family features integrated high-voltage bootstrap MOSFETs that eliminate the need of the
external bootstrap diodes and resistors in many applications.
There is one bootstrap MOSFET for each high-side output channel and it is connected between the V CC supply and
its respective floating supply (i.e., V B1 , V B2 , V B3 ); see Figure 22 for an illustration of this internal connection.
The integrated bootstrap MOSFET is turned on only during the time when LO is ‘high’, and it has a limited source
current due to R BS . The V BS voltage will be charged each cycle depending on the on-time of LO and the value of the
C BS capacitor, the drain-source (collector-emitter) drop of the external IGBT (or MOSFET), and the low-side free-
wheeling diode drop.
The bootstrap MOSFET of each channel follows the state of the respective low-side output stage (i.e., the bootstrap
MOSFET is ON when LO is high, it is OFF when LO is low), unless the V B voltage is higher than approximately 110%
of V CC . In that case, the bootstrap MOSFET is designed to remain off until V B returns below that threshold; this
concept is illustrated in Figure 23.
V B1
V CC
V B2
V B3
Figure 22: Internal bootstrap MOSFET connection
Figure 23: Bootstrap MOSFET state diagram
A bootstrap MOSFET is suitable for most of the PWM modulation schemes and can be used either in parallel with the
external bootstrap network (i.e., diode and resistor) or as a replacement of it. The use of the integrated bootstrap as
a replacement of the external bootstrap network may have some limitations. An example of this limitation may arise
when this functionality is used in non-complementary PWM schemes (typically 6-step modulations) and at very high
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? 2008 International Rectifier