Data Sheet

© 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN7888 • Rev.1.5 11
FAN7888 — 3 Half-Bridge Gate-Drive IC
Application Information
1. Protection Function
1.1 Under-Voltage Lockout (UVLO)
The high- and low-side drivers include under-voltage
lockout (UVLO) protection circuitry for each channel that
monitors the supply voltage (V
DD
) and bootstrap capaci-
tor voltage (V
BS1,2,3
) independently. It can be designed
prevent malfunction when V
DD
and V
BS1,2,3
are lower
than the specified threshold voltage. The UVLO hystere-
sis prevents chattering during power supply transitions.
1.2 Shoot-Through Prevention Function
The FAN7888 has shoot-through prevention circuitry
monitoring the high- and low-side control inputs. It can
be designed to prevent outputs of high and low side from
turning on at same time, as shown
Figure 27 and 28.
Figure 27. Waveforms for Shoot-Through Prevention
Figure 28. Waveforms for Shoot-Through Prevention
2. Operational Notes
The FAN7888 is a three half-bridge gate driver with
internal, typical 270 ns dead-time for the three-phase
brushless DC (BLDC) motor drive system, as shown in
Figure 1.
Figure 29 shows a switching sequence of 120  electrical
commutation for a three-phase BLDC motor drive system.
The waveforms are idealized: they assumed that the
generated back EMF waveforms are trapezoidal with flat
tops of sufficient width to produce constant torque when
the line currents are perfectly rectangular, 120
 electrical
degrees, with the switching sequence as shown in
Figure
29. The operating waveforms of the wye-connection
reveal that repeat every 60 electrical degrees, with each
60
 segment being “commutated” to another phase, as
shown in Figure 29.
After DT
HIN1,2,3/LIN1,2,3
LO1,2,3/HO1,2,3
HO1,2,3/LO1,2,3
After DT
Shoot-Through Prevent
LIN1,2,3/HIN1,2,3
FAN7888 Rev.00
After DT
Shoot-Through Prevent
HIN1,2,3/LIN1,2,3
LO1,2,3/HO1,2,3
HO1,2,3/LO1,2,3
LIN1,2,3/HIN1,2,3
FAN7888 Rev.00