AN296220
MCO-0001021
P0100
APPLICATION INFORMATION
Rearranging:
(−
)
=
Assuming the following illustrative datasheet values:
≈ ≈ 10
= 0.71
(10 − 0.71 × 10 )220
=
150
=4
To increase n to 10 for example, the capacitance Cb can be
recalculated as follows:
=−
10 × 150
= 10 − 0.71 × 10 ≈ 500
bootstrap capacitor charge is delivered directly from VREG as
in a conventional bootstrap arrangement. If operating at high
duty, the bootstrap capacitor charge is partly replenished from
VREG on each switching cycle in the conventional way, with
any shortfall being made up from VCP during periods when
Sx is at supply.
During periods when the Sx node remains permanently high
(100% duty), the bootstrap capacitor charge is maintained
from the VCP supply. In this case, the VCP supply operates like
a high power TOCP with 10 to 100 times the current capability.
It provides an average current during the high-side on-time to
replenish the charge before the next turn off-on event occurs.
This arrangement provides the most efficient gate drive system
capable of operating from 0% to 100% duty without restriction.
The diagram below depicts the operation of the bootstrap circuit.
The above method assumes that the high PWM cycle will only
be run for a limited period of time; hence, it provides a solution
to extend the time before a bootstrap undervoltage fault occurs.
LONGTERM OPERATION
AT HIGH DUTY CYCLE
Allegro offers a number of products that can operate without
duty cycle restriction. These include the AMT49100, AMT49106,
and AMT49107. In these products, there is a second charge
pump (VCP charge pump) that generates a positive voltage,
VCP, with respect to VBB. Under most conditions, most of the
VREG
Boot Diode
High-side
Control
VCP Control
Charge Logic
Pump
Gate Driver
C
CBOOT
GH
S
Figure 3: Diagram showing bootstrap circuit
with additional VCP charge pump.
955 PERIMETER ROAD • MANCHESTER, NH 03103 • USA
+1-603-626-2300 • FAX: +1-603-641-5336 • ALLEGROMICRO.COM
3
