HIP4080A/81AEVALZ Intersil, HIP4080A/81AEVALZ Datasheet - Page 6

• Manufacturer Part Number: HIP4080A/81AEVALZ
• Description: DEMO BOARD FOR HIP4081A
• Manufacturer: Intersil

Specifications of HIP4080A/81AEVALZ

• Main Purpose: Power Management, H Bridge Driver (Internal FET)
• Utilized Ic / Part: HIP4080A, HIP4081A
• Secondary Attributes: -
• Embedded: -
• Primary Attributes: -
• Other names: HIP4080A/81AEVAL; HIP4080A/81AEVAL; Q2670719

pump. The peak charge pump current is only about 30µA to
50µA. The gate voltage waveform, when operating without a
bootstrap capacitor, will appear similar to the dotted line
shown in Figure 6
If a bootstrap capacitor value approximately equal to the
equivalent MOSFET gate capacitance is used, the upper
bias supply (labeled “bootstrap voltage” in Figure 6) will drop
approximately in half when the gate is turned on. The larger
the bootstrap capacitance used, the smaller is the
instantaneous drop in bootstrap supply voltage when an
upper MOSFET is turned on.
Although not recommended, one may employ a bootstrap
capacitor without a bootstrap diode. In this case the charge
pump is used to charge up a capacitor whose value should
be much larger than the equivalent gate-source capacitance
of the driven MOSFET. A value of bootstrap capacitance
about 10 times greater than the equivalent MOSFET gate-
source capacitance is usually sufficient. Provided that
sufficient time elapses before turning on the MOSFET again,
the bootstrap capacitor will have a chance to recharge to the
voltage value that the bootstrap capacitor had prior to turning
on the MOSFET. Assuming 2Ω of series resistance is in the
bootstrap change path, an output frequency of up
to
A bootstrap capacitor 10 times larger than the equivalent
gate-source capacitance of the driven MOSFET prevents the
drop in bootstrap supply voltage from exceeding 10% of the
bias supply voltage during turn-on of the MOSFET. When
operating without the bootstrap diode the time required to
replenish the charge on the bootstrap capacitor will be the
same time as it would take to charge up the equivalent gate
capacitance from 0V. This is because the charge lost on the
bootstrap capacitor is exactly equal to the charge transferred
to the gate capacitance during turn-on. Note that the very
first time that the bootstrap capacitor is charged up, it takes
much longer to do so, since the capacitor must be charged
from 0V. With a bootstrap diode, the initial charging of the
bootstrap supply is almost instantaneous, since the charge
required comes from the low-side bias supply. Therefore,
GATE VOLTAGE
BOOT STRAP
(XHO - XHS)
(XHB - XHS)
------------------------------------ -
5
VOLTAGE
INITIATION
×
SIGNAL
GATE
2Ω
1
×
C BS
should allow sufficient refresh time.
FIGURE 6.
6
Application Note 9404
before any upper MOSFETs can initially be gated, time must
be allowed for the upper bootstrap supply to reach full
voltage. Without a bootstrap diode, this initial “charge” time
can be excessive.
If the switching cycle is assumed to begin when an upper
MOSFET is gated on, then the bootstrap capacitor will
undergo a charge withdrawal when the source driver
connects it to the equivalent gate-source capacitance of the
MOSFET. After this initial “dump” of charge, the quiescent
current drain experienced by the bootstrap supply is
infinitesimal. In fact, the quiescent supply current is more
than offset by the charge pump current.
The charge pump continuously supplies current to the
bootstrap supply and eventually would charge the bootstrap
capacitor and the MOSFET gate capacitance back to its
initial value prior to the beginning of the switching cycle. The
problem is that “eventually” may not be fast enough when
the switching frequency is greater than a few hundred Hz.
Bootstrap Bias Supply Circuit Design
For high frequency applications all bootstrap components,
both diodes and capacitors, are required. Therefore, one
must be familiar with bootstrap capacitor sizing and proper
choice of bootstrap diode.
Just after the switch cycle begins and the charge transfer
from the bootstrap capacitor to the gate capacitance is
complete, the voltage on the bootstrap capacitor is the
lowest that it will ever be during the switch cycle. The charge
lost on the bootstrap capacitor will be very nearly equal to
the charge transferred to the equivalent gate-source
capacitance of the MOSFET as shown in Equation 1.
where:
Were it not for the internal charge pump, the voltage on the
bootstrap capacitor and the gate capacitor (because an
upper MOSFET is now turned on) would eventually drain
down to zero due to bootstrap diode leakage current and the
very small supply current associated with the level-shifters
and upper gate driver sub-circuits.
In PWM switch-mode, the switching frequency is equal to
the reciprocal of the period between successive turn-on (or
turn-off) pulses. Between any two turn-on gate pulses exists
one turn-off pulse. Each time a turn-off pulse is issued to an
upper MOSFET, the bootstrap capacitor of that MOSFET
begins its “refresh” cycle. A refresh cycle ends when the
upper MOSFET is turned on again, which varies depending
on the PWM frequency and duty cycle. As the duty cycle
Q G
V
V
C
Q
BS1
BS2
BS
G
=
= Gate charge transferred during turn-on
= Bootstrap Capacitance
= Bootstrap voltage immediately before turn-on
(
= Bootstrap voltage immediately after turn-on
V BS1 V BS2
–
)
×
C BS
December 11, 2007
AN9404.3
(EQ. 1)