A4940KLPTR-T Allegro Microsystems Inc, A4940KLPTR-T Datasheet - Page 9

• Manufacturer Part Number: A4940KLPTR-T
• Description: FULL BRIDGE MOSFET PRE-DRIVER
• Manufacturer: Allegro Microsystems Inc

Specifications of A4940KLPTR-T

• Configuration: H Bridge
• Input Type: Non-Inverting
• Delay Time: 35ns
• Number Of Configurations: 1
• Number Of Outputs: 4
• Voltage - Supply: 5.5 V ~ 50 V
• Operating Temperature: -40°C ~ 150°C
• Mounting Type: Surface Mount
• Package / Case: 24-TSSOP Exposed Pad, 24-eTSSOP, 24-HTSSOP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Peak: -
• High Side Voltage - Max (bootstrap): -
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: 620-1319-2

A4940
Dead Time
To prevent cross-conduction (shoot-through) in any phase of the
power MOSFET bridge, it is necessary to have a dead time delay,
t
complementary turn-on event. The potential for cross-conduction
occurs when any complementary high-side and low-side pair
of MOSFETs are switched at the same time; for example, when
using synchronous rectification or after a bootstrap capacitor
charging cycle. In the A4940, the dead time for both phases is set
by a single dead-time resistor, R
AGND pins.
For R
(ns), can be approximated by:
where R
t
of R
The I
Figure 1. Dead time versus values of R
DEAD
DEAD
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
DEAD
DEAD
0
DEAD
, between a high-side or low-side turn-off and the next
and R
0
DEAD
between 6 and 60 kΩ .
current can be estimated by:
between 3  a nd 240 kΩ at 25°C, the value of t
DEAD
50
is in kΩ. Figure 1 illustrates the relationship of
I
t
DEAD
DEAD
, with the greatest accuracy obtained for values
100
≈
≈
R
50 +
1.2
DEAD
150
1.2 + ( 200 / R
R
DEAD
DEAD
200
DEAD
(kΩ)
7200
, between the RDEAD and
(full range).
250
DEAD
300
)
Application Information
Automotive Full Bridge MOSFET Driver
350
DEAD
400
The maximum dead time of 6 μs typical can be set by connecting
the RDEAD pin directly to VDD.
Alternatively, the dead time in the A4940 can be disabled by
connecting the RDEAD pin directly to GND. In this case the
required dead time must be supplied by the external controller.
The choice of power MOSFET and external series gate resistance
determine the selection of the dead-time resistor, R
dead time should be long enough to ensure that one MOSFET
in a phase has stopped conducting before the complementary
MOSFET starts conducting. This should also take into account
the tolerance and variation of the MOSFET gate capacitance, the
series gate resistance, and the on-resistance of the A4940 internal
drives.
Dead time will be present only if the on-command for one
MOSFET occurs within t
complementary MOSFET. In the case where one side of a phase
drive is permanently off, for example when using diode rectifica-
tion with slow decay, then the dead time will not occur. In this
case the gate drive will turn on within the specified propagation
delay after the corresponding phase input goes high. (Refer to the
Gate Drive Timing diagrams.)
Braking
The A4940 can be used to perform dynamic braking by forc-
ing all low-side MOSFETs on and all high-side MOSFETs off
(ALO=BLO=1, AHI=BHI=0) or, conversely, by forcing all low-
side off and all high-side on (ALO=BLO=0, AHI= BHI=1). This
effectively short-circuits the back EMF of the motor, creating a
breaking torque.
During braking, the load current can be approximated by:
where V
resistance of the phase winding.
Care must be taken during braking to ensure that maximum rat-
ings of the power MOSFETs are not exceeded. Dynamic braking
is equivalent to slow decay with synchronous rectification.
bemf
is the voltage generated by the motor and R
I
BRAKE
≈
DEAD
V
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
bemf
R
L
after the off-command for its
DEAD
. The
L
is the
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