A4940KLPTR-T Allegro Microsystems Inc, A4940KLPTR-T Datasheet - Page 10

• 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
Bootstrap Capacitor Selection
The bootstrap capacitors, CBOOTx, must be correctly selected
to ensure proper operation of the A4940. If the capacitances are
too high, time will be wasted charging the capacitor, resulting in
a limit on the maximum duty cycle and the PWM frequency. If
the capacitances are too low, there can be a large voltage drop at
the time the charge is transferred from CBOOTx to the MOSFET
gate, due to charge sharing.
To keep this voltage drop small, the charge in the bootstrap
capacitor, Q
by the gate of the MOSFET, Q
able value, and the following formula can be used to calculate the
value for C
where V
The voltage drop across the bootstrap capacitor as the MOSFET
is being turned on, ΔV, can be approximated by:
So for a factor of 20, ΔV would be approximately 5% of V
The maximum voltage across the bootstrap capacitor under
normal operating conditions is V
with a good ceramic capacitor the working voltage can be limited
to 16 V.
Bootstrap Charging
It is good practice to ensure the high side bootstrap capacitor is
completely charged before a high side PWM cycle is requested.
The time required to charge the capacitor, t
approximated by:
where C
ΔV is the required voltage of the bootstrap capacitor.
At power-up and when the drives have been disabled for a long
time, the bootstrap capacitor can be completely discharged. In
this case ΔV can be considered to be the full high-side drive
voltage, 12 V. Otherwise, ΔV is the amount of voltage dropped
BOOT
BOOT
BOOT
BOOT
is the value of the bootstrap capacitor, in nF, and
is the voltage across the bootstrap capacitor.
Q
C
∆V ≈
t
CHARGE
:
BOOT
BOOT
, should be much larger than the charge required
Q
C
=
=
=
BOOT
GATE
≈
C
Q
Q
BOOT
GATE
GATE
C
V
BOOT
BOOT
GATE
100
REG
V
20
20
. A factor of 20 is a reason-
BOOT
(max). In most applications,
∆V
CHARGE
(μs), is
Automotive Full Bridge MOSFET Driver
BOOT
.
during the charge transfer, which should be 400 mV or less.
The capacitor is charged whenever the Sx pin is pulled low and
current flows from VREG through the internal bootstrap diode
circuit to CBOOT.
Bootstrap Charge Management
The A4940 provides automatic bootstrap capacitor charge
management. The bootstrap capacitor voltage for each phase
is continuously checked to ensure that it is above the bootstrap
under-voltage threshold, V
voltage drops below this threshold, when the corresponding
high-side is active, the A4940 will turn on the necessary low-side
MOSFET, and continue charging until the bootstrap capacitor
exceeds the undervoltage threshold plus the hysteresis, V
+ V
If the bootstrap capacitor voltage is below the threshold, when the
corresponding high-side is commanded to turn on, the A4940 will
not attempt to turn on the high-side MOSFET, but will turn on the
necessary low-side MOSFET to charge the bootstrap capacitor
until it exceeds the undervoltage threshold plus the hysteresis.
The minimum charge time is typically 7 μs, but may be longer
for very large values of bootstrap capacitor (>1000 nF). If the
bootstrap capacitor voltage does not reach the threshold within
approximately 200 μs, an undervoltage fault will be flagged.
VREG Capacitor Selection
The internal reference, V
gate drive circuits and the charging current for the bootstrap
capacitors. When a low-side MOSFET is turned on, the gate-
drive circuit will provide the high transient current to the gate that
is necessary to turn on the MOSFET quickly. This current, which
can be several hundred milliamperes, cannot be provided directly
by the limited output of the VREG regulator, and must be sup-
plied by an external capacitor connected to the VREG pin.
The turn-on current for the high-side MOSFET is similar in value
to that for the low-side MOSFET, but is mainly supplied by the
bootstrap capacitor. However the bootstrap capacitor must then
be recharged from the VREG regulator output. Unfortunately the
bootstrap recharge can occur a very short time after the low-side
turn-on occurs. This requires that the value of the capacitor con-
nected between VREG and AGND should be high enough to min-
imize the transient voltage drop on VREG for the combination of
a low-side MOSFET turn-on and a bootstrap capacitor recharge.
A value of 20 × C
working voltage will never exceed V
rated as low as 15 V. This capacitor should be placed as close as
possible to the VREG pin.
BOOTUVHYS
.
BOOT
is a reasonable value. The maximum
REG
BOOTUV
115 Northeast Cutoff
1.508.853.5000; www.allegromicro.com
Allegro MicroSystems, Inc.
Worcester, Massachusetts 01615-0036 U.S.A.
, supplies current for the low-side
. If the bootstrap capacitor
REG
, so the capacitor can be
BOOTUV
10