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| 型号： | A4940KLPTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 9/14页 |
| 文件大小： | 0K |
| 描述： | IC MOSF DVR FULL BRIDGE 24TSSOP |
| 标准包装： | 1 |
| 配置： | 半桥 |
| 输入类型： | 非反相 |
| 延迟时间： | 35ns |
| 配置数： | 1 |
| 输出数： | 4 |
| 电源电压： | 5.5 V ~ 50 V |
| 工作温度： | -40°C ~ 150°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 24-TSSOP（0.173"，4.40mm）裸露焊盘 |
| 供应商设备封装： | 24-TSSOP 裸露焊盘 |
| 包装： | 标准包装 |
| 产品目录页面： | 1141 (CN2011-ZH PDF) |
| 其它名称： | 620-1319-6 |
A4940
Automotive Full Bridge MOSFET Driver
Application Information
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 DEAD , between a high-side or low-side turn-off and the next
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 DEAD , between the RDEAD and
AGND pins.
For R DEAD between 3 ? and 240 k Ω at 25°C, the value of t DEAD
(ns), can be approximated by:
The maximum dead time of 6 μ s 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 . The
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
t DEAD ≈ 50 +
7200
1.2 + ( 200 / R DEAD )
series gate resistance, and the on-resistance of the A4940 internal
drives.
Dead time will be present only if the on-command for one
where R DEAD is in k Ω . Figure 1 illustrates the relationship of
t DEAD and R DEAD , with the greatest accuracy obtained for values
of R DEAD between 6 and 60 k Ω? .
The I DEAD current can be estimated by:
MOSFET occurs within t DEAD after the off-command for its
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
4.5
4.0
3.5
3.0
2.5
2.0
I DEAD ≈
1.2
R DEAD
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:
1.5
1.0
I BRAKE ≈
V bemf
R L
0.5
where V bemf is the voltage generated by the motor and R L is the
0
0
50
100
150
200
250
300
350
400
resistance of the phase winding.
R DEAD (k Ω )
Figure 1. Dead time versus values of R DEAD (full range).
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.
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
9
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