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| 型号： | A4935KJPTR-T |
| 厂商： | Allegro Microsystems Inc |
| 文件页数： | 17/25页 |
| 文件大小： | 0K |
| 描述： | IC MOSFET DVR AUTO 3PH 48-LQFP |
| 标准包装： | 1 |
| 配置： | 3 相桥 |
| 输入类型： | PWM |
| 延迟时间： | 90ns |
| 配置数： | 1 |
| 输出数： | 3 |
| 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： | 48-LQFP 裸露焊盘 |
| 供应商设备封装： | 48-LQFP 裸露焊盘（7x7） |
| 包装： | 标准包装 |
| 产品目录页面： | 1141 (CN2011-ZH PDF) |
| 其它名称： | 620-1300-6 |
A4935
Automotive 3-Phase MOSFET Driver
with either 2-phase or 3-phase excitation. When using fast decay,
a PWM duty cycle of 50% results in zero effective motor torque.
A duty cycle of less than 50% causes negative effective torque,
and greater than 50% causes positive effective torque.
To reduce power dissipation in the external FETs, the A4935 can
be instructed to turn on the appropriate low-side and high-side
drives during the load current recirculation PWM off-cycle. This
synchronous rectification allows current to flow through the
selected FETs, rather than the source-drain body diode, during
the decay time. The body diodes of the recirculating power FETs
conduct only during the dead time that occurs at each PWM
transition.
Dead Time
To prevent cross-conduction (shoot through) in any phase of
the power FET bridge, it is necessary to have a dead time delay,
t DEAD , between a high- or low-side turn-off and the next comple-
mentary turn-on event. The potential for cross-conduction occurs
when any complementary high-side and low-side pair of FETs are
switched at the same time; for example, when using synchronous
rectification or after a bootstrap capacitor charging cycle. In the
A4935, the dead time for all three phases is set by a single dead-
time resistor (R DEAD ) between the RDEAD and AGND pins.
For R DEAD values between 3 k Ω and 240 k Ω , at 25°C the nomi-
nal value of t DEAD in ns can be approximated by:
The choice of power FET 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 FET in a
phase has stopped conducting before the complementary FET
starts conducting. This should also take into account the tolerance
and variation of the FET gate capacitance, the series gate resis-
tance, and the on-resistance of the A4935 internal drives.
Internally-generated dead time will be present only if the on-com-
mand for one FET occurs within t DEAD after the off-command
for its complementary FET. 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.)
Fault Blank Time
To avoid false short fault detection, the output from the VDS
monitor for any FET is ignored when that FET is off and for a
period of time after it is turned on. This period of time is the fault
blank time. Its length is the dead time, t DEAD , plus an additional
period of time that compensates for the delay in the V DS moni-
tors. This additional delay is typically 300 to 600 ns. When t DEAD
1.8
t DEAD (nom) = 50 +
7200
1.2 + (200 / R DEAD )
,
(1)
1.6
1.4
where R DEAD is in k Ω . Greatest accuracy is obtained for values
of R DEAD between 6 and 60 k Ω , which are shown in figure 3.
The I DEAD current can be estimated by:
1.2
1.0
I DEAD =
1.2
R DEAD
.
(2)
0.8
0.6
If the dead time is to be generated externally, for example by
the PWM output of a microcontroller, then connect the RDEAD
pin to the AGND pin to set the internally-generated dead time to
0.4
0.2
zero. Note that this configuration can allow cross-conduction, and
appropriate care should be taken, as described in the Cross-Con-
duction section. The maximum internally-generated dead time,
0.0
0
10
20
30 40
R DEAD (k Ω )
50
60
70
6 μ s typical, can be set by connecting the RDEAD and VDD pins.
Figure 3. Dead time versus R DEAD
Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
17
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