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參數(shù)資料
| 型號： | AN-937 |
| 廠商： | Electronic Theatre Controls, Inc. |
| 英文描述： | Gate Drive Characteristics and Requirements for HEXFET |
| 中文描述： | 門驅(qū)動器的特點(diǎn)及要求的HEXFET |
| 文件頁數(shù)： | 5/21頁 |
| 文件大?。?/td> | 418K |
| 代理商： | AN-937 |
AN-937 (v.Int)
In summary:
MOS-gated transistors should be driven from low impedance (voltage) sources, not only to reduce switching losses,
but to avoid dv/dt induced turn-on and reduce the susceptibility to noise.
4. DRIVING STANDARD HEXFET
S FROM TTL
Table 1 shows the guaranteed sourcing and sinking currents for different TTL families at their respective voltages. From this
table, taking as an example of the 74LS series, it is apparent that, even with a sourcing current as low as 0.4 mA, the guaranteed
logic one voltage is 2.4V (2.7 for 74LS and 74S). This is lower than the possible threshold of a HEXFET
. The use of a pull-up
resistor in the output, as shown in Figure 7, takes the drive voltage up to 5 V, as necessary to drive the gate of Logic Level
HEXFET
s, but is not sufficient to fully enhance standard HEXFET
s. Section 8
covers the drive characteristics of the logic
level devices in detail.
Logic
Conditions
Logic Zero
Min. sink current
for V
OL
54 / 74
54H / 74H
(54L) /
74L
(54LS) /
74LS
74S
16mA
< 0.4V
20mA
< (0.4V) /
20mA
< (0.3V) /
0.4V
(4) / 8
< (0.4V) /
0.5V
20mA
0.5V
Logic One
Max. source
current for V
OH
-0.4mA
>2.4V
-0.5mA
>2.4V
-0.2mA
>2.4V
-0.4mA
> (2.5) /
2.7V
12ns
-1.0mA
>2.7V
Typical Gate
Propagation Delay
10ns
7ns
50ns
4ns
Table 1.
Driving HEXFET
s from TTL (Totem Pole Outputs)
Open collector buffers, like the 7406, 7407, etc., possibly with
several drivers connected in parallel as shown in Figure 9, give
enough voltage to drive standard devices into “full
enhancement”, i.e. data sheet on-resistance. The impedance of
this drive circuit, however, gives relative long switching times.
Whenever better switching performance is required, interface
circuits should be added to provide fast current sourcing and
sinking to the gate capacitances. One simple interface circuit is
the complementary source-follower stage shown in Figure 9. To
drive a MOSFET with a gate charge of 60 nC in 60 ns an average
gate current of 1 A has to be supplied by the gate drive circuit, as
indicated in INT-944. The on-resistance of the gate drive
MOSFETs has to be low enough to support the desired switching
times.
With a gate charge of 60 nC and at a switching frequency is
100kHz, the power lost in the gate drive circuit is approximately:
P = V
GS
x Q
G
x f = 12 x 60 x 10
-9
x 100 x 10
3
= 72mW
The driver devices must be capable of supplying 1A without
significant voltage drop, but hardly any power is dissipated in
them.
5. DRIVING STANDARD HEXFET
S FROM C-MOS
While the same general considerations presented above for TTL would also apply to C-MOS, there are three substantial
differences that should be kept in mind:
1. C-MOS has a more balanced source/sink characteristic that, on a first approximation, can be thought of as a 500 ohm
resistance for operation over 8V and a 1k ohm for operation under 8V (Table 2).
To Order
Figure 7.
Direct Drive from TTL Output
LOAD
V
H
PULL-UP
RESISTOR
TTL
(TOTEM POLE)
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