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參數資料
| 型號： | LM2926 |
| 廠商： | National Semiconductor Corporation |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | Cap-Free, NMOS, 150mA Low Dropout Regulator with Reverse Current Protection |
| 中文描述： | 無電容，NMOS管，150mA的低壓差穩壓器的反向電流保護 |
| 文件頁數： | 6/10頁 |
| 文件大小： | 404K |
| 代理商： | LM2926 |
Applications Information
(Continued)
lay capacitor. When the capacitor voltage increases 3.75V
(
V
) from its discharged value, the reset output is
again set HIGH. The delay time is calculated by:
The constant, 1.9 x 10
6
, has a
±
20% tolerance from device
to device. The total delay time error budget is the sum of the
20% device tolerance and the tolerance of the external ca-
pacitor. For a 20% timing capacitor tolerance, the worst case
total timing variation would amount to
±
40%, or a ratio of
2.33:1. In most applications the minimum expected reset
pulse is of interest. This occurs with minimum C
DELAY
, mini-
mum
V
, and maximum I
.
V
are fully specified in the
Electrical Characteristics.
Graphs
showing the relationship between delay time and both tem-
perature and C
DELAY
are shown in the
Typical Performance
Characteristics.
As shown in Figure 1 the delayed reset output is pulled low
by an NPN transistor (Q2), and pulled high to V
by an inter-
nal 30 k
resistor (R3) and PNP transistor (Q3). The reset
output will operate when V
is sufficient to bias Q2 (0.7V or
more). At lower voltages the reset output will be in a high im-
pedance condition. Because of differences in the V
of Q2
and Q3 and the values of R1 and R2, Q2 is guaranteed by
design to bias before Q3, providing a smooth transition from
the high impedance state when V
O
<
0.7V, to the active low
state when V
O
>
0.7V.
The static reset characteristics are shown in Figure 2 This
shows the relationship between the input voltage, the regul-
tor output and reset output. Plots are shown for various ex-
ternal pull-up resistors ranging in value from 3 k
to an open
circuit. Any external pull-up resistance causes the reset out-
put to follow the regulator output until Q2 is biased ON. C
DE-
LAY
has no effect on this characteristic.
Figure 2 is useful for determing reset performance at any
particular input voltage. Dynamic performance at power-up
will closely follow the characteristics illustrated in Figure 2
except for the delay added by C
DELAY
when V
O
reaches 5V.
The dynamic reset characteristics at power-down are illus-
trated by the curve shown in Figure 3 At time t=0 the input
voltage is instantaneously brought to 0V, leaving the output
powered by C
.As the voltage on C
decays (discharged by
a 100
load resistor), the reset output is held low. As V
O
drops below 0.7V, the reset rises up slightly should there be
any external pull-up resistance. With no external resistance,
the reset line stays low throughout the entire power down
cycle. If the input voltage does not fall instantaneously, the
reset signal will tend to follow the performance characteris-
tics shown in Figure 2
SYSTEM DESIGN CONSIDERATIONS
Many microprocessors are specified for operation at 5V
±
10%, although they often continue operating well outside
this range. Others, such as certain members of the COPS
family of microcontrollers, are specified for operation as low
as 2.4V.
Of particular concern is low voltage operation, which occurs
in battery operated systems when the battery reaches the
end of its discharge cycle. Under this condition, when the
supply voltage is outside the guaranteed operating range,
the clock may continue to run and the microprocessor will at-
tempt to execute instructions. If the supply voltage is outside
the guaranteed operating range, the instructions may not ex-
ecute properly and a hardware reset such as is supplied by
DS010759-6
FIGURE 1. Delay Reset Output
DS010759-7
FIGURE 2. Reset Output Behavior during Power-Up
DS010759-8
FIGURE 3. Reset Output Behavior during Power-Down
www.national.com
6
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