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參數資料
| 型號： | LM2798 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | WIRE |
| 中文描述： | 一二零毫安高效降壓型開關電容電壓轉換器電壓監(jiān)測 |
| 文件頁數： | 11/14頁 |
| 文件大小： | 388K |
| 代理商： | LM2798 |
Operation Description
(Continued)
TABLE 4. Typical BATOK functionality, with 1M
pull-up resistor connected between BATOK and V
IN
V
IN
EN
BATOK State
Internal BATOK
Transistor State
OFF
ON
ON
OFF
V(BATOK)
>
2.85V
H
H
L
X
HIGH
LOW
LOW
LOW
V
IN
0V
0V
>
1.1V,
<
2.65V
>
1.1V
≤
1.1V
V
IN
,
≤
1.1V
BATOK: INPUT VOLTAGE STATUS INDICATOR
The BATOK pin is an NMOS-open-drain-logic signal that
indicates the status of the input voltage. To function properly,
the BATOK pin must be connected to a pull-up resistor, or
other pull-up device. With a pull-up in place, V(BATOK) will
be HIGH when V
is at or above 2.85V. If the output falls
below 2.65V (typ.), V(BATOK) will be 0V. There is hysteresis
of 20mV (typ.) between the thresholds. The BATOK function
is disabled and V(BATOK) is pulled down to 0V when the
LM2797/98 is in shutdown (EN = 0V).
Table 4
is a complete
list of the typical BATOK regions of operation.
SHUTDOWN
The LM2797/98 is in shutdown mode when the voltage on
the active-low logic enable pin (EN) is low. In shutdown, the
LM2797/98 draws virtually no supply current. When in shut-
down, the output of the LM2797/98 is completely discon-
nected from the input, and will be 0V unless driven by an
outside source.
In some applications, it may be desired to disable the
LM2797/98 and drive the output pin with another voltage
source. This can be done, but the voltage on the output pin
of the LM2797/98 must not be brought above the input
voltage. The output pin will draw a small amount of current
when driven externally due the internal feedback resistor
divider connected between V
OUT
and GND.
SOFT START
The LM2797/98 employs soft start circuitry to prevent exces-
sive input inrush currents during startup. At startup, the
output voltage gradually rises from 0V to the nominal output
voltage. This occurs in 400μs (typ.) with the LM2798.
Turn-on time of the LM2797 is 100μs (typ.). Soft-start is
engaged when the part is enabled, including situations
where voltage is established simultaneously on the V
IN
and
EN pins.
THERMAL SHUTDOWN
Protection from overheating-related damage is achieved
with a thermal shutdown feature. When the junction tem-
perature rises to 150
o
C (typ.), the part switches into shut-
down mode. The LM2797/98 disengages thermal shutdown
when the junction temperature of the part is reduced to
130
o
C (typ.). Due to its high efficiency, the LM2797/98
should not activate thermal shutdown (or exhibit related
thermal cycling) when the part is operated within specified
input voltage, output current, and ambient temperature op-
erating ratings.
SHORT-CIRCUIT PROTECTION
The LM2797/98 short-circuit protection circuitry protects the
device in the event of excessive output current and/or output
shorts to ground. A graph of "Short-Circuit Current vs. Input
Voltage" is provided in the
Performance Characteristics
section.
Application Information
OUTPUT VOLTAGE RIPPLE
The voltage ripple on the output of the LM2797/98 is highly
dependent on application conditions. The output capacitor,
the input voltage, and the output current each play a signifi-
cant part in determining the output voltage ripple. Due to the
complexity of LM2797/98 operation, providing equations or
models to approximate the magnitude of the ripple cannot be
easily accomplished. The following general statements can
be made, however
The output capacitor will have a significant effect on output
voltage ripple magnitude. Ripple magnitude will typically be
linearly proportional to the output capacitance present. A
low-ESR ceramic capacitor is recommended on the output to
keep output voltage ripple low. Placing multiple capacitors in
parallel can reduce ripple significantly. Doing this increases
capacitance and reduces ESR (the effective net ESR is
governed by the properties of parallel resistance). Placing
two identical capacitors in parallel have twice the capaci-
tance and half the ESR, as compared to one of these ca-
pacitors all by itself. Similarly, if a large-value, high-ESR
capacitor (tantalum, for example) is to be used as the pri-
mary output capacitor, the net output ESR can be signifi-
cantly reduced by placing a low-ESR ceramic capacitor in
parallel with this primary output capacitor.
Ripple is increased when the LM2797/98 is gain hopping.
With high output currents, ripple is likely to vary significantly
with input voltage, depending on whether on not the part is
gain hopping.
CAPACITORS
The LM2797/98 requires 4 external capacitors for proper
operation. Surface-mount multi-layer ceramic capacitors are
recommended. These capacitors are small, inexpensive and
have very low equivalent series resistance (ESR,
≤
15m
typ.). Tantalum capacitors, OS-CON capacitors, and alumi-
num electrolytic capacitors generally are not recommended
for use with the LM2797/98 due to their high ESR, as com-
pared to ceramic capacitors.
For most applications, ceramic capacitors with an X7R or
X5R temperature characteristic are preferred for use with the
LM2797/98. These capacitors have tight capacitance toler-
ance (as good as
±
10%) and hold their value over tempera-
ture (X7R:
±
15% over -55
o
C to 125
o
C; X5R:
±
15% over
-55
o
C to 85
o
C).
L
www.national.com
11
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