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參數(shù)資料
| 型號(hào)： | IRU3072CH |
| 廠商： | International Rectifier |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | 20-PIN SYNCHRONOUS PWM CONTROLLER/ 3 LDO CONTROLLER |
| 中文描述： | 20引腳同步PWM控制器/ 3 LDO控制器 |
| 文件頁數(shù)： | 9/24頁 |
| 文件大?。?/td> | 332K |
| 代理商： | IRU3072CH |
IRU3072
9
Rev. 1.0
3/25/04
www.irf.com
32
μ
A
×
25K
= 0.8V
Figure 8 - Theoretical operation waveforms
during soft-start.
When the power voltage such as Vcc go into steady
state and the output of voltage UVLO “POR” goes high,
a 20uA current source charges the external soft-start
capacitors. The soft-start voltage ramps up. In the mean
time, the current flowing into pin Fb1 starts to decrease
linearly and so does the voltage at the positive pin of
feedback UVLO comparator and the voltage at the nega-
tive input of Error amplifier. When the soft-start capacitor
voltage is around 1V, the current flowing into the Fb1 pin
is approximately 32
μ
A. The voltage at the positive input
of the Error amplifier is approximately:
The Error Amplifier will start to operate and the output
voltage starts to increase. As the soft-start capacitor
voltage continues to go up, the current flowing into the
Fb1 pin will keep decreasing. Because the voltage at pin
of Error Amplier is regulated to reference voltage 0.8V,
the voltage at the Fb1 pin is:
The feedback voltage increases linearly as the injecting
current goes down. The injecting current drops to zero
when soft-start voltage is around 2V and the output volt-
age goes into steady state.
Figure 8 shows that the voltage at the positive pin of
feedback UVLO comparator is always higher than 0.4V,
therefore, feedback UVLO is not functional during soft-
start.
For PWM: C
SS
= 0.1
μ
F, t
START
5ms
For LDOs: C
SSLDO
= 33nF, t
START
2ms
V
FB1
= 0.8V-25K
×
(Injecting Current)
20
μ
A
×
t
START
/C
SS
2V-1V
C
SS
20
μ
A
×
t
START
/1V ---(1)
From the above analysis, the output start up time is the
period when soft-start capacitor voltage increases from
1V to 2V. The start up time will be dependent on the size
of the external soft-start capacitor. The start up time can
be estimated by:
For a given start up time, the soft-start capacitor can be
estimated as:
For 5ms start up time, a 0.1
μ
F soft-start capacitor is
required. In practice, the 20
μ
A current will slightly de-
crease as the soft-start voltage goes up. Therefore, for a
0.1
μ
F soft-start capacitor, start up time may be slightly
longer, e.g. 6ms.
The soft-start waveforms are shown in Figure 9. In this
figure, the start up time for the buck converter V
OUT1
and
LDOs is different by selecting separate soft-start capaci-
tors.
Figure 9 - Soft-start of buck converter
(PWM) and LDO.
Shutdown
The PWM output and LDO output can be turned on and
off individually by pulling up and down the correspond-
ing soft start capacitors.
Soft-Start
Voltage
Voltage at negative input
of Error Amp and Feedback
UVLO comparator
Voltage at Fb1 pin
Current flowing
into Fb1 pin
64uA
0uA
0V
0.8V
1.6V
0.8V
0V
3V
2V
1V
Output of UVLO
POR
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