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參數資料
| 型號： | TPS2421-1DDAR |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 電源管理 |
| 英文描述： | 1-CHANNEL POWER SUPPLY SUPPORT CKT, PDSO8 |
| 封裝： | PLATSIC, SOP-8 |
| 文件頁數： | 6/24頁 |
| 文件大?。?/td> | 1297K |
| 代理商： | TPS2421-1DDAR |
SLUS907D – JANUARY 2009 – REVISED AUGUST 2010
www.ti.com
Operation
When load current exceeds the user programmed fault limit (ISET) during normal operation the fault timer starts. If
load current drops below the ISET threshold before the fault timer expires, normal operation continues. If load
current stays above the ISET threshold the fault timer expires and a fault is declared. When a fault is declared a
device operating in latch mode turns off and can be restarted by cycling power or toggling the EN signal. A
device operating in retry mode attempts to turn on at a 3% duty cycle until the fault is cleared. When the IMAX
limit is reached during a fault the device goes into current limit and the fault timer keeps running. IMAX is
automatically set to 1.25 times ISET.
Startup
When power is first applied to a load with discharged capacitors there is a large inrush current. The inrush is
controlled by the TPS2421 by initially entering the power limit mode and turning on the fault timer. See Figure 13.
As the charge builds on the capacitor, the current increases to IMAX. When the capacitor is fully charged, current
output is set by the dc load value, The fault timer is turned off. The FET is then fully enhanced and the power
good signal is true.
In order to start properly, the fault timer must be set to exceed the capacitor charge time.
When the load has a resistive component as well as capacitive, the fault time needs to be increased because
current to the resistive load is unavailable to charge the capacitor. The startup time for some selected loading is
given in Table 2.
Table 2 data was taken with ISET equal to 4 A. Lower current settings of TPS2421 do not have a great influence
on the start up timer because of operation at power limit. Load capacitance and dc resistance was selected for a
measured start time. The start time is measured from the assertion of the EN pin to the assertion of the PG pin.
Table 2. Start Time for Input Voltage and Output Loading(1)
LOAD CAPACITANCE
DC LOAD
INPUT VOLTAGE (V)
START TIME (ms)
(mF)
RESISTANCE (
)
OPEN
2.5
220
5
2.7
12
2.6
5
OPEN
4
1000
5
4
12
4
OPEN
4.4
220
5
No start
12
7
12
OPEN
14
1000
5
No start
12
23
(1)
ISET = 4 A
Some combinations of loading and current limit settings exceed the 5-W power limit of the internal MOSFET. The
output voltage will not turn on regardless of the fault time setting. One way to work with the physical limits that
create this problem is to allow the power manager to charge only the capacitive component of the load and use
the PG signal to turn on the resistive component. This is common usage in dc-to-dc converters and other
electrical equipment with power good inputs.
14
Copyright 2009–2010, Texas Instruments Incorporated
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