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參數資料
| 型號： | UCC284SDR-12EP |
| 廠商： | TEXAS INSTRUMENTS INC |
| 元件分類： | 固定負電壓單路輸出LDO穩壓器 |
| 英文描述： | 12 V FIXED NEGATIVE LDO REGULATOR, 0.5 V DROPOUT, PDSO8 |
| 封裝： | PLASTIC, MS-012AA, SOIC-8 |
| 文件頁數： | 6/21頁 |
| 文件大小： | 506K |
| 代理商： | UCC284SDR-12EP |
VIN TO VOUT DELAY TIME
WITH CT CAPACITOR REMOVED
VIN TO VOUT DELAY TIME
WITH QUICK START-UP CIRCUIT
operation of the quick start-up circuit
R4 +
V
IN
(V)
T
D
seconds
1.6
CT (F)
ohms
(7)
t
M +
2.6
1.6
t
D
seconds
(8)
SLVS862A – NOVEMBER 2008 – REVISED NOVEMBER 2008 ....................................................................................................................................... www.ti.com
With the quick start-up circuit, the delay time between VIN and VOUT during start-up can be reduced
dramatically. Figure 11 shows that with the quick start-up circuit, the VIN to VOUT delay time has been reduced
to approximately 1 ms.
Figure 11.
Figure 12.
During normal start-up, the UCC284 does not turn on until the voltage on the SD/CT pin reaches approximately
–1.6 V with respect to ground. It takes a certain amount of time for the CT capacitor to charge to this point. For a
circuit that has a very large load, the CT capacitor needs to be large in order for the overcurrent timing to work
properly. A large value of capacitance on the SD/CT pin increases the VIN to VOUT delay time.
The quick start-up circuit uses Q1 to quickly pull the SD/CT pin in a negative direction during start-up, thus
decreasing the VIN-to-VOUT delay time. When VIN is applied to the circuit, Q1 turns on and starts to charge the
CT capacitor. The current pulled through R4 determines the rate at which CT is charged. R4 can be calculated
as follows:
tD is the approximate VIN-to-VOUT delay time desired.
Q1 needs to be turned off after a fixed time to prevent the SD/CT pin from going too far negative with respect to
GND. If the SD/CT pin is allowed to go too far negative with respect to GND, the output turns off again or
possibly even damages the SD/CT pin. The maximum amount of time that Q1 should be allowed to be on is
referred to as tM and can be calculated as follows:
14
Copyright 2008, Texas Instruments Incorporated
Product Folder Link(s): UCC284-EP
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