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| 型号： | UCC28250PWR |
| 厂商： | Texas Instruments |
| 文件页数： | 20/53页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR PWM CM/VM 20-TSSOP |
| 标准包装： | 2,000 |
| PWM 型： | 电流/电压模式 |
| 输出数： | 1 |
| 频率 - 最大： | 1MHz |
| 占空比： | 100% |
| 电源电压： | 4.7 V ~ 17 V |
| 降压： | 无 |
| 升压： | 无 |
| 回扫： | 无 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 无 |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 20-TSSOP（0.173"，4.40mm 宽） |
| 包装： | 带卷 (TR) |
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SLUSA29C – APRIL 2010 – REVISED JULY 2011
SS (Soft Start Programming Pin) (13/20)
The soft-start circuit gradually increases the converter ’ s output voltage until steady state operation is reached.
This reduces start-up stresses and current surge.
When the UCC28250 reaches its valid operating threshold, the SS pin capacitor is charged with a 27- μ A current
source. The UCC28250 ’ s internal error amplifier non-inverting terminal follows the SS pin voltage on REF/EA+
pin voltage depending on which one is lower. Hence, during soft start, the SS pin voltage is lower than REF/EA+.
The internal error amplifier then uses the SS pin as its reference voltage, until the SS pin voltage rises above the
REF/EA+ level. Once the SS pin voltage is above REF/EA+ voltage, soft-start time is considered finished.
The soft-start implementation scheme and timing is different, depending on the location of the UCC28250 with
respect to the isolation barrier.
For secondary-side control, the internal error amplifier is used to achieve the voltage regulation. The REF/EA+ is
connected to an external reference voltage, FB/EA- is connected to the voltage sensing divider, and the error
amplifier ’ s output pin (COMP) is connected through a compensation filter back to the FB/EA- pin ( Figure 13 ). In
this case, the primary output ’ s start-up is a closed loop soft start (soft-start input reference of error amplifier). The
output soft-start time is determined by the external capacitor connected at SS pin based on the internal 27- μ A
charging current and the voltage set at REF/EA+ pin.
Based on the soft-start time T SS , choose soft start capacitor C SS value as:
C SS =
27 m A ′ T SS
V REF / EA +
VOUT
UCC28250
VSENSE
(9)
R S2
R S1
R Z3
C Z3
R O2
R O1
R Z2
C P1
C Z2
VREF
COMP
FB/EA-
REF/EA+
SS
+
+
C SS
Figure 13. Error Amplifier EAMP Connections for secondary-side Control
20
Product Folder Link(s): UCC28250
Copyright ? 2010 – 2011, Texas Instruments Incorporated
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