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| LM34927_15 Datasheet, PDF (14/28 Pages) Texas Instruments – LM34927 Integrated Secondary-Side Bias Regulator for Isolated DC-DC Converters | |||
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LM34927
SNVS799G â APRIL 2012 â REVISED DECEMBER 2014
www.ti.com
⢠C1 is selected to achieve the desired start-up time which can be determined from Equation 9.
tS = C1 x (R2 +
RFB1
RFB1
x
+
RFB2
RFB2
)
(9)
⢠R1 is used to maintain the node B voltage at zero after the soft start is finished. A value larger than the
feedback resistor divider is preferred. The effect of resistor R1 is ignored.
Using component values shown in Figure 19, selecting C1 = 1 µF, R2 = 1 kâ¦, R1 = 30 k⦠results in a soft-start
time of about 2 ms.
VOUT
VCC
RFB2
To FB
C1
R2
D
D1
B
RFB1
R1
Figure 18. Soft-Start Circuit
7.4 Device Functional Modes
UVLO
< 0.66 V
0.66 V to 1.225 V
> 1.225 V
Table 2. Undervoltage Detector
VCC
Disabled
Enabled
VCC < 4.5 V
VCC > 4.5 V
MODE
Shutdown
Standby
Standby
Operating
DESCRIPTION
VCC regulator disabled.
Switching disabled.
VCC regulator enabled.
Switching disabled.
VCC regulator enabled.
Switching disabled.
VCC enabled.
Switching enabled.
14
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