lm5027mhx National Semiconductor Corporation, lm5027mhx Datasheet - Page 20

• Manufacturer Part Number: lm5027mhx
• Description: Voltage Mode Active Clamp Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
For Applications > 100V
For applications where the system input voltage (V
ceeds 100V, VIN can be powered from an external start-up
regulator as shown in
pins together allows the LM5027 to be operated with VIN be-
UVLO
The under-voltage lockout threshold (UVLO) is internally set
to 2.0V at the UVLO pin. With two external resistors as shown
in
UVLO pin to exceed threshold voltage of 2V. When V
below the threshold, the internal 20 µA current sink is enabled
to reduce the voltage at the UVLO pin. When the UVLO pin
voltage exceeds the 2V threshold, the 20 µA current sink is
turned off causing the UVLO voltage to increase and provid-
ing hysteresis. The values of R1 and R2 can be determined
from the following equation:
Figure
12, the LM5027 is enabled when V
Figure
R1 = V
11. Connecting the VIN and VCC
HYS
/20 µA
FIGURE 11. Start-up Regulator for V
PWR
causes the
PWR
PWR
) ex-
is
20
low 13V. To turn-off the internal start-up regulator the VCC
voltage must be raised above 9.9V. The voltage at the VCC
pin must not exceed 15V. The voltage source at the right side
of
becomes active when the LM5027 outputs are active.
Where V
V
ample, if the LM5027 is to be enabled when V
34V, and the hysteresis is 1.8V, then R1 is 90 kΩ and 5.6
kΩ. For this application R1 was selected to be 90.0 kΩ, R2
was selected to be 6.19 kΩ. The LM5027 can be remotely
shutdown by taking the UVLO pin below 0.4V with an external
open collector or open drain device, as shown in
The outputs and the VCC regulator are disabled in shutdown
mode.
PWR
Figure 11
in the second equation is the turn-on voltage. For ex-
HYS
PWR
is typically derived from the power stage, and
is the desired UVLO hysteresis at V
> 100V
30099014
PWR
Figure
PWR
reaches
, and
12.