lm25037mtx National Semiconductor Corporation, lm25037mtx Datasheet - Page 18

• Manufacturer Part Number: lm25037mtx
• Description: Dual-mode Pwm Controller With Alternating Outputs
• Manufacturer: National Semiconductor Corporation

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UVLO DIVIDER SELECTION
A dedicated comparator connected to the UVLO pin detects
an input under-voltage condition. When the UVLO pin voltage
is below 0.45V, the LM25037 controller is in a low current
shutdown mode. For a UVLO pin voltage greater than 0.45V
but less than 1.25V, the controller is in standby mode with
VCC and REF regulators active but no switching. Once the
UVLO pin voltage is greater than 1.25V, the controller is fully
enabled. When the UVLO pin voltage rises above the 1.25V
threshold, an internal 22 µA current source as shown in Figure
14, is activated thus providing threshold hysteresis. The 22
µA current source is deactivated when the voltage at the UV-
LO pin falls below 1.25V. Resistance values for R1 and R2
can be determined from the following equations:
FIGURE 13. Current Sense Using Current Sense Resistor (R1)
FIGURE 12. Current Sense Using Transformer
18
Where V
desired UVLO hysteresis at V
PWR
is the desired turn-on voltage and V
30065136
30065137
PWR
.
HYS
is the