lm5015mhx National Semiconductor Corporation, lm5015mhx Datasheet - Page 15

• Manufacturer Part Number: lm5015mhx
• Description: High Voltage Monolithic Two-switch Forward Dc-dc Regulator
• Manufacturer: National Semiconductor Corporation

EN / UVLO VOLTAGE DIVIDER SELECTION
Two dedicated comparators connected to the EN pin are used
to implement input under-voltage lockout and a shutdown
condition. When the EN pin voltage is below 0.45V, the con-
troller is in a low current shutdown mode where the VIN
current is reduced to 110 µA. For an EN pin voltage greater
than 0.45V but less than 1.26V the controller is in standby
mode, with all internal circuits operational, but with the power
MOSFETs disabled. Once the EN pin voltage is greater than
1.26V, the controller is fully enabled and the HO and LO out-
puts commence switching. Two external resistors can be
used to program the minimum operational voltage for the
power converter as shown in Figure 6. When the EN pin volt-
age falls below the 1.26V threshold, an internal 100 mV
threshold hysteresis prevents noise from toggling the state.
Therefore, the voltage at the EN pin must be reduced to 1.16V
to transition to the standby state. Resistance values for R1
and R2 can be determined from the following equations,
A remote enable function can be accomplished with open
drain device(s) connected to the EN pin as shown in Figure
7. A MOSFET or an NPN transistor connected to the EN pin
will force the regulator into the low power ‘off’ state. Adding a
FIGURE 7. Remote Standby and Disable Control
FIGURE 6. Basic EN (UVLO) Configuration
15
where V
user-defined current through R1 and R2.
For example, if the LM5015 is to be enabled when V
reaches 16V, I
set R1 to 29.4 kΩ and R2 to 2.49 kΩ. If the voltage at the EN
pin can exceed 6V, then the current into the 6V protection
Zener must be limited below 4 mA by the external resistors.
Be sure to check both the power and voltage rating for the
selected R1 resistor (some 0603 resistors are rated as low as
50V maximum operating voltage).
PN diode in the drain (or collector) provides the offset to
achieve the standby state. The advantage of standby is that
the VCC LDO is not disabled and external circuitry powered
by VCC remains functional.
PWR
is the desired turn-on voltage and I
DIVIDER
could be chosen as 500 µA which would
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DIVIDER
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is the
PWR