Datasheet

EN
LM5015
V
PWR
1.26V
6PA
STANDBY
(DISABLE PWM
CONTROLLER)
R1
R2
0.45V
SHUTDOWN VCC LDO
VIN
6V
1k
V
PWR
- 1.26V
R1 =
I
DIVIDER
1.26V
R2 =
I
DIVIDER
+ 6 PA
LM5015
SNVS538C –NOVEMBER 2007–REVISED APRIL 2013 www.ti.com
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 controller 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 outputs commence
switching. Two external resistors can be used to program the minimum operational voltage for the power
converter as shown in Figure 18. When the EN pin voltage 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, where V
PWR
is the desired turn-on voltage and I
DIVIDER
is the user-defined current through R1
and R2.
(4)
For example, if the LM5015 is to be enabled when V
PWR
reaches 16V, I
DIVIDER
could be chosen as 500 µA which
would 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).
Figure 18. Basic EN (UVLO) Configuration
A remote enable function can be accomplished with open drain device(s) connected to the EN pin as shown in
Figure 19. A MOSFET or an NPN transistor connected to the EN pin will force the regulator into the low power
‘off’ state. Adding a 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.
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