Datasheet

R2 =
1.26V
I
DIVIDER
+ 6 PA
R1 =
V
PWR
- 1.26V
I
DIVIDER
10
VIN
V
PWR
0.1 PF
LM5001
LM5001 , LM5001-Q1
SNVS484G –JANUARY 2007–REVISED APRIL 2014 www.ti.com
Application Information (continued)
Figure 14. Input Transient Protection
8.1.2 SW Pin
Attention must be given to the PC board layout for the SW pin which connects to the power MOSFET drain.
Energy can be stored in parasitic inductance and capacitance which cause switching spikes that negatively effect
efficiency, and conducted and radiated emissions. These connections should be as short as possible to reduce
inductance and as wide as possible to reduce resistance. The loop area, defined by the SW and GND pin
connections, the transformer or inductor terminals, and their respective return paths, should be minimized.
8.1.3 EN / UVLO Voltage Divider Selection
Two dedicated comparators connected to the EN pin are used to detect under-voltage and shutdown conditions.
When the EN pin voltage is below 0.45 V, the controller is in a low current shutdown mode where the VIN current
is reduced to 95 µA. For an EN pin voltage greater than 0.45 V but less than 1.26 V the controller is in standby
mode, with all internal circuits operational, but the PWM gate driver signal is blocked. Once the EN pin voltage is
greater than 1.26 V, the controller is fully enabled. Two external resistors can be used to program the minimum
operational voltage for the power converter as shown in Figure 15. When the EN pin voltage falls below the 1.26
V threshold, an internal 100 mV threshold hysteresis prevents noise from toggling the state, so the voltage must
be reduced to 1.16 V to transition to standby. Resistance values for R1 and R2 can be determined from
Equation 2 and Equation 3:
(2)
(3)
where V
PWR
is the desired turn-on voltage and I
DIVIDER
is an arbitrary current through R1 and R2.
For example, if the LM5001 is to be enabled when V
PWR
reaches 16 V, I
DIVIDER
could be chosen as 501 µA which
would set R1 to 29.4 kΩ and R2 to 2.49 kΩ. The voltage at the EN pin should not exceed 10 V unless the current
into the 6 V protection Zener diode is limited below 4 mA. The EN pin voltage should not exceed 14 V at any
time. Be sure to check both the power and voltage rating (some 0603 resistors are rated as low as 50 V) for the
selected R1 resistor.
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