R2 =

1.26V

R1 =

10

VIN

0.1 2F

LM5001

LM5001, LM5001-Q1

SNVS484H – JANUARY 2007 – REVISED JULY 2015

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)

would set R1 to 29.4 k

Ω 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.

12

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