Datasheet

VIN

PWR

0.1 PF

LM5002

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SNVS496D –JANUARY 2007–REVISED MARCH 2013

APPLICATION INFORMATION

The following information is intended to provide guidelines for the power supply designer using the LM5002.

VIN

The voltage applied to the VIN pin can vary within the range of 3.1V to 75V. The current into the VIN pin

depends primarily on the gate charge of the power MOSFET, the switching frequency, and any external load on

the VCC pin. It is recommended the filter shown in Figure 17 be used to suppress transients which may occur at

the input supply. This is particularly important when VIN is operated close to the maximum operating rating of the

LM5002.

When power is applied and the VIN voltage exceeds 2.8V with the EN pin voltage greater than 0.45V, the VCC

regulator is enabled, supplying current into the external capacitor connected to the VCC pin. When the VIN

voltage is between 2.8V and 6.9V, the VCC voltage is approximately equal to the VIN voltage. When the voltage

on the VCC pin exceeds 6.9V, the VCC pin voltage is regulated at 6.9V. In typical flyback applications, an

auxiliary transformer winding is connected through a diode to the VCC pin. This winding must raise the VCC

voltage above 6.9V to shut off the internal start-up regulator. The current requirements from this winding are

relatively small, typically less than 20 mA. If the VIN voltage is much higher than the auxiliary voltage, the

auxiliary winding will significantly improve conversion efficiency. It also reduces the power dissipation within the

LM5002. The externally applied VCC voltage should never exceed 14V. Also the applied VCC should never

exceed the VIN voltage to avoid reverse current through the internal VCC to VIN diode shown in the LM5002

Block Diagram.

Figure 17. Input Transient Protection

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.

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 0.45V, 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 0.45V but less than 1.26V 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 1.26V, 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 18. When the EN pin voltage falls below the

1.26V threshold, an internal 100 mV threshold hysteresis prevents noise from toggling the state, so the voltage

must be reduced to 1.16V to transition to standby. Resistance values for R1 and R2 can be determined from the

following equations:

Product Folder Links: LM5002