LM5035 datasheet(23/39 Pages) TI

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LM5035

AGND

Power

Transformer

VPWR

VIN

Current

Sense

LM5035

VPWR

8.3V - 15V

(from aux winding)

VIN

VCC

LM5035

www.ti.com

SNVS428H – JANUARY 2006 – REVISED OCTOBER 2015

8.2.2.2 For Applications >100 V

For applications where the system input voltage exceeds 100 V or the IC power dissipation is of concern, the

LM5035 can be powered from an external start-up regulator as shown in Figure 17. In this configuration, the VIN

and the VCC pins must be connected together, which allows the LM5035 to be operated below 13 V. The voltage

at the VCC pin must be greater than 8.3 V, yet not exceed 15 V. An auxiliary winding can be used to reduce the

power dissipation in the external regulator once the power converter is active. The NPN base-emitter reverse

breakdown voltage, which can be as low as 5 V for some transistors, must be considered when selecting the

transistor.

8.2.2.3 Current Sense

The CS pin needs to receive an input signal representative of the primary current of the transformer, either from

a current sense transformer or from a resistor in series with the source of the LO switch, as shown in Figure 18

and Figure 19. In both cases, the sensed current creates a ramping voltage across R1, and the RF / CF filter

suppresses noise and transients. R1, RF and CF must be located as close to the LM5035 as possible, and the

ground connection from the current sense transformer, or R1, must be a dedicated track to the AGND pin. The

current sense components must provide greater than 0.25 V at the CS pin when an overcurrent condition exists.

Figure 17. Start-Up Regulator for VPWR >100 V

Figure 18. Current Sense Using Current Sense Transformer

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