50

VIN

0.1 2F

LM5037

LM5037

www.ti.com

SNVS578D – NOVEMBER 2008 – REVISED MAY 2015

Device Functional Modes (continued)

Sub-harmonic oscillation is normally characterized by observing alternating wide and narrow duty cycles at the

controller output. Adding an artificial ramp (slope compensation) to the current sense signal eliminates this

potential oscillation. Current mode control is also susceptible to noise and layout considerations. It is

inductance. When the converter is operating at low duty cycles and light load, the primary current amplitude is

small and is susceptible to noise. The artificial ramp, added to avoid sub-harmonic oscillations, provides

additional benefits by improving the noise immunity of the converter.

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

8.1.1 Input Supply Voltage (VIN and VCC pins)

The voltage applied to the VIN pin, which may be the same as the system voltage applied to the power

gate charge provided by the output drivers, the switching frequency, and any external loads on the VCC and

REF pins. This design uses the filter shown in Figure 20 to suppress transients that may occur at the input

supply. A filter is particularly important when VIN is operated close to the maximum operating rating of the

LM5037.

When power is applied to VIN and the UVLO pin voltage is greater than 0.45 V, the VCC regulator is enabled

and supplies current into an external capacitor connected to the VCC pin. When the voltage on the VCC pin

reaches the regulation point of 7.7 V, the voltage reference (REF) enables. The reference regulation set point is

5 V. The outputs (OUTA and OUTB) enable when the two bias regulators reach their set point and the UVLO pin

potential is greater than 1.25 V. In typical applications, an auxiliary transformer winding connects through a diode

to the VCC pin. In order to shut off the internal start-up regulator, this winding must raise the VCC voltage above

8.1 V.

After the outputs are enabled and the external VCC supply voltage has begun supplying power to the device, the

current into the VIN pin drops below 1 mA. VIN should remain at a voltage equal to or above the VCC voltage to

avoid reverse current through protection diodes.

Figure 20. Input Transient Protection

8.1.2 100-V (or Higher) Input Voltage Applications

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

LM5037 device can be powered from an external start-up regulator as shown in Figure 21. This configuration

shows the VIN and the VCC pins connected together. The voltage at the VCC and VIN pins must be greater than

dissipation in the external regulator after the power converter activates. The N-P-N base-emitter reverses

breakdown voltage, which can be as low as 5 V for some transistors. Consider this breakdown voltage when

selecting the transistor.

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