VCC1

VCC2

8V - 15V (from

external source)

LM5034

GND1

GND2

C2

C1

VIN

V

PWR

LM5034

www.ti.com

SNVS347A – FEBRUARY 2005 – REVISED APRIL 2013

The average VCC regulator current required to drive the external MOSFETs is a function of the MOSFET gate

capacitance and the switching frequency (see Figure 8). To ensure VCC does not droop below the lower UVT

threshold, an external supply should be diode connected to both VCC pins to provide the required current, as

shown in Figure 31. The applied VCC voltage must be between 8V and 15V. Providing the VCC voltage higher

than the 7.7V regulation level with an external supply shuts off the internal regulator, reducing power dissipation

within the IC. Internally there is a diode from the VCC regulator output to VIN. Typically the applied voltage is

derived from an auxiliary winding on the power transformer, or on the output inductor.

Figure 31. External Power to VCC

OSCILLATOR, SYNC INPUT

The oscillator frequency is generally selected in conjunction with the system magnetic components, and any

other aspects of the system which may be affected by the frequency. The RT resistor at the RT/SYNC pin sets

the frequency according to Equation 1. Each output (OUT1 and OUT2) switches at one-half the oscillator

frequency. If the required frequency tolerance is critical in a particular application, the tolerance of the external

resistor and the frequency tolerance specified in the Electrical Characteristics table must be considered when

selecting the RT resistor.

If the LM5034 is to be synchronized to an external clock, that signal must be coupled into the RT/SYNC pin

through a 100 pF capacitor. The external synchronizing frequency must be at least 4% higher than the free

running frequency set by the RT resistor and no higher than twice the free running frequency. The RT/SYNC pin

voltage is nominally regulated at 2.0V and the external pulse amplitude should lift the pin to between 3.8V and

5.0V on the low-to-high transition. The synchronization pulse width should be between 15 and 150 ns. The RT

resistor is always required, whether the oscillator is free running or externally synchronized.

VOLTAGE FEEDBACK, COMP1, COMP2

Each COMP pin is designed to accept a voltage feedback signal from the respective regulated output via an

error amplifier and (typically) an opto-coupler. A typical configuration is shown in Figure 21. VOUT is compared to

a reference by the error amplifier which has an appropriate frequency compensation network. The amplifier’s

output drives the opto-coupler, which in turn drives the COMP pin.

When the LM5034’s two controller channels are configured to provide a single high current output, COMP1 and

COMP2 are typically connected together, and to the feedback signal from the optocoupler.

CURRENT SENSE, CS1, CS2

Each CS pin receives an input signal representative of its transformer’s primary current, either from a current

sense transformer or from a resistor in series with the source of the primary switch, as shown in Figure 32 and

Figure 33. 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 should be as physically close to the LM5034 as possible, and

the ground connection from the current sense transformer, or R1, should be a dedicated track to the appropriate

GND pin. The current sense components must provide >0.5V at the CS pin when an over-current condition

exists.

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