LM5034 datasheet(21/29 Pages) NSC

21 / 29 page

Applications Information (Continued)

VCC1, VCC2

The capacitors at each VCC pin provide not only regulator

noise filtering and stability, but also prevents VCC from

dropping to the lower under-voltage threshold level (UVT =

6.2V) when the output drivers source current surges to the

external MOSFET gates. Additionally, the capacitors provide

a necessary time delay during startup. The time delay allows

the internal circuitry of the LM5034 and associated external

circuitry to stabilize before V

CC reaches the upper UVT

threshold level (7.6V), at which time the outputs are enabled

and the soft-start sequence begins. V

CC is nominally regu-

lated at 7.7V. The delay to the UVT level (Figure 3)is

calculated from the following:

where C1 and C2 are the capacitors at VCC1 and VCC2,

and I

CC(Lim) is the VCC regulator’s current limit. If the capaci-

tors are 0.1 µF each, the nominal I

CC(Lim) of 22 mA provides

a delay of approximately 69 µs. The V

CC capacitor values

should range between 0.1 µF and 25 µF, and they should be

the same value. Experimentation with the final design may

be necessary to determine the optimum value for the V

capacitors.

The average V

CC regulator current required to drive the

external MOSFETs is a function of the MOSFET gate capaci-

tance and the switching frequency (see Figure 6). 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 29. The

applied V

CC voltage must be between 8V and 15V. Providing

the V

CC 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 V

CC regulator output to VIN. Typically the applied

voltage is derived from an auxiliary winding on the power

transformer, or on the output inductor.

OSCILLATOR, SYNC INPUT

The oscillator frequency is generally selected in conjunction

with the system magnetic components, and any other as-

pects of the system which may be affected by the frequency.

The R

T 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 toler-

ance specified in the Electrical Characteristics table must be

considered when selecting the R

T 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

T resistor and no higher than twice the free running fre-

quency. 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 R

T 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 configura-

tion is shown in Figure 19.V

OUT 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 30 and Figure 31. In both

cases the sensed current creates a ramping voltage across

R1, and the R

F/CF filter suppresses noise and transients.

R1, R

F 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.

20136829

FIGURE 28. Shutdown Control

20136830

FIGURE 29. External Power to V

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