LM5032 datasheet(17/22 Pages) NSC

17 / 22 page

Applications Information (Continued)

VCC

The capacitor at VCC provides 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 capacitor provides a nec-

essary time delay during startup. The time delay allows the

internal circuitry of the LM5032 and associated external

circuitry to stabilize before VCC reaches the upper UVT

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

and the soft-start sequence begins. VCC is nominally regu-

lated at 7.7V. The delay to the UVT level (Figure 2) is

calculated from the following:

where C1 is the capacitor at VCC and I

CC(Lim) is the VCC

regulator’s current limit. If the capacitor is 0.1 µF, the nominal

CC(Lim) of 22 mA provides a delay of approximately 35 µs.

The capacitor value should range between 0.1 µF and 25 µF.

Experimentation with the final design may be necessary to

determine the optimum value for the VCC capacitor.

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 5). To ensure

VCC does not droop below the lower UVT threshold, an

external supply should be diode connected to VCC to pro-

vide the required current, as shown in Figure 25. The applied

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

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 LM5032 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 16. 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 LM5032’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 Figures 26 and 27. In both cases

the sensed current creates a ramping voltage across R1,

and the R

F/CF filter suppresses noise and transients. R1, RF

and C

F should be as physically close to the LM5032 as

possible, and the ground connection from the current sense

transformer, or R1, should be a dedicated track to the ap-

propriate GND pin. The current sense components must

provide >0.5V at the CS pin when an over-current condition

exists.

20135029

FIGURE 24. Shutdown Control

20135030

FIGURE 25. External Power to V

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