Datasheet

V
CC1
V
CC2
8V - 15V (from
external source)
LM5034
GND1
GND2
C2
C1
V
IN
V
PWR
LM5034
www.ti.com SNVS347A –FEBRUARY 2005–REVISED APRIL 2013
The average V
CC
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 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.
Figure 31. External Power to V
CC
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 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 tolerance 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 R
T
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 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 configuration is shown in Figure 21. 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 32 and
Figure 33. In both cases the sensed current creates a ramping voltage across R1, and the R
F
/C
F
filter
suppresses noise and transients. R1, R
F
and C
F
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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