Datasheet

VCC
8V - 15V (from
external source)
LM5032
GND1
GND2
C1
VIN
V
PWR
t
VCC
=
C1 x 7.6V
I
CC(Lim)
UVLO
LM5032
1.25V
20 PA
Max. Duty
Cycle Limiter
R1
R2
V
PWR
Shutdown
Control
LM5032
SNVS344A –MARCH 2005–REVISED APRIL 2013 www.ti.com
Figure 26. Shutdown Control
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 necessary 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 regulated at 7.7V. The delay to the UVT level (Figure 4) is calculated from the
following:
(5)
where C1 is the capacitor at VCC and I
CC(Lim)
is the V
CC
regulator’s current limit. If the capacitor is 0.1 µF, the
nominal I
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
capacitance and the switching frequency (see Figure 7). To ensure VCC does not droop below the lower UVT
threshold, an external supply should be diode connected to VCC to provide the required current, as shown in
Figure 27. 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 27. 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.
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