Datasheet

VIN
VCC
from aux winding
LM25037
9V
V
PWR
5.5V to 14V
50
VIN
V
PWR
0.1
PF
LM 25037
LM25037, LM25037-Q1
www.ti.com SNVS572D –JULY 2008–REVISED MARCH 2013
VIN and VCC
The voltage applied to the VIN pin, which may be the same as the system voltage applied to the power
transformer’s primary (V
PWR
), can vary in the range from 5.5V to 75V. The current into the VIN pin depends
primarily on the gate charge provided by the output drivers, the switching frequency, and any external loads on
the VCC and REF pins. It is recommended that the filter shown in Figure 23 be used to suppress transients that
may occur at the input supply. This is particularly important when VIN is operated close to the maximum
operating rating of the LM25037.
When power is applied to VIN and the UVLO pin voltage is greater than 0.45V, the VCC regulator is enabled and
supplies current into an external capacitor connected to the VCC pin. When the voltage on the VCC pin reaches
the regulation point of 7.7V, the voltage reference (REF) is enabled. The reference regulation set point is 5V. The
outputs (OUTA and OUTB) are enabled when the two bias regulators reach their set point and the UVLO pin
potential is greater than 1.25V. In typical applications, an auxiliary transformer winding is connected through a
diode to the VCC pin. This winding must raise the VCC voltage above 8.1V to shut off the internal start-up
regulator.
After the outputs are enabled and the external VCC supply voltage has begun supplying power to the IC, the
current into the VIN pin drops below 1 mA. VIN should remain at a voltage equal to or above the VCC voltage to
avoid reverse current through protection diodes.
Figure 23. Input Transient Protection
FOR APPLICATIONS WITH >75V INPUT
For applications where the system input voltage exceeds 75V or the IC power dissipation is of concern, the
LM25037 can be powered from an external start-up regulator as shown in Figure 24. In this configuration, the
VIN and the VCC pins should be connected together. The voltage at the VCC and VIN pins must be at least 5.5V
(> Max VCC UV voltage) yet not exceed 14V. An auxiliary winding can be used to reduce the power dissipation
in the external regulator once the power converter is active. The NPN base-emitter reverses breakdown voltage,
which can be as low as 5V for some transistors, should be considered when selecting the transistor.
Figure 24. Start-up Regulator for VPWR >75V
CURRENT SENSE
The CS pin should receive an input signal representative of the transformer’s primary current, either from a
current sense transformer or from a resistor in series with the source of the OUTA and OUTB MOSFET switches.
In both cases, the sensed current creates a voltage ramp across R1, and the R
F
/C
F
filter suppresses noise and
transients as shown in Figure 25 and Figure 26. R1, R
F
and C
F
should be located as close to the LM25037 as
possible, and the ground connection from the current sense transformer, or R1, should be a dedicated track to
the AGND pin. The current sense components must provide greater than 220 mV at the CS pin when an over-
current condition exists.
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