lm5035mhx National Semiconductor Corporation, lm5035mhx Datasheet - Page 17

• Manufacturer Part Number: lm5035mhx
• Description: Pwm Controller With Integrated Half-bridge And Syncfet Drivers
• Manufacturer: National Semiconductor Corporation

Applications Information
The following information is intended to provide guidelines
for the power supply designer using the LM5035.
VIN
The voltage applied to the VIN pin, which may be the same
as the system voltage applied to the power transformer’s
primary (V
current into VIN depends primarily on the gate charge pro-
vided to the output drivers, the switching frequency, and any
external loads on the VCC and REF pins. It is recommended
the filter shown in Figure 6 be used to suppress transients
which may occur at the input supply. This is particularly
important when VIN is operated close to the maximum op-
erating rating of the LM5035.
When power is applied to VIN and the UVLO pin voltage is
greater than 0.4V, 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 7.6V, the voltage reference (REF) is enabled. The
reference regulation set point is 5V. The HO, LO, SR1 and
SR2 outputs 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 wind-
ing is connected through a diode to the VCC pin. This
winding must raise the VCC voltage above 8.3V 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
VIN 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 6. Input Transient Protection
PWR
), can vary in the range of 13 to 105V. The
FIGURE 7. Start-up Regulator for V
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FOR APPLICATIONS
For applications where the system input voltage exceeds
100V or the IC power dissipation is of concern, the LM5035
can be powered from an external start-up regulator as
shown in Figure 7. In this configuration, the VIN and the VCC
pins should be connected together, which allows the
LM5035 to be operated below 13V. The voltage at the VCC
pin must be greater than 8.3V yet not exceed 15V. An
auxiliary winding can be used to reduce the power dissipa-
tion in the external regulator once the power converter is
active. The NPN base-emitter reverse breakdown voltage,
which can be as low as 5V for some transistors, should be
considered when selecting the transistor.
CURRENT SENSE
The CS pin needs to 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 LO switch, as shown in Figure 8 and Figure 9. In both
cases, the sensed current creates a ramping voltage across
R1, and the R
R1, R
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 0.25V at the CS pin when an over-current condition
exists.
F
and C
PWR
F
>
F
should be located as close to the LM5035 as
100V
/C
F
filter suppresses noise and transients.
>
100V
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