lm5037mtx National Semiconductor Corporation, lm5037mtx Datasheet - Page 17

• Manufacturer Part Number: lm5037mtx
• Description: Dual-mode Pwm Controller With Alternating Outputs
• Manufacturer: National Semiconductor Corporation

(F
current sense resistor (R
of 80 mV will suffice. The slope compensation "volts" that re-
sults from the above expression is the maximum voltage of
the artificial ramp added linearly to the RAMP pin till the end
of maximum switching period. For circuits where a current
sense tramsformer is used for primary current sensing, the
turns-ratio of the current sense transformer has to be taken
into account.
C
by the internal RAMP discharge FET. Capacitor values rang-
ing from 100 pF to 1500 pF are recommended. The value
must be small enough such that the capacitor can be dis-
charged within the clock (CLK) pulse width each cycle.
R
For example, with a C
erence voltage of 5V (V
and R
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 pri-
mary (V
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 recom-
mended that the filter shown in Figure 10 be used to suppress
transients that may occur at the input supply. This is particu-
larly important when VIN is operated close to the maximum
operating rating of the LM5037.
When power is applied to VIN and the UVLO pin voltage is
greater than 0.45V, the VCC regulator is enabled and sup-
plies 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 con-
nected 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.
slope
slope
OSC
) of 250 kHz, a filter inductance of 4 µH (L
filter
should be selected such that it can be fully discharged
can be selected from the following formula:
PWR
= 25Ω results in R
), can vary in the range from 8V to 100V. The
slope
REF
CS
of 1500 pF, F
) of 32 mΩ, slope compensation
), slope compensation of 80 mV
slope
value of 165 kΩ.
OSC
of 250 kHz, ref-
Filter
) and a
17
FOR APPLICATIONS WITH >100V INPUT
For applications where the system input voltage exceeds
100V or the IC power dissipation is of concern, the LM5037
can be powered from an external start-up regulator as shown
in Figure 11. In this configuration, the VIN and the VCC pins
should be connected together. The voltage at the VCC and
VIN pins must be greater than 8.1V (> Max VCC reference
voltage) yet not exceed 15V. 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 tran-
sistors, should be considered when selecting the transistor.
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
ure 12 and Figure 13. R1, R
close to the LM5037 as possible, and the ground connection
from the current sense transformer, or R1, should be a dedi-
cated 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.
F
/C
FIGURE 11. Start-up Regulator for VPWR >100V
F
filter suppresses noise and transients as shown in Fig-
FIGURE 10. Input Transient Protection
F
and C
F
should be located as
30069434
www.national.com
30069435