lm5045sqx National Semiconductor Corporation, lm5045sqx Datasheet - Page 12

• Manufacturer Part Number: lm5045sqx
• Description: Full-bridge Pwm Controller With Integrated Mosfet Drivers
• Manufacturer: National Semiconductor Corporation

www.national.com
Functional Description
The LM5045 PWM controller contains all of the features nec-
essary to implement a Full-Bridge topology power converter
using either current mode or voltage mode control. This de-
vice is intended to operate on the primary side of an isolated
dc-dc converter with input voltage up to 100V. This highly in-
tegrated controller-driver provides dual 2A high and low side
gate drivers for the four external bridge MOSFETs plus con-
trol signals for secondary side synchronous rectifiers. Exter-
nal resistors program the leading and trailing edge dead-time
between the main and synchronous rectifier control signals.
Intelligent startup of synchronous rectifier allows turn-on of
the power converter into the pre-bias loads. Cycle-by-cycle
current limit protects the power components from load tran-
sients while hiccup mode protection limits average power
dissipation during extended overload conditions. Additional
features include programmable soft-start, soft-start of the
synchronous rectifiers, and a 2 MHz capable oscillator with
synchronization capability and thermal shutdown.
High-Voltage Start-Up Regulator
The LM5045 contains an internal high voltage start-up regu-
lator that allows the input pin (VIN) to be connected directly
to the supply voltage over a wide range from 14V to 100V.
The input can withstand transients up to 105V. When the UV-
LO pin potential is greater than 0.4V, the VCC regulator is
enabled to charge an external capacitor connected to the
VCC pin. The VCC regulator provides power to the voltage
reference (REF) and the gate drivers (HO1/HO2 and LO1/
LO2). When the voltage on the VCC pin exceeds its Under
Voltage (UV) threshold, the internal voltage reference (REF)
reaches its regulation set point of 5V and the UVLO voltage
is greater than 1.25V, the soft-start capacitor is released and
normal operation begins. The regulator output at VCC is in-
ternally current limited. The value of the VCC capacitor de-
pends on the total system design, and its start-up character-
istics. The recommended range of values for the VCC
capacitor is 0.47μF to 10µF.
The internal power dissipation of the LM5045 can be reduced
by powering VCC from an external supply. The output voltage
of the VCC regulator is initially regulated to 9.5V. After the
synchronous rectifiers are engaged (which is approximately
when the output voltage in within regulation), the VCC voltage
is reduced to 7.7V. In typical applications, an auxiliary trans-
former winding is connected through a diode to the VCC pin.
This winding must raise the VCC voltage above 8V to shut off
the internal start-up regulator. Powering VCC from an auxil-
iary winding improves efficiency while reducing the
controller’s power dissipation. The VCC UV circuit will still
function in this mode, requiring that VCC never falls below its
UV threshold during the start-up sequence. The VCC regula-
tor series pass transistor includes a diode between VCC and
VIN that should not be forward biased in normal operation.
Therefore, the auxiliary VCC voltage should never exceed the
VIN voltage.
An external DC bias voltage can be used instead of the inter-
nal regulator by connecting the external bias voltage to both
the VCC and the VIN pins. This implementation is shown in
the Applications Information section. The external bias must
be greater than 10V and less than the VCC maximum voltage
rating of 14V.
Line Under-Voltage Detector
The LM5045 contains a dual level Under-Voltage Lockout
(UVLO) circuit. When the UVLO pin voltage is below 0.4V, the
12
controller is in a low current shutdown mode. When the UVLO
pin voltage is greater than 0.4V but less than 1.25V, the con-
troller is in standby mode. In standby mode the VCC and REF
bias regulators are active while the controller outputs are dis-
abled. When the VCC and REF outputs exceed their respec-
tive under-voltage thresholds and the UVLO pin voltage is
greater than 1.25V, the soft-start capacitor is released and the
normal operation begins. An external set-point voltage divider
from VIN to GND can be used to set the minimum operating
voltage of the converter. The divider must be designed such
that the voltage at the UVLO pin will be greater than 1.25V
when VIN enters the desired operating range. UVLO hystere-
sis is accomplished with an internal 20μA current sink that is
switched on or off into the impedance of the set-point divider.
When the UVLO threshold is exceeded, the current sink is
deactivated to quickly raise the voltage at the UVLO pin.
When the UVLO pin voltage falls below the 1.25V threshold,
the current sink is enabled causing the voltage at the UVLO
pin to quickly fall. The hysteresis of the 0.4V shutdown com-
parator is internally fixed at 50mV.
The UVLO pin can also be used to implement various remote
enable / disable functions. Turning off the converter by forcing
the UVLO pin to standby condition (0.4V < UVLO < 1.25V)
provides a controlled soft-stop. Refer to the Soft Stop section
for more details.
Over Voltage Protection
An external voltage divider can be used to set either an over
voltage or an over temperature protection. During an OVP
condition, the SS and SSSR capacitors are discharged and
all the outputs are disabled. The divider must be designed
such that the voltage at the OVP pin is greater than 1.25V
when over voltage/temperature condition exists. Hysteresis is
accomplished with an internal 20μA current source. When the
OVP pin voltage exceeds 1.25V, the 20μA current source is
activated to quickly raise the voltage at the OVP pin. When
the OVP pin voltage falls below the 1.25V threshold, the cur-
rent source is deactivated causing the voltage at the OVP to
quickly fall. Refer to the Applications Information section for
more details.
Reference
The REF pin is the output of a 5V linear regulator that can be
used to bias an opto-coupler transistor and external house-
keeping circuits. The regulator output is internally current
limited to 15mA. The REF pin needs to be locally decoupled
with a ceramic capacitor, the recommended range of values
are from 0.1μF to 10μF
Oscillator, Sync Input
The LM5045 oscillator frequency is set by a resistor connect-
ed between the RT pin and AGND. The RT resistor should be
located very close to the device. To set a desired oscillator
frequency (F
calculated from the following equation:
For example, if the desired oscillator frequency is 400 kHz i.e.
each phase (LO1 or LO2) at 200 kHz, the value of R
25kΩ. If the LM5045 is to be synchronized to an external
clock, that signal must be coupled into the RT pin through a
100pF capacitor. The RT pin voltage is nominally regulated
OSC
), the necessary value of RT resistor can be
T
will be