Datasheet

LM5041
SNVS248D –AUGUST 2003–REVISED MARCH 2013 www.ti.com
DETAILED OPERATING DESCRIPTION
The LM5041 PWM controller contains all of the features necessary to implement either current-fed or voltage-fed
push-pull or bridge power converters. These “Cascaded” topologies are well suited for multiple output and higher
power applications. The LM5041’s four control outputs include: the buck stage controls (HD and LD) and the
push-pull control outputs (PUSH and PULL). Push-pull outputs are driven at 50% nominal duty cycle at one half
of the switching frequency of the buck stage and can be configured for either a specified overlap time (for
current-fed applications) or a specified both-off time (for voltage-fed applications). Push-pull stage MOSFETs can
be driven directly from the internal gate drivers while the buck stage requires an external driver such as the
LM5102. The LM5041 includes a high-voltage start-up regulator that operates over a wide input range of 15V to
100V. The PWM controller is designed for high-speed capability including an oscillator frequency range up to 1
MHz and total propagation delays of less than 100ns. Additional features include: line Under-Voltage Lockout
(UVLO), soft-start, an error amplifier, precision voltage reference, and thermal shutdown.
High Voltage Start-Up Regulator
The LM5041 contains an internal high-voltage start-up regulator, thus the input pin (Vin) can be connected
directly to the line voltage. The regulator output is internally current limited to 15mA. When power is applied, the
regulator is enabled and sources current into an external capacitor connected to the Vcc pin. The recommended
capacitance range for the Vcc regulator is 0.1uF to 100uF. When the voltage on the Vcc pin reaches the
regulation point of 9V and the internal voltage reference (REF) reaches its regulation point of 5V, the controller
outputs are enabled. The Buck stage outputs will remain enabled until Vcc falls below 7V or the line Under-
Voltage Lockout detector indicates that Vin is out of range. The push-pull outputs continue switching until the
REF pin voltage falls below approximately 3V. In typical applications, an auxiliary transformer winding is
connected through a diode to the Vcc pin. This winding must raise the Vcc voltage above 9.3V to shut off the
internal start-up regulator. Powering V
CC
from an auxiliary winding improves efficiency while reducing the
controller's power dissipation. The recommended capacitance range for the Vref regulator output is 0.1uF to
10uF.
The external V
CC
capacitor must be sized such that the capacitor maintains a V
CC
voltage greater than 7V during
the initial start-up. During a fault mode when the converter auxiliary winding is inactive, external current draw on
the V
CC
line should be limited so the power dissipated in the start-up regulator does not exceed the maximum
power dissipation of the controller.
An external start-up or other bias rail can be used instead of the internal start-up regulator by connecting the V
CC
and the V
IN
pins together and feeding the external bias voltage into the two pins.
Line Under-Voltage Detector
The LM5041 contains a line Under-Voltage Lockout (UVLO) circuit. An external set-point resistor divider from V
IN
to ground sets the operational range of the converter. The divider must be designed such that the voltage at the
UVLO pin will be greater than 2.5V when V
IN
is in the desired operating range. If the Under-Voltage threshold is
not met, all functions of the controller are disabled and the controller will enter a low-power state with input
current <300µA. ULVO hysteresis is accomplished with an internal 20µA current source that is switched on or off
into the impedance of the set-point divider. When the UVLO threshold is exceeded, the current source is
activated to instantly raise the voltage at the UVLO pin. When the UVLO pin falls below the 2.5V threshold, the
current source is turned off causing the voltage at the UVLO pin to fall. The UVLO pin can also be used to
implement a remote enable / disable function. By shorting the UVLO pin to ground, the converter can be
disabled. The controller can also be disabled through the soft-start pin (SS). The controller will enter a low-power
off state if the SS pin is forced below the 0.45V shutdown threshold.
Buck Stage Control Outputs
The LM5041 Buck switch maximum duty cycle clamp ensures that there will be sufficient off time each cycle to
recharge the bootstrap capacitor used in the high side gate driver. The Buck switch is specified to be off, and the
sync switch on, for at least 250ns per switching cycle. The Buck stage control outputs (LD and HD) are CMOS
buffers with logic levels of 0 to 5V.
During any fault state or Under-Voltage off state, the buck stage control outputs will default to HD low and LD
high.
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