LM5041A

SNVS350B – MARCH 2005 – REVISED MARCH 2013

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DETAILED OPERATING DESCRIPTION

The LM5041A 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 LM5041A’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 an overlap time (for current-fed

applications) or a 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

LM5041A 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 LM5041A 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

controller's power dissipation. The recommended capacitance range for the Vref regulator output is 0.1uF to

10uF.

the initial start-up. During a fault mode when the converter auxiliary winding is inactive, external current draw on

power dissipation of the controller.

Line Under-Voltage Detector

The LM5041A contains a line Under-Voltage Lockout (UVLO) circuit. An external set-point resistor divider from

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.

Buck Stage Control Outputs

The LM5041A 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 remains off, and the sync

switch remains 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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