LM5041

SNVS248C – MAY 2004 – REVISED MAY 2004

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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

to ground sets the operational range of the converter. The divider must be designed such that the voltage at the

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 guaranteed 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.

Push-Pull Outputs

The push pull outputs operate continuously at a nominal 50% duty cycle. A distinguishing feature of the LM5041

is the ability to accurately configure either dead time (both-off) or overlap time (both-on) on the complementary

push-pull outputs. The overlap/dead time magnitude is controlled by a resistor connected to the TIME pin on the

controller. The TIME pin holds one end of the resistor at 2.5V and the other end of the resistor should be

connected to either REF for dead time control setting or to GND for overlap control. The polarity of the current in

the TIME is detected by the LM5041 The magnitude of the overlap/dead time can be calculated as follows:

Current-fed designs require a period of overlap to insure there is a continuous path for the buck inductor current.

Voltage-fed designs require a period of dead time to insure there is no time when the push-pull transformer acts

as a shorted turn to the low impedance sourcing node. The push-pull outputs alternate continuously under all

conditions provided REF the voltage is greater than 3V.

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