PUSH

DEADTIME

WAVEFORMS

PULL

PUSH

PULL

OVERLAP

WAVEFORMS

LM5041B

www.ti.com

SNVS605 – MAY 2009

Push-Pull Outputs

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

LM5041B 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 LM5041B 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.

PWM Comparator

The PWM comparator compares the slope compensated current ramp signal to the loop error voltage from the

internal error amplifier (COMP pin). This comparator is optimized for speed in order to achieve minimum

controllable duty cycles. The comparator polarity is such that 0V on the COMP pin will produce zero duty cycle in

the buck stage.

Error Amplifier

An internal high gain wide-bandwidth error amplifier is provided within the LM5041B. The amplifier’s non-inverting

input is tied to a 0.75V reference. The inverting input is connected to the FB pin. In non-isolated applications the

power converter output is connected to the FB pin via the voltage setting resistors. Loop compensation

components are connected between the COMP and FB pins. For most isolated applications the error amplifier

function is implemented on the secondary side of the converter and the internal error amp is not used. The

internal error amplifier is configured as an open drain output and can be disabled by connecting the FB pin to

ground. An internal 5 k

Ω pull-up resistor between the 5V reference and the COMP pin can be used as the pull-up

for an opto-coupler in isolated applications.

Copyright © 2009, Texas Instruments Incorporated

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Product Folder Links: LM5041B