Datasheet

K1 * R
SET
PUSH
DEADTIME
WAVEFORMS
PULL
PUSH
PULL
K1 * R
SET
K2 * R
SET
K2 * R
SET
OVERLAP
WAVEFORMS
LM5041A
www.ti.com SNVS350B –MARCH 2005–REVISED MARCH 2013
Push-Pull Outputs
The push pull outputs operate continuously at a nominal 50% duty cycle. A distinguishing feature of the
LM5041A 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 LM5041A The magnitude of the overlap/dead time can be calculated as
follows:
Overlap Time (ns) = (3.66 x R
SET
) + 7
Overlap Time in ns, R
SET
connected to GND, R
SET
in kΩ
Dead Time (ns) = (3.69 x R
SET
) + 21
Dead Time in ns, R
SET
connected to REF, R
SET
in kΩ
Recommended R
SET
programming range: 10kΩ to 100kΩ
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.
Figure 13.
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 LM5041A. 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 5kΩ pull-up resistor between the 5V reference and COMP can be used as the pull-up for an
opto-coupler in isolated applications.
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