LM5041ASD datasheet(12/16 Pages) NSC

12 / 16 page

Slope Compensation

The PWM comparator compares the current sense signal to

the voltage at the COMP pin. The output stage of the internal

error amplifier generally drives the COMP pin. At duty cycles

greater than 50 percent, current mode control circuits are

subject to sub-harmonic oscillation. By adding an additional

fixed ramp signal (slope compensation) to the current sense

ramp, oscillations can be avoided. The LM5041A integrates

this slope compensation by buffering the internal oscillator

ramp and summing a current ramp generated by the oscil-

lator internally with the current sense signal. Additional slope

compensation may be provided by increasing the source

impedance of the current sense signal.

Soft-start and Shutdown

The soft-start feature allows the power converter to gradually

reach the initial steady state operating point, thereby reduc-

ing start-up stresses and surges. At power on, a 10uA cur-

rent is sourced out of the soft-start pin (SS) to charge an

external capacitor. The capacitor voltage will ramp up slowly

and will limit the maximum duty cycle of the buck stage. In

the event of a fault as indicated by V

CC Under-voltage, line

Under-voltage the output drivers are disabled and the soft-

start capacitor is discharged to 0.7V. When the fault condi-

tion is no longer present, a soft-start sequence will begin

again and buck stage duty cycle will gradually increase as

the soft-start capacitor is charged.

Thermal Protection

Internal Thermal Shutdown circuitry is provided to protect the

integrated circuit in the event that the maximum junction

temperature is exceeded. When activated, typically at 165

degrees Celsius, the controller is forced into a low-power

standby state, disabling the output drivers and the bias

regulator. This feature is provided to prevent catastrophic

failures from accidental device overheating.

LM5041ASD Datasheet(PDF) 12 Page - National Semiconductor (TI)