20074904

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 pro-

vided within the LM5041. 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 con-

verter output is connected to the FB pin via the voltage

setting resistors. Loop compensation components are con-

nected 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 5k

Ω pull-up resistor between

the 5V reference and COMP can be used as the pull-up for

an opto-coupler in isolated applications.

Current Limit/Current Sense

The LM5041 contains two levels of over-current protection. If

the voltage at the CS pin exceeds 0.5V the present buck

stage duty cycle is terminated (cycle by cycle current limit). If

the voltage at the CS pin overshoots the 0.5V threshold and

exceeds 0.6V, then the controller will terminate the present

cycle and fully discharge the soft-start capacitor. A small RC

filter located near the controller is recommended to filter

current sense signals at the CS pin. An internal MOSFET

discharges the external CS pin for an additional 50ns at the

beginning of each cycle to reduce the leading edge spike

that occurs when the buck stage MOSFET is turned on.

The LM5041 current sense and PWM comparators are very

fast, and may respond to short duration noise pulses. Layout

considerations are critical for the current sense filter and

sense resistor. The capacitor associated with the CS filter

must be placed close to the device and connected directly to

the pins of the controller (CS and GND). If a current sense

transformer is used, both leads of the transformer secondary

should be routed to the sense resistor, which should also be

located close to the IC. A resistor may be used for current

sensing instead of a transformer, located in the push-pull

transistor sources, but a low inductance type of resistor is

required. When designing with a sense resistor, all of the

noise sensitive low power grounds should be connected

together around the IC and a single connection should be

made to the high current power ground (sense resistor

ground point).

The second level current sense threshold is intended to

protect the power converter by initiating a low duty cycle

hick-up mode when abnormally high currents are sensed. If

the second level threshold is reached, the soft-start capaci-

tor will be discharged and a start-up sequence will com-

mence when the soft-start capacitor is determined to be fully

discharged. The second level threshold will only be reached

when a high dV/dt is present at the current sense pin. The

current sense transient must be fast enough to reach the

second level threshold before the first threshold detector

turns off the buck stage driver. Very high current sense dV/dt

can occur with a saturated power inductor or shorted load.

Excessive filtering on the CS pin such as an extremely low

value current sense resistor or an inductor that does not

saturate with excessive loading, may prevent the second

level threshold from being reached. If the second level

threshold is never exceeded during an overload condition,

the first level current sense will continue cycle by cycle

limiting and the output characteristic of the converter will be

that of a current source. However, a sustained overload

current level can cause excessive temperatures in the power

train especially the output rectifiers.

Oscillator and Sync Capability

The LM5041 oscillator is set by a single external resistor

connected between the RT pin and GND. To set a desired

oscillator frequency (F), the necessary RT resistor can be

calculated from:

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