LM5041ASD datasheet(11/16 Pages) NSC

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Push-Pull Outputs (Continued)

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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 LM5041A. The amplifier’s non-inverting in-

put is tied to a 0.75V reference. The inverting input is con-

nected 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 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 LM5041A provides cycle-by-cycle over-current protec-

tion. If the voltage at the CS comparator (CS pin voltage plus

slope comp voltage) exceeds 0.5V the present buck stage

duty cycle is terminated (cycle by cycle current limit). 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 LM5041A 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 trans-

former 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).

Oscillator and Sync Capability

The LM5041A 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:

The buck stage will switch at the oscillator frequency and

each push-pull output will switch at half the oscillator fre-

quency in a push-pull configuration. The LM5041A can also

be synchronized to an external clock. The external clock

must have a higher frequency than the free running fre-

quency set by the RT resistor. The clock signal should be

capacitively coupled into the RT pin with a 100pF capacitor.

A peak voltage level greater than 3V is required for detection

of the sync pulse. The sync pulse width should be set in the

15 to 150ns range by the external components. The RT

resistor is always required, whether the oscillator is free

running or externally synchronized. The voltage at the RT pin

is internally regulated to 2V. The RT resistor should be

located very close to the device and connected directly to the

pins of the IC (RT and GND).

LM5041ASD Datasheet(PDF) 11 Page - National Semiconductor (TI)

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