Slope-Comp =

50 x 10

-9

3 x 25:

CS

OUTB

OUTA

Vin

Power Transformer

R1

LM5037

Q1

Q2

LM5037

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SNVS578D – NOVEMBER 2008 – REVISED MAY 2015

Application Information (continued)

Figure 23. Current Sense Using Current Sense Resistor (R1)

Configuration and component selection for current mode control is recommended as follows: The current sense

resistor is selected such that during over current condition, the voltage across the current sense resistor is above

(6)

slope compensation required to avoid sub-harmonic oscillation is equal to at least one-half the down-slope of the

output inductor current, transformed to the primary. To mitigate sub-harmonic oscillation after one switching

period, the slope compensation has to be equal to one times the down slope of the filter inductor current

transformed to primary. This is known as deadbeat control. For circuits where primary current is sensed using a

resistor, the amount of slope compensation for dead-beat control required can be calculated from:

where

•

turns-ratio is referred with respect to the primary

(7)

For example, for a 5-V output converter with a turns ratio between secondary and primary of 1:2, an oscillator

compensation of 80 mV suffices. The slope compensation "volts" that results from the above expression is the

maximum voltage of the artificial ramp added linearly to the RAMP pin till the end of maximum switching period.

For circuits where a current sense transformer is used for primary current sensing, the turns-ratio of the current

sense transformer has to be taken into account.

values ranging from 100 pF to 1500 pF are recommended. The value must be small enough such that the

capacitor can be discharged within the clock (CLK) pulse width each cycle.

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