LM3101 datasheet(10/20 Pages) NSC

10 / 20 page

Functional Description (Continued)

To operate under current mode or charge mode control

insert a resistor RMC between the MC pin and ground and

connect the CMI pin to the Current Mode Sense Circuit (see

Figure 4 ) At the MC pin a voltage proportional to the oscil-

lator ramp voltage develops (see

Figure 3 ) The voltage

ramp applied to RMC generates a current ramp which is

duplicated on the CMI pin due to the current mirror The

current ramp which flows out of the CMI pin to the resistor

RF is the compensation ramp needed to stabilize convert-

ers operating at duty cycles above 50% The slope of the

compensating ramp can be scaled by RF which is connect-

ed between the CMI pin and the terminating resistor RS of

the current sense transformer In all practical cases RF will

be much greater than RS For both control modes the cur-

rent ramp provides slope compensation according to the

equation

ICOMP Dt e 24 103 (RT RMC)

A ms X

So with RT equal 25 kX and RMC equal 6 kX the compen-

sation ramp slope is

ICOMP Dt e 24 103 (25 kX 6kX) e 160 mA ms

The resistor RF together with the capacitor CF serves as an

RC filter for the leading edge spike of the current sense

waveform (the spike is caused by the output rectifier reverse

recovery andor the winding capacitance of the power

transformer)

CHARGE MODE CONTROL

Under charge mode control the current sense transformer

drives a capacitor C1 that integrates the sensed switch

current on a cycle-by-cycle basis

Figure 5 shows the inte-

grating current sense circuitry and the simplified details of

the associated internal circuitry of the LM3101 Transistor

Q1 discharges the integrating capacitor C1 once every

switch cycleduring the switch off-time (Q1 can provide up

to 20 mA of discharge current) Charge mode control yields

the fastest average current control loop

TLH11436 – 10

TLH11436 – 11

FIGURE 5 Charge Mode Operation

LM3101 Datasheet(PDF) 10 Page - National Semiconductor (TI)

LM3101 Datasheet(HTML) 10 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL