Data Sheet G14309EJ2V0DS00

24

µµµµPC1909

4.4.2 Dead time adjustment

The dead time between the fall of OUT1 and the rise of OUT2 (tqc) and the dead time between the fall of OUT2 and the

rise of OUT1 (tqd) is determined by the oscillation frequency and the amount of level shift of the triangle wave.

Although usually tqc = tqd, if these values differ, connect a suitable resistor between the CT pin and the VREF pin, as well

as between the CT pin and GND, and adjust the dead time by making the oscillation waveform asymmetrical, as shown

in Figure 4-8.

Figure 4-8. Dead Time Adjustment

VREF

R2

RT

RT

CT

CT

GND

16

15

3

14

VREF

R1

RT

RT

CT

CT

GND

16

15

3

14

FB

(a) tqc < tqd

(b) tqc > tqd

OUT1

OUT2

tqc

tqd

CT

dCT

FB

OUT1

OUT2

tqc

tqd

CT

dCT

The charge current (ICT) of the timing capacitor (CT) is expressed as follows.

ICT [A] =

800

+ RT [Ω]

4.2

If RT is taken as 20 k

Ω, ICT will be approximately 200

µA.

To reduce tqc, connect a resistor (R1) between the VREF and CT pins. If the value of the resistor is set so that the current

charged in CT is about 10% more than ICT, tqc can be reduced and tqd increased by about 10% compared to when R1 is

not connected.

R1 here can be calculated from the following equation.

R1 [

Ω] =

ICT [A]

VREF [V]

− VOSC [V]

∆

Remark VOSC : Central voltage value of triangle wave

∆ICT : Value of ICT current increased (decreased) by R1 (R2)

Note also that connecting a resistor (R2) between CT and GND makes it possible to reduce the current charged to CT.

If a resistor is selected that allows about 10% current to flow, tqc can be increased and tqd reduced by about 10%

compared to when R2 is not connected.

R2 here can be calculated from the following equation.

R2 [

Ω] =

ICT [A]

VOSC [V]

∆