RT8805C datasheet(13/18 Pages) RICHTEK

13 / 18 page

RT8805C

Preliminary

DS8805C-03 August 2007

www.richtek.com

Figure 8. Type 2 Bode Plot

Frequency

10Hz

100Hz

1.0KHz

10KHz

100KHz

1.0MHz

vdb(vo) vdb(comp2) vdb(lo)

-40

-60

100

10k

100k

-20

-40

-60

Loop Gain

Compensation

Gain

Modulator

Gain

Frequency (Hz)

There is another type of compensation called Type 3

compensation that adds a pole-zero pair to the Type 2

network. It's used to compensate output capacitor whose

ESR value is much lower (pure MLCC or OSCON

Capacitors).

As shown in Figure 9, to insert a network between VOUT

and FB in the original Type 2 compensation network can

result in Type 3 compensation. Figure 10 shows the

difference of their AC response. Type 3 compensation has

an additional pole-zero pair that causes a gain boost at

the flat gain region. But the gain boosted is limited by the

ratio (R1+R4)/R4; if R3 << R4.

COMP

OUT

Figure 9. Additional Network of Type 3 Compensation

(Add between VOUT and FB)

Figure 10. AC Response Curves of Type 2 and 3

Type 3 will induce additional one pole and one zero.

Zeros :

×+

(R1 R3) C3

2R3 C3

××

Poles :

which is in the origin.

We recommend FZ1 placed in 0.5 x FP(LC); FZ2 placed in

FP(LC); FP3 placed in FESR and FP2 placed in 0.5 x FSW.

Figure 11 shows Type 3 Bode Plot.

Figure 11. Type 3 Bode Plot

Protection

OCP

The RT8805C uses

“ Cycle by Cycle” current comparison.

The over current level is set by IMAX pin. When OC

function occurs and SS > 3.7V, a constant current of 10

μA

starts to discharge the capacitor connected to SS pin

right away. When OC occurs, UGATE and LGATE will be

off.

Loop Gain

-80

-60

-40

-20

Log Frequency

Compensation Gain

Gain

Modulator Gain

Pole

FZ2

Add Type 3 compensation

Original Type 2 compensation

FP3

FP2

FZ1

FP1

FP2

RT8805C Datasheet(PDF) 13 Page - Richtek Technology Corporation

RT8805C Datasheet(HTML) 13 Page - Richtek Technology Corporation

Html Pages

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

Datasheet Download

Link URL