For the stability of the voltage loop see the AN938.

The open loop transfer functions is composed of the following blocks :

- Error amplifier and compensation block :

⋅ (1 + s

s

2

in which Ro = 1.2M

Ω and Co = 220pF are internal capacitance and resistance of the Error Amplifier

- Output LC filter:

Gfil

1

RL

⋅

s

s

2

(ESR + RL) ⋅ L

RL

+ Rs

+ s ⋅







Cout

⋅ Rs ⋅ (RL + ESR) + L + RL ⋅ ESR ⋅ Cout

RL

+ Rs







+ 1

Rs is the sensing resistor, RL is the load resistance.

PWM gain:

Gpwm

= Vcc ⁄ Vct =

Vcc

⋅ 6

Vcc

≈ 6

where Vct is the peak to peak saw tooth oscillator.

Ro

gm

Cc

Rc

Co

D99IN1022

Figure 17. Error Amplifier Compensation Cir-

cuit

Rs

Cout

I +

Vout

ESR

L

RL

Figure 18. Output filter

LM358

5

6

+

7

-

Rint

Cint

FB

Rs

Cout

I +

Rs

I -

Cout

Vout

R

Figure 19. Current feedback.

AN1126 APPLICATION NOTE

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