AN901 Datasheet, PDF (13/25 Pages) STMicroelectronics – Electromagnetic compatibility

AN901
3.3. Proportional Mode
Once the secondary side senses 90% of V_OUT, the control loop begins its P-mode operation. During this mode of
operation, the dc-dc converter closes the loop (dc-dc converter secondary side communicates with the primary
side), and therefore, analyzing the loop stability is required.
Figure 7 shows a simplified block diagram of the dc-dc control feedback loop. gm_p represents the equivalent
modulator and power stage transconductance of the dc-dc converter, and resistors R5 and R6 are the feedback
resistors used to sense V_OUT. C10 is the output capacitor, and R_LOAD represents output load. Parameter gm_fb and
R_o,gmfb are the effective error amplifier transconductance and the error amplifier output resistance, respectively.
During the P-Mode, an integrated resistor R_INT is connected to the COMP pin.
Figure 7. Simplified Feedback Loop
For stability analysis, the loop at the input of the error amplifier is broken to obtain the small-signal transfer function
from V_fb,in to V_fb,out:
H_PS = -^V-_V---^f-_fb--_b-^--_^o--_i-^un---^t = A_{DC P}---₁-----+--¹---_{----S-----p}----_-
Equation 33.
_p  _R-----_l-_o---_a--¹-_d---_C-----₁--₀-
Equation 34.
A_{DC P} = –_R-----₅---^R--+--⁶---_R-----₆- gm_fbR_INT  R_{O gmfb}  gm_pR_LOAD  R5 + R6
Equation 35.
Rev. 0.1
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