AN938 Datasheet, PDF (22/33 Pages) STMicroelectronics – The L4973 family is a 3.5 A monolithic DC-DC converter

Typical application
Figure 30. Block diagram compensation loop
V_REF
A(s)
Vc/Vct
LC
+-
AN938
Figure 31. LC output filter
α
L_O
Cout
Resr
Figure 32. Error amplifier equivalent circuit
Cc
gm
Ro
Co
Rc
AM03512v1
AM03513v1
3.7
3.8
AM03514v1
Error amplifier and compensation blocks
The open loop gain is:
Equation 13
A(s)
=
g
m
⋅
------------------------------------------------^R-----^O-----^â‹---⁽---¹----+-----^s-----^â‹---^R----^c----^â‹---^C-----^c---⁾------------------------------------------------
s² ⋅ R_o ⋅ C_o ⋅ R_c ⋅ C_c + s ⋅ (R_o ⋅ C_c + R_o ⋅ C_o + R_c ⋅ C_c) + 1
where Cout is the parallel between the output and the external capacitance of the error
amplifier and R_o is the E/A output impedance. R_c and C_c are the compensation values.
LC filter
Equation 14
A_o(s)
=
-----------¹----+-----^R-----^e---^s--^r----^â‹---^C----^o---^u---^t---^â‹---^s-----------
L ⋅ C_out ⋅ s² + R_esr ⋅ s + 1
22/33
Doc ID 5410 Rev 10