Datasheet

DocID17928 Rev 5 9/29
ST1S40 Functional description
29
Figure 4 shows the simple small signal model for the peak current mode control loop.
Figure 4. Block diagram of the loop for the small signal analysis
Three main terms can be identified to obtain the loop transfer function:
1. from control (output of E/A) to output, G
CO
(s)
2. from output (Vout) to the FB pin, G
DIV
(s)
3. from the FB pin to control (output of E/A), G
EA
(s).
The transfer function from control to output G
CO
(s) results:
Equation 1
where R
LOAD
represents the load resistance, R
i
(0.3 ) the equivalent sensing resistor of
the current sense circuitry, 
p
the single pole introduced by the LC filter and 
z
the zero
given by the ESR of the output capacitor.
F
H
(s) accounts for the sampling effect performed by the PWM comparator on the output of
the error amplifier that introduces a double pole at one half of the switching frequency.
Equation 2
L
Cout
Curr ent sense
Logic
And
Driver
Slope
Compensati on
PW M compar ator
Error Amp
Rc
Cc
R1
R2
0.8 V
High side
Switch
Low si de
Switch
G
CO
(s)
G
DIV
(s)
G
EA
(s)
VIN
V
C
V
OUT
V
FB
G
CO
s
R
LOAD
R
i
------------------
1
1
R
out
T
SW

L
--------------------------- - m
C
1D–0.5–+
---------------------------------------------------------------------------------------------
1
s

z
------+


1
s

p
------+


---------------------- F
H
s=

Z
1
ESR C
OUT

-------------------------------=