Datasheet

DocID023654 Rev 2 9/27
ST1S41 Functional description
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 FB pin, G
DIV
(s);
3. from 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
the equivalent sensing resistor of the
current sense circuitry, ω
p
the single pole introduced by the LC filter and ω
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
Current sense
Logic
and
Driver
Slope
Compensation
PW M com parator
Error Amp
Rc
Cc
R1
R2
0.8 V
High side
Switch
Low side
Switch
G
CO
(s)
G
DIV
(s )
G
EA
(s)
VIN
V
C
V
OUT
V
FB
AM15061v1
G
CO
s()
R
LOAD
R
i
------------------
1
1
R
out
T
SW
⋅
L
------------------------------
m
C
1D–()0.5–⋅[]⋅+
-----------------------------------------------------------------------------------------------------
1
s
ω
z
-------+
⎝⎠
⎛⎞
1
s
ω
p
-------+
⎝⎠
⎛⎞
----------------------
F
H
s()⋅⋅⋅=
ω
Z
1
ESR C
OUT
⋅
----------------------------------=