Datasheet

Functional description ST1S40
10/29 DocID17928 Rev 5
Equation 3
where:
Equation 4
S
n
represents the ON time slope of the sensed inductor current, S
e
the slope of the external
ramp (V
PP
peak-to-peak amplitude 1.25 V) that implements the slope compensation to
avoid sub-harmonic oscillations at duty cycle over 50%.
The sampling effect contribution F
H
(s) is:
Equation 5
where:
Equation 6
and
Equation 7
The resistor to adjust the output voltage gives the term from output voltage to the FB pin.
G
DIV
(s) is:
The transfer function from FB to Vcc (output of E/A) introduces the singularities (poles and
zeros) to stabilize the loop. Figure 5 shows the small signal model of the error amplifier with
the internal compensation network.

p
1
R
LOAD
C
OUT

------------------------------------- -
m
C
1D–0.5–
LC
OUT
f
SW

---------------------------------------------+=
m
C
1
S
e
S
n
------ +=
S
e
V
pp
f
SW
=
S
n
V
IN
V
OUT
–
L
----------------------------- - R
i
=








F
H
s
1
1
s

n
Q
P

-------------------
s
2

n
2
------++
------------------------------------------ -=
Q
P
1
 m
C
1D–0.5–
----------------------------------------------------------=

n
 f
SW
=
G
DIV
s
R
2
R
1
R
2
+
--------------------=