Datasheet

1
Z
P_EA
=
R
COMP
x C
HF
1
Z
P_EA
=
R
COMP
x (C
HF
// C
COMP
)
Z
Z_EA
=
R
COMP
x C
COMP
1
Z
Z_EA
=
R
COMP
x C
COMP
1
A
FB_MID
=
R
COMP
R
FB2
A
FB_MID
=
R
COMP
R
FB2
A
FB
=
1
R
FB2
x (C
COMP
+ C
HF
)
A
FB
=
1
R
FB2
x (C
COMP
+ C
HF
)
=
s
A
FB
x
Z
P_EA
V
OUT
^
V
COMP
^
-
Z
Z_EA
1 +
s
s x (1+ )
=
s
A
FB
x
Z
P_EA
V
OUT
^
V
COMP
^
-
Z
Z_EA
1 +
s
s x (1+ )
K = 1
K =
L
O
R
RAMP
x C
RAMP
x R
S
x A
S
2
f
SW
Z
n
=
Z
SW
2
=Sx f
SW
or f
n
=
Q =
1
S(K - 0.5)
Z
P_HF
Q x Z
n
=
Z
P_HF
f
SW
K  0.5
=
or
Z
P_LF
=
R
LOAD
x C
OUT
1
Z
P_LF
=
(R
LOAD
+ R
ESR1
) x (C
OUT1
+ C
OUT2
)
L
O
x (C
OUT1
+ C
OUT2
) x Z
P_HF
1 1
+
R
ESR1
1
Z
P_ESR
=
x (C
OUT1
// C
OUT2
)
Z
Z_ESR
=
R
ESR
1
x C
OUT
Z
Z_ESR
=
R
ESR1
1
x C
OUT1
A
M
=
R
LOAD
R
S
x A
S
x L
O
Z
P_HF
x
1
1 +
R
LOAD
A
M
=
R
LOAD
R
S
x A
S
(1 + )
Z
Z_ESR
=
s
V
OUT
^
V
COMP
^
A
M
x
Z
P_LF
1 +
s
=
V
OUT
^
V
COMP
^
A
M
x
Z
Z_ESR
1 +
s
s
(1 + )
Z
P_LF
x
(1 + )
Z
P_ESR
x
(1 + + )
s
s
Z
P_HF
s
2
Z
n
2
LM25117
LM25117-Q1
www.ti.com SNVS714E –APRIL 2011–REVISED MARCH 2013
Table 1. LM25117 Frequency Analysis Formulas
(1)
SIMPLE FORMULA COMPREHENSIVE FORMULA*
Modulator
Transfer
Function
Modulator DC
Gain
ESR Zero
ESR Pole Not considered
Dominant Load
Pole
Sampled Gain
Not considered
Inductor Pole
Quality Factor Not considered
Sub-harmonic
Not considered
Double Pole
K Factor
Feedback
Transfer
Function
Feedback DC
Gain
Mid-band Gain
Low Frequency
Zero
High Frequency
Pole
(1) Comprehensive Equation includes an inductor pole and a gain peaking at f
SW
/2, which caused by sampling effect of the current mode
control. Also it assumes that a ceramic capacitor C
OUT2
(No ESR) is connected in parallel with C
OUT1
. R
ESR1
represents ESR of C
OUT1
.
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