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
=
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
(1 + )
Z
Z_ESR
=
s
V
OUT
^
V
COMP
^
A
M
x
Z
P_LF
1 +
s
LM5117, LM5117-Q1
www.ti.com SNVS698E –APRIL 2011–REVISED MARCH 2013
The highest power dissipating components are the two power switches. Selecting NMOS switches with exposed
pads aids the power dissipation of these devices.
Table 1. LM5117 Frequency Analysis Formulas
SIMPLE FORMULA COMPREHENSIVE FORMULA
(1)
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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