Datasheet

100
1k 10k 100k 1M
FREQUENCY (Hz)
-150
-120
-90
-60
-30
0
PHASE (
o
)
100
1k 10k 100k 1M
FREQUENCY (Hz)
-60
-44
-28
-12
4
20
GAIN (dB)
V
IN
x R
O
G
PS
=
V
RAMP
sC
O
R
C
+ 1
as
2
+ bs + c
x
1
f
ESR
=
2SC
O
ESR
= 20.3 kHz
= 4.5 kHz
R
O
+ R
L
f
DP
=
1
2S
LC
O
(R
O
+ ESR)
V
IN
A
DC
=
V
RAMP
=
3.3
1.0
= 10.4 dB
LM2745, LM2748
www.ti.com
SNOSAL2E APRIL 2005REVISED APRIL 2013
The power stage modulator provides a DC gain A
DC
that is equal to the input voltage divided by the peak-to-peak
value of the PWM ramp. This ramp is 1.0V
pk-pk
for the LM2745/8. The inductor and output capacitor create a
double pole at frequency f
DP
, and the capacitor ESR and capacitance create a single zero at frequency f
ESR
. For
this example, with V
IN
= 3.3V, these quantities are:
In the equation for f
DP
, the variable R
L
is the power stage resistance, and represents the inductor DCR plus the
on resistance of the top power MOSFET. R
O
is the output voltage divided by output current. The power stage
transfer function G
PS
is given by the following equation, and Figure 30 shows Bode plots of the phase and gain in
this example.
a = LC
O
(R
O
+ R
C
)
b = L + C
O
(R
O
R
L
+ R
O
R
C
+ R
C
R
L
)
c = R
O
+ R
L
Figure 30. Power Stage Gain and Phase
Copyright © 2005–2013, Texas Instruments Incorporated Submit Documentation Feedback 21
Product Folder Links: LM2745 LM2748