Datasheet

EVAL-ADP2118
Rev. 0 | Page 8 of 16
The feedforward capacitor can be used to improve dynamic
response with the following conditions: V
IN
= 5 V, V
OUT
= 3.3 V,
f
S
= 1.2 MHz, L = 1 µH, C
OUT
= 100 µF, R
TOP
= 10 kΩ, R
BOT
=
2.21 kΩ.
Figure 5 and Figure 6 show the bode plot and load dynamic
response without the C
FF
capacitor.
From Table 7, f
C
(cross frequency) = 45 kHz.
From Equation 8,
( )
( )
pF831
kΩ21.2kΩ10
kΩ21.2kΩ10
kHz452
1
2
=
×
+
×
×π×
=
FF
C
Therefore, C
FF
= 1000 pF.
Figure 7 and Figure 8 show the bode plot and load dynamic
response with C
FF
= 1000 pF.
The cross frequency with C
FF
added is improved to 171 kHz
with a phase margin of 67°.
Comparing Figure 6 and Figure 8, the load dynamic response is
significantly improved with the addition of the C
FF
capacitor.
60
–60
200
–200
–160
–120
–80
–40
0
40
80
120
160
–48
–36
–24
–12
0
12
24
36
48
100 1k 10k 100k 1M
MAGNITUDE (dB)
PHASE (Degrees)
FREQUENCY (Hz)
M1 M2
MAGNITUDE
PHASE
DATA M1 M2 M2 – M1
FREQUENCY 44.54kHz 446.87kHz 402.33kHz
MAGNITUDE –0.002dB –30.127dB –30.124dB
PHASE 73.821 Degrees –0.910 Degrees –74.731 Degrees
08742-005
Figure 5. Bode Plot Without C
FF
CH3 200mV CH4 2.00A
M200µs A CH4 2.16A
T –593.600µs
3
4
T
V
OUT
(AC)
I
O
08742-006
Figure 6. Load Dynamic Without C
FF
60
–60
200
–200
–160
–120
–80
–40
0
40
80
120
160
–48
–36
–24
–12
0
12
24
36
48
100 1k 10k 100k 1M
MAGNITUDE (dB)
PHASE (Degrees)
FREQUENCY (Hz)
M1 M2
MAGNITUDE
PHASE
DATA M1 M2 M2 – M1
FREQUENCY 171.03kHz 982.91kHz 811.88kHz
MAGNITUDE –0.003dB –23.383dB –23.380dB
PHASE 67.053 Degrees –0.403 Degrees –67.457 Degrees
08742-007
Figure 7. Bode Plot with C
FF
= 1000 pF
CH3 200mV CH4 2.00A
M200µs A CH4 2.16A
T –593.600µs
3
4
T
V
OUT
(AC)
I
O
08742-008
Figure 8. Load Dynamic with C
FF
= 1000 pF