AAT1415-Q8-T AAT [Advanced Analog Technology, Inc.], AAT1415-Q8-T Datasheet - Page 29

• Manufacturer Part Number: AAT1415-Q8-T
• Description: FIVE-CHANNEL DC-DC CONVERTER WITH A 2.5V LDO
• Manufacturer: AAT [Advanced Analog Technology, Inc.]

When the C
what’s calculated above, better output voltage ripple
can be achieved.
Buck Converter Compensation
The buck converter employs current-mode control,
thereby simplifying the control-loop compensation.
When the buck converter operates with continuous
inductor current (typically the case), a R
appears in the loop-gain frequency response. To
ensure stability, set the compensation
9a) to zero to compensate for the R
Then set the loop crossover frequency below 1/5 of the
switching frequency. The compensation resistor and
capacitor are then chosen to optimize control-loop
stability.
Choose the compensation capacitor
desired crossover frequency
by the following equation:
where
(typ),
0.5V/A (typ),
70µS
transient-droop (
C
R
C
C
C
C
C
OUT
U
C
=
=
=
=
R
0.64
2
0.25V / A
I
=
LOAD
CS
π ×
V
1.25V
(typ).
250mA
IN
3.3V
V
is the current-sense amplifier transresistance,
164k
×
_
IN
OUT
×
0.05 1.25 70µ A / V
is the feedback regulation voltage, 1.25V
G is the error amplifier transconductance,
_
R
m
Select
Ω ×
CS
value is two to three times greater than
×
×
×
TD
2
36k
70µ A / V
0.25V / A
×
35kHz (
π ⋅
%
2
35kHz
Ω ×
G
⋅ π
1
) requirements by the following
×
m
R
f
C
3.3nF 10µF
×
C
1.25V
×
based
(1 0.39)
f . Determine the value
C
250mA
≈
Advanced Analog Technology, Inc.
−
3.3V
≈
36k
on
)
LOAD
C
≈
R C
Ω
≈
LOAD
–
C
3.3nF
C C
the
100pF
,
C
to set the
–
Advanced Analog Technology, Inc . –
C
OUT
台 灣 類 比 科 技 股 份 有 限 公 司
OUT
allowed
(Figure
,
pole.
pole
Page 29 of 32
Version 3.00
equation:
Where
The output filter capacitor (typically ceramic capacitor)
is then chosen to cancel the
If the output filter capacitor (typically electrolytic
capacitor) has significant equivalent series resistance
(ESR), a zero occurs at the following:
If
If the system wants better transient response, it can
parallel a capacitor
If
be omitted.
So, for a 1.5V/250mA output with V
3%:
R
C
C
C
C
R
Z
f , it should be cancelled with a pole set by capacitor
V
C
OUT _
ESR
U
UPPER
C
OUT
P
P
Z
C
=
=
=
ESR
P
connected from EO_ to GND:
I
C
2
( L
=
=
or
π ×
:
OUT
PK
I
2
( L
= 20 kΩ ,
V
>>
I
–
LOAD
π
PK
OUT _
R
)
C
R
×
×
f
UPPER _
U
×
C
)
C
C
TD
R
is the inductor peak current.
, it can be ignored. If
OUT
is calculated to be less than 10pF, it can
ESR
×
%
1
R C
×
×
f
C C
SW
AAT1415/AAT1415A
R
×
V
R
CS
C
IN
f
ESR
C
1
U
= 500kHz and transient-droop
_
× 
×


with
G
V
IN _
m
R C
C C
R
V
UPPER _
OUT _
I
= 3.0V, L = 10µH,
zero:
Z
ESR



from IN_ to
is less than
May 2008