Advanced Analog Technology, Inc.
October 2007
AAT7301
Compensation
from EO to GND:
The AAT7301 employs current-mode control, thereby
simplifying the control-loop compensation. When the
AAT7301 operates with continuous inductor current
(typically the case), a RLOAD COUT pole appears in the
loop-gain frequency response. To ensure stability, set
the compensation RCCC to zero to compensate for the
RLOAD COUT pole. Set the loop crossover below the
lower of 1/10 the switching frequency (380kHz). The
compensation resistor and capacitor are then chosen
to optimize control-loop stability.
C
=
×
R
ESR
OUT
C
P
R
C
If CP is calculated to be < 10pF, it can be omitted.
LAYOUT CONSIDERATIONS
Conductors carrying discontinuous currents and any
high-current path should be made as short and wide as
possible. The compensation network should be very
close to the EO pin and avoid through VIA. The IC
must be bypassed with a 0.22µF ceramic capacitor
placed close to the VDD. Tie the feedback resistor
divider to be very close to output capacitor and far
away from the inductor or Schottky diode. Keep the
feedback network IN close to the IC. Switching nodes
(SW) should be kept as small as possible and should
be routed away from high-impedance nodes such as
IN.
Choose the compensation resistor RC to set the
desired crossover frequency fC. Determine the value by
the following equation:
2π×
C
×
V
×
R
×
f
C
OUT
OUT
CS
R
=
C
G
×
V
IN
m
Where COUT is the output capacitance, VOUT is the
output voltage, RCS is the current-sense amplifier
transresistance (0.2V/A),
G
is the error amplifier
m
transconductance (80 µS ) and VIN is the feedback
threshold voltage (0.9V).
If the value calculated for RC is greater than 100kΩ
recommend 100kΩ
,
.
Choose the compensation capacitor CC to set the zero
to 1/4 of the crossover frequency.
C
<
×
V
OUT
OUT
C
C
4×R
C
If the output filter capacitor COUT has significant ESR, a
zero occurs at the following:
1
Z
=
ESR
2
π×
C
×
R
ESR
OUT
If ZESR > fC / 4, it can be ignored, as is typically the case
with ceramic output capacitors. If ZESR < fC / 4, it should
be cancelled with a pole set by capacitor CP connected
–
–
–
Advanced Analog Technology, Inc. –
Version 1.00
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