MAX16809EVKIT+ Maxim Integrated Products, MAX16809EVKIT+ Datasheet - Page 9

• Manufacturer Part Number: MAX16809EVKIT+
• Description: Power Management Modules & Development Tools EVAL KIT FOR MAX16809
• Manufacturer: Maxim Integrated Products

Specifications of MAX16809EVKIT+

• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The DC gain of the power circuit is expressed as the
change in the output voltage, with respect to the
change in error-amplifier output voltage. As the boost
converter in the MAX16809 EV kit drives a constant-
current load, the power-circuit DC gain is calculated
based on a constant-current load:
Calculate the power-circuit DC gain using the following
equation:
where R
switching frequency, and the factor 3 is to account for
the attenuation of error-amp output before it is fed to
the current-sense comparator.
The power-circuit gain is lowest at the minimum input
supply voltage and highest at the maximum input sup-
ply voltage. Any input supply voltage between 9V and
16V can be used for power-circuit gain calculation, as
the final compensation values obtained are the same.
Calculate the frequency F
gain starts falling, at -20dB/decade using the following
equation:
where C
parallel combination of C16, C17, C18, and C24. Adjust
the output capacitance so that the product of F
G
obtained this way will be much greater than the value
obtained using the maximum output voltage ripple
specification.
The compensation strategy is as follows. The gain-fre-
quency response of the feedback loop should cross 0dB
at or below half of the RHP zero frequency, with a slope of
-20dB/decade for the feedback to be stable and have
sufficient phase margin. The compensation network from
COMP pin to FB pin of the MAX16809 (formed by R5,
C28, C29, and R11) offers one dominant pole (P1), a zero
(Z1), and a high-frequency pole (P3). There are two very
low frequency poles and a zero in the loop before the
crossover frequency. The function of the zero (Z1) is to
compensate for the output pole and to reduce the slope
of the loop gain from -40dB/decade to -20dB/decade,
and also to reduce the phase lag by 90 degrees.
P
is below F
G
P
OUT
CS
=
F
P
⎛
⎜
⎝
is the current-sense resistor, F
2
is the output filter capacitor, which is the
2
ZRHP
× ×
=
L F
2
_______________________________________________________________________________________
π
/ 6. The value of output capacitance
G
SW
×
V
P
IN
C
=
2
OUT
×
(
Δ
1
VLED
P2
Δ
EA
−
VLED
, at which the power-circuit
D
× ×
OUT
1
MAX
3
2
R
+
)
CS
V
I
O
IN
×
⎞
⎟ ×
⎠
G
P
R
CS
SW
×
3
P2
is the
and
MAX16809 Evaluation Kit
Choose the crossover frequency to be half of the worst-
case RHP zero frequency:
Place the zero (Z1) at one-third of the crossover fre-
quency, so that the phase margin starts improving from
a sufficiently lower frequency:
Use the following equation to calculate the dominant
pole location, so that the loop gain crosses 0dB at F
Since the open-loop gain of the error amplifier can have
variations, the dominant pole location can also vary
from device to device. In the MAX16809 EV kit, the
dominant pole location is decided by the error-amplifier
gain, so the combined effect is a constant-gain-band-
width product.
Select the value of R11 such that the input bias current
of the error amplifier does not cause considerable drop
across it. The effective AC impedance seen from the
FB pin is the sum of R11 and R7. It is preferable to
keep R7 much lower, compared to R11, to have better
control on the AC impedance. Find C29 using the fol-
lowing equation:
The location of the zero (Z1) decided by R5 and C29 is
given by the following equation:
Place the high-frequency pole (P
C29, and R5, at half the switching frequency to provide
further attenuation to any high-frequency signal propa-
gating through the system. The location of the high-fre-
quency pole (F
and should be used to calculate the value of C28:
C
F
29
P3
=
P3
=
F
F
P
) is given by the following equation,
2
Z1
1
2π
π
=
F
=
×
F
×
C
Z
G
2
R
1
2π
=
EA
F
5
×
=
ZRHP
G
×
F
×
×
⎛
⎜
⎝
ZRHP
TOT
F
R
3
(
C
C
2
R
5
1
1
1
1
28
11
×
×
×
F
C
+
+
Z
F
29
P
R
1
3
C
2
7
), formed by C28,
1
29
)
×
F
⎞
⎟
⎠
P
−
1
1
C
:
9