MAX8646
The zero-cross frequency of the close-loop, f
C
should
be between 10% and 20% of the switching frequency,
f
S
. A higher zero-cross frequency results in faster tran-
sient response. Once f
C
is chosen, C1 is calculated
from the following equation:
Due to the underdamped nature of the output LC dou-
ble pole, set the two zero frequencies of the type III
compensation less than the LC double-pole frequency
to provide adequate phase boost. Set the two zero fre-
quencies to 80% of the LC double-pole frequency.
Hence:
Setting the second compensation pole, f
P2_EA
, at
f
Z_ESR
yields:
Set the third compensation pole at 1/2 of the switching
frequency. Calculate C2 as follows:
The above equations provide application compensation
when the zero-cross frequency is significantly higher
than the double-pole frequency. When the zero-cross
frequency is near the double-pole frequency, the actual
zero-cross frequency is higher than the calculated fre-
quency. In this case, lowering the value of R1 reduces
the zero-cross frequency. Also, set the third pole of the
type III compensation close to the switching frequency
if the zero-cross frequency is above 200kHz to boost
the phase margin. The recommended range for R3 is
2kΩ to 10kΩ. Note that the loop compensation remains
unchanged if only R4’s resistance is altered to set dif-
ferent outputs.
Soft-Starting into a Prebiased Output
When the PREBIAS pin is left unconnected, the
MAX8646 is capable of soft-starting up into a prebi-
ased output without discharging the output capacitor.
This type of operation is also termed monotonic start-
up. However, in order to avoid output voltage glitches
during soft-start it should be ensured that the inductor
current is in continuous conduction mode during the
end of the soft-start period. this is done by satisfying
the following equation:
where C
O
is the output capacitor, V
O
is the output volt-
age, t
SS
is the soft-start time set by the soft-start capac-
itor C
SS
, and I
P-P
is the peak inductor ripple current (as
defined in the
Output-Capacitor Selection
section).
Depending on the application, one of these parameters
may drive the selection of the others. See
Starting into
.
.
(.)
