Datasheet
the MOSFET, it is the inductance from the drain to the
source lead.
Alternately, to save board space and cost, the RC net-
works above can be omitted; however, the value of
R
ILIM
should be raised to account for the voltage step
caused by the inductive divider.
An additional switching noise filter may be needed at
ILIM by connecting a capacitor in parallel with R2 (in
the case of R
DS(ON)
sensing) or from ILIM to LX (in the
case of resistor sensing). For the case of R
DS(ON)
sens-
ing, the value of the capacitor should be:
C3 > 15 / (π x f
S
x R2)
For the case of resistor sensing:
C3 < 25 x 10
-9
/ R2
Selecting the Soft-Start Capacitor
An external capacitor from SS to GND is charged by an
internal 5µA current source, to the corresponding feed-
back threshold. Therefore, the soft-start time is calculat-
ed as:
t
SS
= C
SS
x V
FB
/ 5µA
For example, 0.033µF from SS to GND yields approxi-
mately a 3.96ms soft-start period.
In the tracking application (see Figure 3), the output
voltage is required to track REFIN during REFIN rise
and fall time. C
SS
must be chosen so that t
ss
is less
than REFIN rise and fall time.
Compensation Design
The MAX8597/MAX8598/MAX8599 use a voltage-mode
control scheme that regulates the output voltage by
comparing the error-amplifier output (COMP) with a
fixed internal ramp to produce the required duty cycle.
The error amplifier is an operational amplifier with
25MHz bandwidth to provide fast response. The output
lowpass LC filter creates a double pole at the resonant
frequency that introduces a gain drop of 40dB per
decade and a phase shift of 180 degrees per decade.
The error amplifier must compensate for this gain drop
and phase shift to achieve a stable high-bandwidth
closed-loop system. The Type III compensation
scheme (Figure 6) is used to achieve this stability.
The basic regulator loop can be thought of as consist-
ing of a power modulator and an error amplifier. The
power modulator has a DC gain set by V
IN
/ V
RAMP
,
with a double pole, f
P_LC
, and a single zero, f
Z_ESR
, set
by the output inductor (L), the output capacitor (C
O
),
and its equivalent series resistance (R
ESR
). Below are
the equations that define the power modulator:
where C
O
is the total output capacitance and R
ESR
is
the total ESR of the output capacitors.
G
V
V
where V V typ
f
LC
f
RC
MOD DC
IN
RAMP
RAMP
PLC
O
Z ESR
ESR O
()
_
_
, ( )
==
=
×
=
××
1
1
2
1
2
π
π
MAX8597/MAX8598/MAX8599
Low-Dropout, Wide-Input-Voltage,
Step-Down Controllers
______________________________________________________________________________________ 19
R2
C3
R
C
R
DS(ON)
DL
LX
DH
ILIM
R2
R
C
R
SENSE
DL
LX
DH
ILIM
C3
Figure 5. Adding RC for More Accurate Sensing