Datasheet
MAX8597/MAX8598/MAX8599
Low-Dropout, Wide-Input-Voltage,
Step-Down Controllers
16 ______________________________________________________________________________________
Setting the Output Voltage
Fixed Output Voltage
The output voltage is set by a resistor-divider network
from the output to GND with FB at the center tap (R4
and R5 in Figure 4). Select R4 between 5kΩ and 15kΩ
and calculate R5 by:
R5 = R4 x [( V
OUT
/ V
FB
) - 1]
Live Adjustable Output Voltage (see Figure 1)
Using the uncommitted operational amplifier, the
MAX8597 can be configured such that the output volt-
age is adjustable using a voltage source (V
ADJ
). The
following parameters must be defined before starting
the design:
• The minimum desired output voltage, V
OUT_MIN
• The maximum desired output voltage, V
OUT_MAX
• The desired input that corresponds to the minimum
output voltage, V
ADJ_MIN
• The desired input that corresponds to the maximum
output voltage, V
ADJ_MAX
Select V
AOUT
(uncommitted operational-amplifier out-
put) between 0.05V and 3V and V
AOUT_MAX
higher
than V
AOUT_MIN
. Calculate the required AIN+ reference
(V
AIN+
) as:
V
AIN+
is set using a resistor-divider from REFOUT to
GND (R6 and R7). Select R7 to be approximately 50kΩ
as a starting point and then calculate R6 as:
R6 = R7 x [(2.5V / V
AIN+
) - 1]
Select R4 to be 100kΩ and calculate R5 as:
Select R9 between 5kΩ and 15kΩ, then calculate R8
and R10 as follows:
where V
FB
is the feedback regulation voltage (0.6V with
REFIN connected to AVL).
Additionally, to minimize error, R6 and R7 should be
chosen such that:
Inductor Selection
There are several parameters that must be examined
when determining which inductor is to be used: input
voltage, output voltage, load current, switching fre-
quency, and LIR. LIR is the ratio of inductor current rip-
ple to DC load current. A higher LIR value allows for a
smaller inductor but results in higher losses and higher
output ripple. A good compromise between size and
efficiency is a 30% LIR. Once all the parameters are
chosen, the inductor value is determined as follows:
where f
S
is the switching frequency. Choose a standard
value close to the calculated value. The exact inductor
value is not critical and can be adjusted in order to
make trade-offs among size, cost, and efficiency.
Lower inductor values minimize size and cost, but also
increase the output ripple and reduce the efficiency
due to higher peak currents. On the other hand, higher
inductor values increase efficiency, but eventually
resistive losses due to extra turns of wire exceed the
benefit gained from lower AC current levels. Find a low-
loss inductor having the lowest possible DC resistance
that fits the allotted dimensions. Ferrite cores are often
the best choice, although powdered iron is inexpensive
and can work well up to 300kHz. The chosen inductor’s
saturation current rating must exceed the peak inductor
current determined as:
Input Capacitor
The input filter capacitor reduces peak currents drawn
from the power source and reduces noise and voltage
ripple on the input caused by the circuit’s switching.
The input capacitor must meet the ripple current
requirement (I
RMS
) imposed by the switching currents
defined by the following equation:
I
IVVV
V
RMS
LOAD OUT IN OUT
IN
=
××
()
−
II
LIR
I
PEAK LOAD MAX LOAD MAX
=+
⎛
⎝
⎜
⎞
⎠
⎟
×
() ()
2
L
VxVV
V x f x I x LIR
OUT IN OUT
IN S LOAD MAX
()
=
()
−
RR
RR
RR
RR
67
67
45
45
×
+
=
×
+
R
RR V V
VR VV R
OUT MAX FB
FB FB AOUT MIN
10
89
89
_
_
=
××
()
×
()
+
()
×
[]
−
−
R
VVVV V VV V
VVVVV
R
OUT MIN FB FB AOUT MIN OUT MAX FB AOUT MAX FB
OUT MAX FB OUT MIN FB FB
8 9
____
__
=
()
×
()
+
()
×
()
[ ]
()()
()
×
×
−− − −
−− −
R
VV R
VV
AIN AOUT MIN
ADJ MAX AIN
5
4
( )
( )
_
_
=
×
+
+
−
−
V
VVVV
VV V V
AIN
AOUT MAX ADJ MAX AOUT MIN ADJ MIN
ADJ MAX ADJ MIN AOUT MAX AOUT MIN
+
=
××
+
−
−−
()( )
__ __
__ _ _