Datasheet

MAX16955
36V, 1MHz Step-Down Controller
with Low Operating Current
16
Maxim Integrated
Setting the Output Voltage
Connect FB to BIAS to enable the fixed step-down con-
troller output voltage (5V), set by a preset, internal
resistive voltage-divider connected between the output
(OUT) and SGND.
To achieve other output voltages between 1V to 10V,
connect a resistive divider from OUT to FB to SGND
(Figure 2). Select R
FB2
(FB to SGND resistor) less than
or equal to 100kΩ. Calculate R
FB1
(OUT to FB resistor)
with the following equation:
where V
FB
= 1V (typ) (see the Electrical Characteristics
table) and V
OUT
can range from 1V to 10V.
Setting the Switching Frequency
The switching frequency, f
SW
, is set by a resistor
(R
FOSC
) connected from FOSC to SGND. See Figure 3
to select the correct R
FOSC
value for the desired
switching frequency.
For example, a 400kHz switching frequency is set with
R
FOSC
= 76.8kΩ. Higher frequencies allow designs with
lower inductor values and less output capacitance.
Consequently, peak currents and I
2
R losses are lower
at higher switching frequencies, but core losses, gate-
charge currents, and switching losses increase.
Inductor Selection
Three key inductor parameters must be specified for
operation with the MAX16955: inductance value (L),
inductor saturation current (I
SAT
), and DC resistance
(R
DCR
). To select inductance value, the ratio of inductor
peak-to-peak AC current to DC average current (LIR)
must be selected first. A good compromise between
size and loss is a 30% peak-to-peak ripple current to
average-current ratio (LIR = 0.3). The switching fre-
quency, input voltage, output voltage, and selected LIR
then determine the inductor value as follows:
where V
SUP(MIN)
is the minimum supply voltage, V
OUT
is
the typical output voltage, and I
OUT(MAX)
is the maximum
load current. The switching frequency is set by R
FOSC
(see the Setting the Switching Frequency section).
The MAX16955 uses internal frequency independent
slope compensation to ensure stable operation at duty
cycles above 50%. The maximum slope compensation
ramp voltage over a full clock period is 200mV. Use the
equation below to select the inductor value:
However, if it is necessary, higher inductor values can
be selected.
VV
LH f MHz
OUT
SW
[]
[] [ ]
%
μ×
=±125
L
VV V
VfI
OUT SUP MIN OUT
SUP MIN SW OUT MA
=
−
()
××
()
() (
XX
LIR
)
×
RR
V
V
FB FB
OUT
FB
12
1=
⎛
⎝
⎜
⎞
⎠
⎟
−
⎡
⎣
⎢
⎢
⎤
⎦
⎥
⎥
Figure 2. Adjustable Output Voltage
MAX16955
R
FB2
R
FB1
OUT
FB
Figure 3. Switching Frequency vs. R
FOSC
SWITCHING FREQUENCY vs. R
FOSC
MAX16955 toc07
R
FOSC
(kI)
FREQUENCY (kHz)
135125105 11545 55 65 75 85 9535
250
300
350
400
450
500
550
600
650
700
750
800
850
900
950
1000
1050
1100
1150
1200
200
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