Datasheet

12Maxim Integrated
MAX16909
36V, 220kHz to 1MHz Step-Down Converter
with Low Operating Current
Skip Mode/Standby Mode
During light-load operation the device enters skip mode
operation. Skip mode turns off the majority of circuitry
and allows the output to drop below regulation voltage
before the switch is turned on again. The lower the load
current, the longer it takes for the regulator to initiate
a new cycle. Because the converter skips unneces-
sary cycles and turns off the majority of circuitry, the
converter efficiency increases. When the high-side FET
stops switching for more than 50Fs, most of the internal
circuitry, including LDO, draws power from V
OUT
(V
OUT
= 3V to 5.5V), allowing current consumption from the bat-
tery to drop to only 30FA.
Overtemperature Protection
Thermal-overload protection limits the total power dis-
sipation in the device. When the junction temperature
exceeds +175NC (typ), an internal thermal sensor
shuts down the internal bias regulator and the step-
down converter, allowing the IC to cool. The thermal
sensor turns on the IC again after the junction tempera-
ture cools by 15NC.
Applications Information
Setting the Output Voltage
Connect FB to BIAS for a fixed 5V output voltage. To set
the output to other voltages between 1V and 10V, con-
nect a resistive divider from output (OUT) to FB to GND
(Figure 1). Calculate R
FB1
(OUT to FB resistor) with the
following equation:
OUT
FB1 FB2
FB
V
RR 1
V


= −





where V
FB
= 1V (see the Electrical Characteristics table).
Internal Oscillator
The switching frequency, f
SW
, is set by a resistor (R
FOSC
)
connected from FOSC to GND. See Figure 2 to select the
correct R
FOSC
value for the desired switching frequency.
For example, a 400kHz switching frequency is set with
R
FOSC
= 65kI. 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 device: 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 frequency, input voltage,
output voltage, and selected LIR then determine the
inductor value as follows:
OUT SUP OUT
SUP SW OUT
V(V V)
L
V f I LIR
−
=
where V
SUP
, V
OUT
, and I
OUT
are typical values (so that
efficiency is optimum for typical conditions). The switch-
ing frequency is set by R
FOSC
(see the Internal Oscillator
section). The exact inductor value is not critical and can
be adjusted to make trade-offs among size, cost, efficien-
cy, and transient response requirements. Table 1 shows
a comparison between small and large inductor sizes.
Figure 1. Adjustable Output-Voltage Setting
Figure 2. Switching Frequency vs. R
FOSC
R
FB2
R
FB1
FB
MAX16909
V
OUT
SWITCHING FREQUENCY vs. R
FOSC
MAX16909 toc05
R
FOSC
(kI)
SWITCHING FREQUENCY (MHz)
1009070 8040 50 6030
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
0
20 110
V
IN
= 14V