Datasheet

MAX16977
36V, 2A, 2.2MHz Step-Down Converter
with Low Operating Current
12Maxim Integrated
Skip Mode/Standby Mode
During light-load operation, I
INDUCTOR
P 185mA, 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 con-
verter skips unnecessary 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
(for V
OUT
= 3V to 5.5V), allowing current consump-
tion from the battery to drop to only 30FA.
Spread Spectrum
The IC has an internal speread-spectrum option to
optimize EMI performance. This is factory set and the
S-version of the IC should be ordered. For spread-spec-
trum-enabled ICs, the operating frequency is varied ±6%
up from the base 2.2MHz frequency. The modulation sig-
nal is a triangular wave with a period of 400μs. Therefore,
fOSC ramps up 6% in 200μs and then ramps down 6%
and back to 2.2MHz in 200μs. The cycle repeats. The
400μs modulation period is fixed for other fOSC frequen-
cy. The internal spread spectrum is disabled if the IC is
synced to an external clock. However, the IC accepts an
external spread-spectrum clock.
Overtemperature Protection
Thermal-overload protection limits the total power dissipa-
tion 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, allow-
ing the IC to cool. The thermal sensor turns on the IC again
after the junction temperature 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 2.2MHz switching frequency is set with
R
FOSC
= 12kI. 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
Figure 1. Adjustable Output-Voltage Setting
Figure 2. Switching Frequency vs. R
FOSC
R
FB2
R
FB1
FB
MAX16977
V
OUT
SWITCHING FREQUENCY vs. R
FOSC
MAX16977 toc06
R
FOSC
(kI)
SWITCHING FREQUENCY (MHz)
211815
0.5
1.0
1.5
2.0
2.5
3.0
0
12 24
V
IN
= 14V
I
LOAD
= 1.5A