MAX618EEE View Datasheet(PDF) - Maxim Integrated
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MAX618EEE Datasheet PDF : 15 Pages
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MAX618
28V Internal Switch, Step-Up DC-DC Converter
Shutdown Mode
In shutdown mode (SHDN = 0), the MAX618’s feedback
and control circuit, reference, and internal biasing circuitry
turn off and reduce the IN supply current to 3μA (10μA
max). When in shutdown, a current path remains from
the input to the output through the external inductor and
diode. Consequently, the output falls to VIN less one
diode drop in shutdown.
SHDN may not exceed VL. For always-on operation,
connect SHDN to VL. To add on/off control to the circuit
of Figure 1 or 4, pull SHDN to VL with a resistor (10kΩ
to 100kΩ) and drive SHDN with an open-drain logic gate
or switch as shown in Figure 5. Alternatively, the circuit of
Figure 3 allows direct SHDN drive by any logic-level gate
powered from the same supply that powers VL and IN, as
shown in Figure 6.
Design Procedure
The MAX618 operates in a number of DC-DC converter
configurations including step-up, SEPIC, and flyback. The
following design discussion is limited to step-up converters.
Setting the Output Voltage
Two external resistors (R1 and R2) set the output voltage.
First, select a value for R2 between 10kΩ and 200kΩ.
Calculate R1 with:
= R1
R
2
VOUT
VFB
− 1
where VFB is 1.5V.
Determining the Inductor Value
The MAX618’s high switching frequency allows the use of
a small value inductor. The recommended inductor value
is proportional to the output voltage and is given by the
following:
L
=
VOUT
7 ×105
After solving for the above equation, round down as
necessary to select a standard inductor value.
When selecting an inductor, choose one rated to 250kHz,
with a saturation current exceeding the peak inductor
current, and with a DC resistance under 200mΩ. Ferrite
core or equivalent inductors are generally appropriate
(see MAX618 EV kit data sheet). Calculate the peak
inductor current with the following equation:
= ILX(PEAK )
IOUT
VOUT
VIN
+
2µs
VIN
L
(VOUT − VIN)
VOUT
Note that the peak inductor current is internally limited
to 2A.
Diode Selection
The MAX618’s high switching frequency demands a high-
speed rectifier. Schottky diodes are preferred for most
applications because of their fast recovery time and low
forward voltage. Make sure that the diode’s peak current
rating exceeds the 2A peak switch current, and that its
breakdown voltage exceeds the output voltage.
OPEN-DRAIN
LOGIC
100kΩ
ON/OFF
CONTROL
MAX618
VL
SHDN
Figure 5. Adding On/Off Control to Circuit of Figure 1 or 4
SYSTEM
LOGIC SUPPLY
MAX618
IN
VL
SYSTEM LOGIC
ON/OFF
CONTROL
SHDN
Figure 6. Adding On/Off Control to Circuit of Figure 3
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