Shutdown Mode

In shutdown mode (SHDN = 0), the MAX618’s feed-

back 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 exter-

nal inductor and diode. Consequently, the output falls

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 (10k

Ω

to 100k

Ω) 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 volt-

age. First, select a value for R2 between 10k

Ω and

200k

Ω. Calculate R1 with:

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:

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

200m

Ω. Ferrite core or equivalent inductors are gener-

ally appropriate (see MAX618 EV kit data sheet).

Calculate the peak inductor current with the following

equation:

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 cur-

rent, and that its breakdown voltage exceeds the out-

put voltage.

I

I

V

V

2s

V

L

VV

V

LX(PEAK)

OUT

OUT

IN

IN

OUT

IN

OUT

=+













−

()

















µ

L

RR

V

V

OUT

FB

12

1

=−









28V, PWM, Step-Up DC-DC Converter

_______________________________________________________________________________________

9

MAX618

VL

100k

ON/OFF

CONTROL

OPEN-DRAIN

LOGIC

SHDN

MAX618

IN

VL

SYSTEM LOGIC

ON/OFF

CONTROL

SHDN

SYSTEM

LOGIC SUPPLY

Figure 5. Adding On/Off Control to Circuit of Figure 1 or 4

Figure 6. Adding On/Off Control to Circuit of Figure 3