Shutdown

The MAX1947 enters shutdown mode when SHDN is

driven low. During shutdown, the synchronous rectifier

disconnects the output from the input, eliminating the DC

conduction path that normally exists with traditional boost

converters in shutdown mode. The output is actively dis-

charged to ground through an internal 500Ω resistor. The

quiescent current is reduced to 2µA while in shutdown

mode. Drive SHDN high for normal operation. The output

reaches regulation approximately 650µs after SHDN

goes high.

Applications Information

Inductor Selection

An inductor value of 4.7µH performs well in most appli-

cations. The MAX1947 also works with inductors in the

2.2µH to 6.8µH range. Smaller inductance values typi-

cally offer a smaller physical size for a given series

resistance, allowing the smallest overall circuit dimen-

sions but with lower output-current capability. Circuits

using larger inductance values exhibit higher output-

current capability, but are larger for the same series

resistance and current rating.

The inductor’s incremental saturation current rating

should be greater than the peak switch-current limit.

However, it is generally acceptable to bias the inductor

into saturation by as much as 20%, although this

slightly reduces efficiency (see the Electrical

Characteristics for the MAX1947 NFET current limit).

Table 1 lists the suggested components for several typ-

ical applications. Also, the inductor’s DC resistance sig-

nificantly affects efficiency.

Low Input/Output Voltage

8

_______________________________________________________________________________________

APPLICATION

LOAD

INPUT CAPACITOR (µF)

OUTPUT CAPACITOR (µF)

INDUCTOR (µH)

≤50% full load

2.2

4.7

3.3

1-Cell Input,

Output < 2.7V

>50% full load

2.2

10

3.3

≤50% full load

2.2

4.7

4.7

1-Cell Input,

Output > 2.7V

>50% full load

2.2

10

4.7

≤50% full load

2.2

4.7

6.8

2-Cell Input

>50% full load

2.2

10

6.8

Table 1. Suggested Component Values

MANUFACTURER

PART

PART NUMBER

PHONE

WEBSITE

Sumida

Inductor

CDRH3D16 series

81-3-3667-3381

www.sumida.com

Output capacitors

4.7µF JMK212BJ475MG,

10µF JMK212BJ106MG

Taiyo Yuden

Input capacitors

2.2µF LMK107BJ225MA

408-573-4150

www.t-yuden.com

Output capacitors

4.7µF C2012X5R0J475K,

10µF C2012X5R0J106K

TDK

Input capacitors

2.2µF C1608X5R03225M

888-835-6646

www.TDK.com

Table 2. Suggested Component Manufacturers