Rainbow Electronics MAX1947 - Manual (Page 8)

MAX1947
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
Step-Up DC-DC Converter with
RESET
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