Rainbow Electronics MAX1760 - Manual (Page 10)

MAX1760
time constant is:
where R SS
≥ 470k.
Placing a capacitor across the lower resistor of the cur-
rent-limiting resistive divider provides both current-limit
and soft-start features simultaneously (Figures 4 and 5).
Inductor Selection
The MAX1760’s high switching frequency allows the
use of a small 3.3µH surface-mount inductor. The cho-
sen inductor should generally have a saturation current
rating exceeding the N-channel switch current limit;
however, it is acceptable to bias the inductor current
into saturation by as much as 20% if a slight reduction
in efficiency is acceptable. Lower current-rated induc-
tors may be used if ISET is employed to reduce the
peak inductor current (see Setting the Switch Current
Limit and Soft-Start). For high efficiency, choose an
inductor with a high-frequency ferrite core material to
reduce core losses. To minimize radiated noise, use a
toroid or shielded inductor. See Table 3 for suggested
components and Table 4 for a list of component suppli-
ers. Connect the inductor from the battery to the LX pin
as close to the IC as possible.
External Diode
For output voltages greater than 4V, an external
Schottky diode must be connected from LX to POUT, in
parallel with the on-chip synchronous rectifier (Figure
6). The diode should be rated for 0.5A. Representative
devices are Motorola MBR0520L, Nihon EP05Q03L, or
generic 1N5817. This external diode is also recom-
mended for applications that must start with input volt-
ages at or below 1.8V. The Schottky diode carries
current during startup and after the synchronous rectifi-
er turns off; thus, its current rating only needs to be
500mA. Connect the diode as close to the IC as possi-
ble. Do not use ordinary rectifier diodes; their slow
switching speeds and long reverse-recovery times ren-
der them unacceptable. For circuits that do not require
startup with inputs below 1.8V, and have an output of
4V or less, no external diode is needed.
Input and Output Filter Capacitors
Choose input and output filter capacitors that will ser-
vice the input and output peak currents with accept-
able voltage ripple. Choose input capacitors with
working voltage ratings over the maximum input volt-
t R C
SS SS SS
=
0.8A, Low-Noise, 1MHz,
Step-Up DC-DC Converter
10 ___________________________________________________
Table 3. Component Selection Guide
Sanyo POSCAP series Sumida CDRH5D18
Kemet T510 series Coilcraft LPT3305
— Sumida CD43
Surface Mount
AVX TPS series Coilcraft DS3316P
CAPACITORS INDUCTORS PRODUCTION METHOD
—
Motorola MBR0520L
—
EIC SB series
DIODES
Table 4. Component Suppliers
USA: 916-941-0712 EIC
USA: 847-639-6400 Coilcraft
USA: 847-956-0666
Japan: 011-81-3-3667-3302
Sumida
USA: 843-448-9411 AVX
PHONE SUPPLIER
USA: 408-629-4789
Japan: 81-45-474-7030
Motorola
USA: 810-287-2536 Kemet
Note: Please indicate that you are using the MAX1760 when
contacting these component suppliers.
FB PGND GND
POUT
MRB0520L
LX
ISET
OUT
REF
CLK/SEL
V IN = 0.7V
TO VOUT
V OUT ON
MAX1760
100 µF
3.3 µH
0.68 µF
4.7 Ω
33 µF
0.22 µF
Figure 6. Connection with External Schottky Diode for Output
Voltages Greater than 4V, or to Assist Low-Voltage Startup