Rainbow Electronics MAX1677 - Manual (Page 14)

Use inductors with a ferrite core or equivalent; powder
iron cores are not recommended for use with the
MAX1677’s high switching frequencies. The inductor’s
incremental saturation rating ideally should exceed the
selected current limit, however it is generally accept-
able to bias most inductors into saturation by as much
as 20% (although this may reduce efficiency).
For best efficiency, select inductors with resistance no
greater than the internal N-channel FET resistance in
each boost converter (220m
Ω for the MBC, and 1Ω for
the LCD). The inductor is effectively in series with the
input at all times, so inductor wire losses can be rough-
ly approximated by I
IN2
× R
L . See Table 4 for a list of
inductor suppliers.
The LCD boost converter (LCD) features selectable
inductor/switch current limit of 350mA or 225mA. The
higher current setting provides the greatest output cur-
rent, while the lower setting allows the smallest inductor
size.
External Diodes
The MAX1677’s on-chip synchronous rectifier allows
the normally required external Schottky diode to be
omitted from the MBC in designs whose input exceeds
1.4V. In circuits that need to operate below 1.4V (1-cell
inputs for example), connecting a Schottky diode in
parallel with the internal synchronous rectifier (from LX
to POUT) provides the lowest start-up voltage. Suitable
devices are the 1N5817 or MBR0520L, however the
diode current rating need not match the peak switch
current, since most of the current is handled by the on-
chip synchronous rectifier.
Since the LCD boost converter (LCD) does not have
synchronous rectification, an external diode is always
needed. High switching speed demands a high-speed
rectifier. For best efficiency, Schottky diodes such as
the 1N5818 and MBR0530L are recommended. Be
sure that the diode current rating exceeds the peak
current set by LCDPOL, and that the diode voltage rat-
ing exceeds the LCD output voltage. In particularly
cost-sensitive applications, and if the LCD’s 225mA
peak current is set, a high-speed silicon signal diode
(such as an 1N4148) may be used instead of a
Schottky diode, but with reduced efficiency.
Input Bypass Capacitors
A low-ESR input capacitor connected in parallel with
the battery will reduce peak currents and input-reflected
noise. Battery bypassing is especially helpful at low input
voltages and with high-impedance batteries (such as
alkaline types). Benefits include improved efficiency
and lower useful end-of-life voltage for the battery.
100µF is typically recommended for 2-cell applications.
Small ceramic capacitors may also be used for light
loads or in applications that can tolerate higher input
ripple. Only one input bypass capacitor is typically
needed for both the MBC and LCD.
Output Filter Capacitors
For most applications, a 100µF, 10V, low-ESR output fil-
ter capacitor is recommended for the MBC output. A
surface-mount tantalum capacitor typically exhibits
30mV ripple when the MBC is stepping up from 1.2V to
3.3V at 100mA. OS-CON and ceramic capacitors offer
lowest ESR, while low-ESR tantalums offer a good bal-
ance between cost and performance.
The LCD output typically exhibits less than 1% peak-to-
peak ripple with 4.7µF of filter capacitance. This can be
either a ceramic or tantalum type, but be sure that the
capacitor voltage rating exceeds the LCD output volt-
age. If the LCD’s 225mA peak switch current setting is
used, the designer can choose lower output ripple or
MAX1677
Compact, High-Efficiency, Dual-Output
Step-Up and LCD Bias DC-DC Converter
14 ______________________________________________________________________________________
Table 4. Component Suppliers
PHONE FAX
Sanyo: OS-CON
and GX series
619-661-6835 619-661-1055
Murata: LQH4 and
LQH3C series
814-237-1431 814-238-0490
SUPPLIER
INDUCTORS
CAPACITORS
AVX: TPS series 803-946-0690 803-626-3123
TDK: NLC Series 847-390-4373 847-390-4428
Matsuo:
267 series
714-969-2591 714-960-6492
Sprague: 595D
series
603-224-1961 603-224-1430
Motorola:
MBR0520
602-303-5454 602-994-6430
Nihon: EC11 FS1
series
805-867-2555 805-867-2698
Coilcraft: DO and
DT series
847-639-6400 847-639-1469
Sumida: CD, CDR,
and RCH series
847-956-0666 847-956-0702
INDUCTORS
CAPACITORS
DIODES