MAX1703 Maxim, MAX1703 Datasheet - Page 12

• Manufacturer Part Number: MAX1703
• Description: 1-Cell to 3-Cell / High-Power 1.5A / Low-Noise / Step-Up DC-DC Converter
• Manufacturer: Maxim

1-Cell to 3-Cell, High-Power (1.5A),
Low-Noise, Step-Up DC-DC Converter
Table 3. Component Selection Guide
Set the output voltage between 2.5V and 5.5V by con-
necting a resistor voltage-divider to FB from OUT to
GND, as shown in Figure 2. The resistor values are then
as follows:
where V
1.24V. Since the input bias current into FB is less than
20nA, R2 can have a large value (such as 270kΩ or
less) without sacrificing accuracy. Connect the resistor
voltage-divider as close to the IC as possible, within
0.2in. (5mm) of the FB pin.
The MAX1703’s high switching frequency allows the
use of a small surface-mount inductor. A 4.7µH induc-
tor should have a saturation-current rating that exceeds
the N-channel switch current limit. However, it is gener-
ally acceptable to bias the inductor current into satura-
tion by as much as 20%, although this will slightly
reduce efficiency. For high efficiency, choose an induc-
tor with a high-frequency core material, such as ferrite,
to reduce core losses. To minimize radiated noise, use
a toroid, pot core, or shielded bobbin inductor. See
Table 3 for suggested components and Table 4 for a
list of component suppliers. Connect the inductor from
the battery to the LX pins as close to the IC as possible.
Use a Schottky diode such as a 1N5817, MBR0520L, or
equivalent. The Schottky diode carries current during
both start-up and PFM mode after the synchronous recti-
fier turns off. Thus, its current rating only needs to be
500mA. Connect the diode between LXN/LXP and
POUT, as close to the IC as possible. Do not use ordi-
nary rectifier diodes, since slow switching speeds and
long reverse recovery times will compromise efficiency
and load regulation.
12
__________________Design Procedure
Surface Mount
Through Hole
______________________________________________________________________________________
PRODUCTION
FB
, the boost-regulator feedback setpoint, is
R1 = R2(V
Setting the Output Voltages
OUT
Sumida CDR125
Coilcraft DO3316
Sumida RCH654 series
/ V
Inductor Selection
FB
INDUCTORS
- 1)
Output Diode
Matsuo 267 series
Sprague 595D series
AVX TPS series
Sanyo OS-CON series
Nichicon PL series
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-
age, and output capacitors with working voltage ratings
higher than the output.
A 330µF, 100mΩ, low-ESR tantalum capacitor is recom-
mended for a 5V output. For full output load current,
one 470µF or two 220µF, 100mΩ low-ESR tantalum
capacitors are recommended for a 3.3V output. The
input filter capacitor (C
drawn from the input source and reduces input switch-
ing noise. The input voltage source impedance deter-
mines the required size of the input capacitor.
When operating directly from one or two NiCd cells
placed close to the MAX1703, use a 100µF, low-ESR
input filter capacitor.
Sanyo OS-CON and Panasonic SP/CB-series ceramic
capacitors offer the lowest ESR. Low-ESR tantalum
capacitors are a good choice and generally offer a
good tradeoff between price and performance. Do not
exceed the ripple current ratings of tantalum capaci-
tors. Avoid most aluminum-electrolytic capacitors,
because their ESR is often too high.
Table 4. Component Suppliers
AVX
Coilcraft
Matsuo
Motorola
Sanyo
Sumida
SUPPLIER
CAPACITORS
Input and Output Filter Capacitors
USA: [phone]
USA: [phone]
USA: [phone]
USA: [phone]
USA: [phone]
Japan: 81-7-2070-6306
USA: [phone]
Japan: 81-3-3607-5111
[phone]
PHONE
IN
) also reduces peak currents
Motorola MBR0520L
1N5817
DIODES
[phone]
[phone]
[phone]
[phone]
[phone]
81-7-2070-1174
[phone]
81-3-3607-5144
FAX