1 - t o 3 -Ce ll, Hig h -Cu rre n t , Lo w -No is e ,
S t e p -Up DC-DC Co n ve rt e rs w it h Lin e a r Re g u la t o r
5/MAX1706
To set the low-dropout linear-regulator output, use a
resistor voltage-divider connected to FBLDO from LDO
to GND. Set the output to a value at least 300mV less
than the step-up converter output using the following
formula:
MUR150 or EC11FS1). Do not use ordinary rectifier
d iod e s , s inc e s low s witc hing s p e e d s a nd long re -
verse recovery times compromise efficiency and load
regulation.
Ch o o s e In p u t a n d Ou t p u t
Filt e r Ca p a c it o rs
V
LDO
Choose input and output filter capacitors that service
the input and output peak currents with acceptable
voltage ripple. Choose input capacitors with working
voltage ratings over the maximum input voltage, and
output capacitors with working voltage ratings higher
than the output.
R3 = R4
- 1
V
FBLDO
where V
, the linear-regulator feedback trip point,
FBLDO
is 1.250V. Since the input bias current into FBLDO is
le s s tha n 50nA, R4 c a n b e a la rg e va lue (s uc h a s
270kΩ or less). Connect the resistor voltage-divider as
close to the IC as possible, within 0.2in. (5mm) of the
FBLDO pin.
A 100µF, 100mΩ, low-ESR tantalum capacitor is recom-
me nd e d a t the MAX1706’s s te p -up outp ut. For the
MAX1705, use two in parallel or a 220µF low-ESR tanta-
lum c a p a c itor. The inp ut filte r c a p a c itor (C7) a ls o
reduces peak currents drawn from the input source
and reduces input switching noise. The input voltage
source impedance determines the size required for the
input capacitor. When operating directly from one or
two NiCd cells placed close to the MAX1705/MAX1706,
us e a 22µF, low-ESR inp ut filte r c a p a c itor. Whe n
operating from a power source placed farther away, or
In d u c t o r S e le c t io n
The MAX1705/MAX1706’s high switching frequency
allows the use of a small surface-mount inductor. Use a
10µH inductor for the MAX1705 and a 22µH inductor
for the MAX1706. Make sure the saturation-current rat-
ing exceeds the N-channel switch current limit of 1.55A
for the MAX1705 and 950mA for the MAX1706. For high
efficiency, chose an inductor with a high-frequency
core material, such as ferrite, to reduce core losses. To
minimize radiated noise, use a torroid, pot core, or
shielded-bobbin inductor. See Table 3 for suggested
p a rts a nd Ta b le 4 for a lis t of ind uc tor s up p lie rs .
Connect the inductor from the battery to the LX pin as
close to the IC as possible.
LINEAR-
REGULATOR
OUTPUT
STEP-UP
OUTPUT
LDO
POUT
OUT
MAX1705
MAX1706
C2*
C1*
R3
R4
R1
R2
At t a c h in g t h e Ou t p u t Dio d e
Use a Schottky diode, such as a 1N5817, MBR0520L,
or equivalent. The Schottky diode carries current during
start-up, and in PFM mode after the synchronous rectifi-
er turns off. Thus, the current rating only needs to be
500mA. Attach the diode between the LX and POUT
pins, as close to the IC as possible.
FB
FBLDO
GND
PGND
In hig h-te mp e ra ture a p p lic a tions , s ome Sc hottky
diodes may be unsuitable due to high reverse-leakage
currents. Try substituting a Schottky diode with a higher
reverse voltage rating, or use an ultra-fast silicon rectifi-
er with reverse recover times less than 60ns (such as a
* OPTIONAL COMPENSATION CAPACITORS
Figure 8. Feedback Connections for the MAX1705/MAX1706
Table 3. Component Selection Guide
PRODUCTION
INDUCTORS
CAPACITORS
Matsuo 267 series
Sprague 595D series
AVX TPS series
DIODES
Sumida CDR63B, CD73, CDR73B, CD74B series
Coilcraft DO1608, DO3308, DT3316 series
Surface Mount
Motorola MBR0520L
Sanyo OS-CON series
Nichicon PL series
Through Hole
Sumida RCH654 series
Motorola 1N5817
______________________________________________________________________________________ 15
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
