Rainbow Electronics MAX1724 - Manual (Page 10)

For most applications, the peak inductor current equals
the current limit. However, for applications using large
inductor values or low input voltages, the maximum on-
time limits the peak inductor current (see Inductor
Selection section).
Capacitor Selection
Choose input and output capacitors to supply the input
and output peak currents with acceptable voltage rip-
ple. The input filter capacitor (C
IN ) reduces peak cur-
rents drawn from the battery and improves efficiency.
Low equivalent series resistance (ESR) capacitors are
recommended. Ceramic capacitors have the lowest
ESR, but low ESR tantalum or polymer capacitors offer
a good balance between cost and performance.
Output voltage ripple has two components: variations
in the charge stored in the output capacitor with each
LX pulse, and the voltage drop across the capacitor’s
ESR caused by the current into and out of the capaci-
tor:
where I PEAK is the peak inductor current (see Inductor
Selection section). For ceramic capacitors, the output
voltage ripple is typically dominated by V
RIPPLE(C) . For
example, a 10µF ceramic capacitor and a 10µH induc-
tor typically provide 75mV of output ripple when step-
ping up from 3.3V to 5V at 50mA. Low input-to-output
voltage differences (i.e. two cells to 3.3V) require high-
er output capacitor values.
Capacitance and ESR variation of temperature should
be considered for best performance in applications
with wide operating temperature ranges. Table 2 lists
suggested capacitors and suppliers.
PC Board Layout Considerations
Careful PC board layout is important for minimizing
ground bounce and noise. Keep the IC’s GND pin and
the ground leads of the input and output capacitors
less than 0.2in (5mm) apart using a ground plane. In
addition, keep all connections to FB
(MAX1722/MAX1723 only) and LX as short as possible.
V V V
V
V
L
V V C
I
RIPPLE RIPPLE C RIPPLE ESR
RIPPLE ESR
RIPPLE C
OUT BATT OUT
= +
≈
≈
( )






( )
( ) ( )
( ) ( )
( )
PEAK ESR COUT
PEAK OUT
I R
-
I - 2
1
2
2
MAX1722/MAX1723/MAX1724
1.5µA I Q , Step-Up DC-DC Converters
in Thin SOT23-5
10 ______________________________________________________________________________________
Table 2. Suggested Surface-Mount Capacitors and Manufacturers (C1 and C2)
MANUFACTURER
CAPACITOR
VALUE
DESCRIPTION
PHONE
WEBSITE
1 µF to 10µF X7R Ceramic
AVX
10 µF to 330µF
TAJ Tantalum Series
TPS Tantalum Series
843-448-9411
www.avxcorp.com
1 µF to 22µF X5R/X7R Ceramic
10 µF to 330µF T494 Tantalum Series Kemet
68 µF to 330µF T520 Tantalum Series
864-963-6300
www.kemet.com
Sanyo 33 µF to 330µF TPC Polymer Series
408-749-9714
www.secc.co.jp
Taiyo Yuden 33 µF to 330µF X5R/X7R Ceramic
800-368-2496
www.t-yuden.org
TDK 1 µF to 10µF X7R Ceramic
847-803-6100
www.tdk.com
Vishay Sprague 10 µF to 330µF
594D Tantalum Series
595D Tantalum Series 203-452-5664
www.vishay.com
Chip Information
TRANSISTOR COUNT: 863