Lo w -No is e , 5 .5 V-In p u t ,
P WM S t e p -Do w n Re g u la t o r
MAX1692
ESR values are met:
Ca p a c it o r S e le c t io n
Choose input- and output-filter capacitors to service
inductor currents with acceptable voltage ripple. The
input-filter capacitor also reduces peak currents and
noise at the voltage source. In addition, connect a low-
ESR bulk capacitor (>10µF suggested) to the input.
Se le c t this b ulk c a p a c itor to me e t the inp ut rip p le
requirements and voltage rating, rather than capacitor
size. Use the following equation to calculate the maxi-
mum RMS input current:
C2 > 2V (1 + V
/V
) / (V
· R · f
SENSE OSC
)
REF
OUT IN(MIN)
OUT
R
ESR
< (R
)(V
) / (V
)
SENSE OUT
REF
where C2 is the output filter capacitor, V
nal reference voltage of 1.25V, V (min) is the minimum
input voltage (2.7V), R
tance of 0.1Ω, and f
quency (typically 750kHz). These equations provide the
minimum requirements. The value of C2 may need to
be increased for operation at duty-cycle extremes.
is the inter-
REF
IN
is the internal sense resis-
is the internal oscillator fre-
SENSE
OSC
1/2
I
= I
[V
(V - V
)] · V
RMS
OUT OUT IN
OUT IN
Tables 1 and 2 provide recommended inductor and
capacitor sizes at various external sync frequencies.
Table 3 lists suppliers for the various components used
with the MAX1692.
When selecting an output capacitor, consider the out-
put-ripple voltage and approximate it as the product of
the ripple current and the ESR of the output capacitor.
V
=
[V
[2 · f
(V - V
)] /
RIPPLE
OUT IN
OUT
S t a n d a rd Ap p lic a t io n Circ u it s
(L) (V )] · ESR
OSC
IN C2
Figures 2 and 3 are standard application circuits opti-
mized for power and board space respectively. The cir-
cuit of Figure 2 is the most general of the two, and
generates 1.8V at 600mA.
where ESR is the equivalent-series resistance of the
C2
output capacitor.
The MAX1692’s loop gain is relatively low, enabling the
use of small, low-value output filter capacitors. Higher
values provide improved output ripple and transient
response. Lower oscillator frequencies require a larger-
value output capacitor. When PWM/PFM is used, verify
capacitor selection with light loads during PFM opera-
tion, since output ripple is higher under these condi-
tions. Low-ESR capacitors are recommended.
The circuit of Figure 3 is optimized for smallest overall
size. Cellular phones are using low voltage for base-
band logic and have critical area and height restric-
tions. This circuit operates from a single Li-ion battery
(2.9V to 4.5V) and delivers up to 200mA at 1.8V. It uses
small ceramic capacitors at the input and output and a
tiny chip inductor such as the NLC322522T series from
TDK. With the MAX1692 in a 10-pin µMAX package, the
entire circuit can fit in only 60mm2 and have less than
2.4mm height.
Capacitor ESR is a major contributor to output ripple
(usually more than 60%). Ordinary aluminum-electrolyt-
ic capacitors have high ESR and should be avoided.
Low-ESR aluminum-electrolytic capacitors are accept-
a b le a nd re la tive ly ine xp e ns ive . Low-ESR ta nta lum
capacitors are better and provide a compact solution
for space-constrained surface-mount designs. Do not
exceed the ripple-current ratings of tantalum capaci-
tors. Ceramic capacitors have the lowest ESR overall,
and OS-CON™ capacitors have the lowest ESR of the
high-value electrolytic types.
L1
V
IN
10µH
+2.9V TO +4.5V
V
OUT
= 1.8V @ 200mA
C2
IN
LX
C5
4.7µF
10µF
10µF
BP
It is generally not necessary to use ceramic or OS-CON
capacitors for the MAX1692; consider them only in very
compact, high-reliability, or wide-temperature applica-
tions where the expense is justified. When using very-
low-ESR c a p a c itors , s uc h a s c e ra mic or OS-CON,
c he c k for s ta b ility while e xa mining loa d -tra ns ie nt
response. The output capacitor is determined by ensur-
ing that the minimum capacitance value and maximum
MAX1692
/
ON OFF
SHDN
C5
47pF
R1
138k
C4
0.1µF
REF
LIM
FB
R2
301k
SYNC/
PWM
GND PGND
Figure 3. Miniaturized 200mA Output Circuit Fits in 60mm2
OS-CON is a trademark of Sanyo Corp.
_______________________________________________________________________________________
9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
