5 V/3 .3 V o r Ad ju s t a b le , 1 0 0 % Du t y-Cyc le ,
Hig h -Effic ie n c y, S t e p -Do w n DC-DC Co n t ro lle rs
current feedback signal and causing jitter. 0.47µF is
recommended. Increase the value as necessary in
high-power applications.
Byp a s s REF with 0.1µF. This c a p a c itor s hould b e
placed within 0.2 inches (5mm) of the IC, next to REF,
with a direct trace to GND (Figure 7).
R
SENSE
MAX1626
4x
SCALE
La yo u t Co n s id e ra t io n s
High-frequency switching regulators are sensitive to PC
board layout. Poor layout introduces switching noise into
the current and voltage feedback signals, resulting in jit-
ter, instability, or degraded performance. The current-
sense resistor must be placed within 0.2 inches (5mm)
of the controller IC, directly between V+ and CS. Place
voltage feedback resistors (MAX1627) next to the FB pin
(no more than 0.2") rather than near the output. Place
the 0.47µF input and 0.1µF reference bypass capacitors
within 0.2 inc he s (5mm) of V+ a nd REF, a nd route
directly to GND. Figure 7 shows the recommended lay-
out and routing for these components.
C
REF
C
V+ BYPASS
Figure 7. Recommended Placement and Routing of the
Current-Sense Resistor, 0.1µF Reference, and 0.47µF Input
Bypass Capacitors
and output ripple similar to or less than that seen dur-
ing no-load conditions.
/MAX1627
Instability can also be caused by excessive stray capaci-
tance on FB when using the MAX1627. Compensate for
this by adding a 0pF to 330pF feed-forward capacitor
across the upper feedback resistor (R2 in Figure 5).
High-power traces, highlighted in the Typical Operating
Circuit (Figure 1), should be as short and as wide as
possible. The supply-current loop (formed by C2, C3,
R , U1, L1, and C1) and commutation-current loop
SENSE
(D1, L1, and C1) should be as tight as possible to
reduce radiated noise. Place the anode of the commuta-
tion diode (D1) and the ground pins of the input and
output filter capacitors close together, and route them to
a common “star-ground” point. Place components and
route ground paths so as to prevent high currents from
causing large voltage gradients between the ground pin
of the output filter capacitor, the controller IC, and the
reference bypass capacitor. Keep the extra copper on
the component and solder sides of the PC board, rather
than etching it away, and connect it to ground for use as
a p s e ud o-g round p la ne . Re fe r to the MAX1626
Evaluation Kit manual for a two-layer PC board example.
MAX1 6 2 6 /MAX1 6 2 7 vs .
MAX1 6 4 9 /MAX1 6 5 1 vs .
MAX6 4 9 /MAX6 5 1
The MAX1626/MAX1627 are specialized, third-genera-
tion upgrades to the MAX649/MAX651 step-down con-
trollers. They feature improved efficiency, a reduced
c urre nt-s e ns e thre s hold (100mV), s oft-s ta rt, a nd a
100% d uty c yc le for lowe s t d rop out. The MAX649/
MAX651 ha ve a two-s te p (210mV/110mV) c urre nt-
sense threshold. The MAX1649/MAX1651 are second-
generation upgrades with a 96.5% maximum duty cycle
for improved dropout performance and a reduced cur-
rent-sense threshold (110mV) for higher efficiency,
e s p e c ia lly a t low inp ut volta g e s . The MAX1649/
MAX1651 are preferable for special applications where
a 100% duty cycle is undesirable, such as flyback and
SEPIC circuits.
S t a b ilit y a n d MAX1 6 2 7 Fe e d b a c k
Co m p e n s a t io n
Use proper PC board layout and recommended exter-
nal components to ensure stable operation. In one-
shot, sequenced PFM DC-DC converters, instability is
ma nife s te d a s “Motorb oa t Ins ta b ility.” It is us ua lly
caused by excessive noise on the current or voltage
feedback signals, ground, or reference, due to poor PC
b oa rd d e s ig n or e xte rna l c omp one nt s e le c tion.
Motorb oa t ins ta b ility is c ha ra c te rize d b y g roup e d
switching pulses with large gaps and excessive low-
fre q ue nc y outp ut rip p le . It is norma l to s e e s ome
grouped switching pulses during the transition from
discontinuous to continuous current mode. This effect
is associated with small gaps between pulse groups
Since the MAX1626’s pinout is similar to those of the
MAX649 and MAX1649 family parts, the MAX1626 can
be substituted (with minor external component value
changes) into fixed-output mode applications, provided
the PC board layout is adequate. The MAX1627 can
also be substituted when MAX649 or MAX1649 family
parts are used in adjustable mode, but the feedback
resistor values must be changed, since the MAX1627
has a lower reference voltage (1.3V vs. 1.5V). Reduce
the current-sense resistor value by 50% when substitut-
ing for the MAX649 or MAX651.
12 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
