Mu lt i-Ou t p u t , Lo w -No is e P o w e r-S u p p ly
Co n t ro lle rs fo r No t e b o o k Co m p u t e rs
0–MAX1635
Cu rre n t -S e n s e Re s is t o r Va lu e
The current-sense resistor value is calculated according
to the worst-case-low current-limit threshold voltage
(from the Electrical Characteristics table) and the peak
inductor current:
Byp a s s in g VL
Bypass the VL output with a 4.7µF tantalum capacitor
paralleled with a 0.1µF ceramic capacitor, close to the
device.
Ou t p u t Filt e r Ca p a c it o r Va lu e
The output filter capacitor values are generally deter-
mined by the ESR and voltage rating requirements, rather
than actual capacitance requirements for loop stability. In
other words, the low-ESR electrolytic capacitor that meets
the ESR requirement usually has more output capaci-
tance than is required for AC stability. Use only special-
ized low-ESR capacitors intended for switching-regulator
applications, such as AVX TPS, Sprague 595D, Sanyo
OS-CON, or Nichicon PL series. To ensure stability, the
capacitor must meet both minimum capacitance and
maximum ESR values as given in the following equations:
80mV
R
=
SENSE
I
PEAK
Use I
from the second equation in the Inductor
PEAK
Value section
Use the calculated value of R
to size the MOSFET
SENSE
switches and specify inductor saturation-current ratings
according to the worst-case high-current-limit threshold
voltage:
120mV
I
=
PEAK(MAX)
R
V
(1 + V
/ V
)
SENSE
REF
OUT
IN(MIN)
C
R
>
OUT
ESR
V
x R
x f
SENSE
Low-ind uc ta nc e re s is tors , s uc h a s s urfa c e -mount
metal-film, are recommended.
OUT
R
x V
OUT
SENSE
V
<
In p u t Ca p a c it o r Va lu e
Connect low-ESR bulk capacitors and small ceramic
capacitors (0.1µF) directly to the drains on the high-
side MOSFETs. The bulk input filter capacitor is usually
selected according to input ripple current requirements
a nd volta g e ra ting , ra the r tha n c a p a c itor va lue .
Electrolytic capacitors with low enough effective series
resistance (ESR) to meet the ripple current requirement
inva ria b ly ha ve s uffic ie nt c a p a c ita nc e va lue s .
Aluminum e le c trolytic c a p a c itors , s uc h a s Sa nyo
OS-CON or Nic hic on PL, a re s up e rior to ta nta lum
types, which carry the risk of power-up surge-current
fa ilure , e s p e c ia lly whe n c onne c ting to rob us t AC
adapters or low-impedance batteries. RMS input ripple
REF
(can be multiplied by 1.5; see text below)
These equations are worst case, with 45 degrees of
p ha s e ma rg in to e ns ure jitte r-fre e , fixe d -fre q ue nc y
op e ra tion a nd p rovid e a nic e ly d a mp e d outp ut
response for zero to full-load step changes. Some cost-
conscious designers may wish to bend these rules with
less-expensive capacitors, particularly if the load lacks
large step changes. This practice is tolerable if some
b e nc h te s ting ove r te mp e ra ture is d one to ve rify
acceptable noise and transient response.
No well-defined boundary exists between stable and
unstable operation. As phase margin is reduced, the
first symptom is a bit of timing jitter, which shows up as
blurred edges in the switching waveforms where the
scope won’t quite sync up. Technically speaking, this
jitter (usually harmless) is unstable operation, since the
duty factor varies slightly. As capacitors with higher
ESRs are used, the jitter becomes more pronounced,
and the load-transient output voltage waveform starts
looking ragged at the edges. Eventually, the load-tran-
sient waveform has enough ringing on it that the peak
noise levels exceed the allowable output voltage toler-
ance. Note that even with zero phase margin and gross
instability present, the output voltage noise never gets
current (I
) is determined by the input voltage and
RMS
load current, with the worst case occurring at V = 2 x
IN
V
:
OUT
V
(V - V
)
OUT IN OUT
I
= I
x
RMS
LOAD
V
IN
Therefore, when V is 2 x V
:
IN
OUT
I
LOAD
2
I
=
RMS
Byp a s s in g V+
Bypass the V+ input with a 4.7µF tantalum capacitor
paralleled with a 0.1µF ceramic capacitor, close to the
much worse than I
x R
(under constant loads).
PEAK
ESR
Designers of RF communicators or other noise-sensi-
tive analog equipment should be conservative and stay
within the guidelines. Designers of notebook computers
a nd s imila r c omme rc ia l-te mp e ra ture -ra ng e d ig ita l
IC. A 10Ω series resistor to V is also recommended.
IN
______________________________________________________________________________________ 19
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
