Lo w -Vo lt a g e , P re c is io n S t e p -Do w n
Co n t ro lle r fo r P o rt a b le CP U P o w e r
L = V
(V
- V
) / (V
x f x I
x LIR)
OUT IN(MAX)
OUT
IN(MIN)
OUT
__________________De s ig n P ro c e d u re
The five p re d e s ig ne d s ta nd a rd a p p lic a tion c irc uits
(Figure 1 and Table 1) contain ready-to-use solutions
for c ommon a p p lic a tion ne e d s . Us e the following
design procedure to optimize these basic schematics
for different voltage or current requirements. But before
beginning a design, firmly establish the following:
where f = switching frequency, normally 200kHz or
300kHz, and I = maximum DC load current. The
peak current can be calculated by:
OUT
I
= I
+ [V
(V
- V
) / (2 x f x L x V
)]
PEAK
LOAD
OUT IN(MAX)
OUT
IN(MAX)
The inductor's DC resistance should be low enough
that R x I < 100mV, as it is a key parameter for
DC
PEAK
• Ma ximum inp ut (b a tte ry) volta g e , V
. This
IN(MAX)
1
e ffic ie nc y p e rforma nc e . If a s ta nd a rd , off-the -s he lf
value should include the worst-case conditions, such
as no-load operation when a battery charger or AC
adapter is connected but no battery is installed.
2
inductor is not available, choose a core with an LI rat-
2
ing greater than L x I
and wind it with the largest
PEAK
diameter wire that fits the winding area. For 300kHz
applications, ferrite-core material is strongly preferred;
for 200kHz applications, Kool-Mu® (aluminum alloy) or
even powdered iron is acceptable. If light-load efficien-
c y is unimp orta nt (in d e s ktop PC a p p lic a tions , for
example), then low-permeability iron-powder cores may
be acceptable, even at 300kHz. For high-current appli-
cations, shielded-core geometries, such as toroidal or
pot core, help keep noise, EMI, and switching-wave-
form jitter low.
V
must not exceed 30V.
IN(MAX)
• Minimum input (battery) voltage, V
. This should
IN(MIN)
be taken at full load under the lowest battery condi-
tions. If V is less than 4.5V, use an external cir-
IN(MIN)
cuit to externally hold VL above the VL undervoltage
lockout threshold. If the minimum input-output differ-
ence is less than 1.5V, the filter capacitance required
to maintain good AC load regulation increases (see
Low-Voltage Operation section).
In d u c t o r Va lu e
The exact inductor value is not critical and can be
freely adjusted to make trade-offs between size, cost,
and efficiency. Lower inductor values minimize size
and cost but reduce efficiency due to higher peak-cur-
rent levels. The smallest inductor is achieved by lower-
ing the ind uc ta nc e until the c irc uit op e ra te s a t the
border between continuous and discontinuous mode.
Furthe r re d uc ing the ind uc tor va lue b e low this
crossover point results in discontinuous-conduction
operation even at full load. This helps lower output filter
capacitance requirements, but efficiency suffers due to
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 accord-
ing to the worst-case, low-current-limit threshold volt-
age (from the Electrical Characteristics table) and the
peak inductor current:
R
= 80mV / I
PEAK
SENSE
Use I
from the second equation in the Inductor
PEAK
Value section. Use the calculated value of R
to
SENSE
size the MOSFET switches and specify inductor satura-
tion-current ratings according to the worst-case high-
current-limit threshold voltage:
2
high I R losses. On the other hand, higher inductor val-
I
= 120mV / R
SENSE
PEAK
ues mean greater efficiency, but resistive losses due to
extra wire turns eventually exceed the benefit gained
from lower peak-current levels. Also, high inductor val-
Low-inductance resistors, such as surface-mount metal
film, are recommended.
ues can affect load-transient response (see the V
SAG
equation in the Low-Voltage Operation section). The
equations in this section are for continuous-conduction
operation.
In p u t Ca p a c it o r Va lu e
Connect low-ESR bulk capacitors directly to the drain
on the high-side MOSFET. The bulk input filter capaci-
tor is usually selected according to input ripple current
requirements and voltage rating, rather than capacitor
value. Electrolytic capacitors with low enough equiva-
lent series resistance (ESR) to meet the ripple-current
requirement invariably have sufficient capacitance val-
ues. Aluminum electrolytic capacitors, such as Sanyo
OS-CON or Nic hic on PL, a re s up e rior to ta nta lum
types, which risk power-up surge-current failure, espe-
cially when connecting to robust AC adapters or low-
Thre e ke y ind uc tor p a ra me te rs mus t b e s p e c ifie d :
inductance value (L), peak current (I ), and DC
PEAK
resistance (R ). The following equation includes a
DC
constant, LIR, which is the ratio of inductor peak-to-
peak AC current to DC load current. A higher LIR value
allows smaller inductance but results in higher losses
and higher ripple. A good compromise between size
and losses is a 30% ripple-current to load-current ratio
(LIR = 0.3), which corresponds to a peak inductor cur-
rent 1.15 times higher than the DC load current.
impedance batteries. RMS input ripple current (I
) is
RMS
Kool-Mu is a registered trademark of Magnetics, Inc.
18 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
