Design procedure – Rainbow Electronics MAX1635 User Manual
Page 18
MAX1630–MAX1635
Multi-Output, Low-Noise Power-Supply
Controllers for Notebook Computers
18
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Kool-Mu is a registered trademark of Magnetics Div., Spang & Co.
sistor can be added. Figure 6’s circuit delivers more
than 200mA. Total output current is constrained by the
V+ input voltage and the transformer primary load (see
Maximum 15V V
DD
Output Current vs. Supply Voltage
graphs in the
Typical Operating Characteristics
).
__________________Design Procedure
The three predesigned 3V/5V standard application cir-
cuits (Figure 1 and Table 1) contain ready-to-use solu-
tions for common application needs. Also, two standard
flyback transformer circuits support the 12OUT linear
regulator in the
Applications Information
section. Use
the following design procedure to optimize these basic
schematics for different voltage or current require-
ments. But before beginning a design, firmly establish
the following:
Maximum input (battery) voltage, V
IN(MAX)
. This
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.
V
IN(MAX)
must not exceed 30V.
Minimum input (battery) voltage, V
IN(MIN)
. This
should be taken at full load under the lowest battery
conditions. If V
IN(MIN)
is less than 4.2V, use an external
circuit 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).
Inductor Value
The exact inductor value isn’t 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-current
levels. The smallest inductor is achieved by lowering
the inductance until the circuit operates at the border
between continuous and discontinuous mode. Further
reducing the inductor value below this crossover point
results in discontinuous-conduction operation even at
full load. This helps lower output filter capacitance
requirements, but efficiency suffers due to high I
2
R
losses. On the other hand, higher inductor values mean
greater efficiency, but resistive losses due to extra wire
turns will eventually exceed the benefit gained from
lower peak-current levels. Also, high inductor values
can affect load-transient response (see the V
SAG
equa-
tion in the
Low-Voltage Operation
section). The equa-
tions that follow are for continuous-conduction
operation, since the MAX1630 family is intended mainly
for high-efficiency, battery-powered applications. See
Appendix A in Maxim’s
Battery Management and DC-
DC Converter Circuit Collection
for crossover-point and
discontinuous-mode equations. Discontinuous conduc-
tion doesn’t affect normal Idle Mode operation.
Three key inductor parameters must be specified:
inductance value (L), peak current (I
PEAK
), and DC
resistance (R
DC
). The following equation includes a
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 found at a 30% ripple-current to load-
current ratio (LIR = 0.3), which corresponds to a peak
inductor current 1.15 times higher than the DC load
current.
where: f = switching frequency, normally 200kHz or
300kHz
I
OUT
= maximum DC load current
LIR = ratio of AC to DC inductor current, typi-
cally 0.3; should be selected for >0.15
The nominal peak inductor current at full load is 1.15 x
I
OUT
if the above equation is used; otherwise, the peak
current can be calculated by:
The inductor’s DC resistance should be low enough that
R
DC
x I
PEAK
< 100mV, as it is a key parameter for effi-
ciency performance. If a standard off-the-shelf inductor
is not available, choose a core with an LI
2
rating greater
than L x I
PEAK
2 and wind it with the largest-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 pow-
dered iron is acceptable. If light-load efficiency is unim-
portant (in desktop PC applications, for example), then
low-permeability iron-powder cores, such as the
Micrometals type found in Pulse Engineering’s 2.1µH
PE-53680, may be acceptable even at 300kHz. For
high-current applications, shielded-core geometries,
such as toroidal or pot core, help keep noise, EMI, and
switching-waveform jitter low.
I
= I
+
V
(V
- V
2 x f x L x V
PEAK
LOAD
OUT
IN(MAX)
OUT
IN(MAX)
)
L =
V
V
- V
V
x f x I
x LIR
OUT
IN(MAX)
OUT
IN(MAX)
OUT
(
)