MAX1637 Datasheet, PDF (15/20 Pages) Maxim Integrated Products – Miniature, Low-Voltage, Precision Step-Down Controller

Miniature, Low-Voltage,
Precision Step-Down Controller
Inductor Value
The exact inductor value is not critical and can be
freely adjusted to allow trade-offs among size, cost,
and efficiency. Lower inductor values minimize size
and cost, but reduce efficiency due to higher peak-
current levels. The smallest inductor value is obtained
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 under
these conditions, due to high I²R losses. On the other
hand, higher inductor values produce greater efficien-
cy, but also result in resistive losses due to extra wire
turns—a consequence that eventually overshadows the
benefits gained from lower peak current levels. High
inductor values can also affect load-transient response
(see the V_SAG equation in the Low-Voltage Operation
section). The equations in this section are for continu-
ous-conduction 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 lower inductance, but results in higher losses
and ripple. A good compromise is 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.
L = V_OUT(V_IN(MAX) - V_OUT) / (V_IN(MIN) x ƒ x I_OUT x
LIR)
where ƒ = switching frequency (normally 200kHz or
300kHz), and I_OUT = maximum DC load current.
The peak current can be calculated as follows:
I_PEAK = I_LOAD + [V_OUT(V_IN(MAX) - V_OUT) / (2 x ƒ x L
x V_IN(MAX))]
The inductor’s DC resistance should be low enough
that R_DC x I_PEAK < 100mV, as it is a key parameter for
efficiency performance. If a standard, off-the-shelf
inductor is not available, choose a core with an LI² rat-
ing greater than L x IPEAK² 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 powdered iron is acceptable. If light-load efficien-
cy is unimportant (in desktop PC applications, for
example), then low-permeability iron-powder cores can
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-
waveform jitter low.
Current-Sense Resistor Value
The current-sense resistor value is calculated accord-
ing to the worst-case, low-current limit threshold volt-
age (from the Electrical Characteristics) and the peak
inductor current:
R_SENSE = 80mV / I_PEAK
Use I_PEAK from the second equation in the Inductor
Value section. Use the calculated value of R_SENSE to
size the MOSFET switches and specify inductor satura-
tion-current ratings according to the worst-case high-
current-limit threshold voltage:
I_PEAK = 120mV / R_SENSE
Low-inductance resistors, such as surface-mount metal
film, are recommended.
Input Capacitor Value
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 Nichicon PL, are superior to tantalum
types, which risk power-up surge-current failure, espe-
cially when connecting to robust AC adapters or low-
impedance batteries. RMS input ripple current (I_RMS) is
determined by the input voltage and load current, with
the worst case occurring at V_IN = 2 x V_OUT. Therefore,
when V_IN is 2 x V_OUT:
I_RMS = I_LOAD / 2
V_CC and V_GG should be isolated from each other with a
20Ω resistor and bypassed to ground independently.
Place a 0.1µF capacitor between V_CC and GND, as
close to the supply pin as possible. A 4.7µF capacitor
is recommended between V_GG and PGND.
Output Filter Capacitor Value
The output filter capacitor values are generally deter-
mined by the ESR and voltage-rating requirements,
rather than by actual capacitance requirements for loop
stability. In other words, the low-ESR electrolytic capac-
itor that meets the ESR requirement usually has more
output capacitance than is required for AC stability.
Kool-Mu is a trademark of Magnetics, Inc.
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