Rainbow Electronics MAX799 User Manual
Page 20
MAX796/MAX797/MAX799
Step-Down Controllers with
Synchronous Rectifier for CPU Power
20
______________________________________________________________________________________
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 value of
LIR allows smaller inductance, but results in higher
losses and ripple. A good compromise between size
and losses is found at a 30% ripple current to load cur-
rent ratio (LIR = 0.3), which corresponds to a peak
inductor current 1.15 times higher than the DC load
current.
V
OUT
(V
IN(MAX)
- V
OUT
)
L = ———————————
V
IN(MAX)
x f x I
OUT
x LIR
where:
f = switching frequency, normally 150kHz or
300kHz
I
OUT
= maximum DC load current
LIR = ratio of AC to DC inductor current,
typically 0.3
The 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:
V
OUT
(V
IN(MAX)
- V
OUT
)
I
PEAK
= I
LOAD
+ ———————————
2 x f x L x V
IN(MAX)
The inductor’s DC resistance is a key parameter for effi-
ciency performance and must be ruthlessly minimized,
preferably to less than 25m
Ω
at I
OUT
= 3A. If a stan-
dard off-the-shelf inductor is not available, choose a
core with an LI
2
rating greater than L x I
PEAK2
and wind
it with the largest diameter wire that fits the winding
area. For 300kHz applications, ferrite core material is
strongly preferred; for 150kHz applications, Kool-mu
(aluminum alloy) and even powdered iron can be
acceptable. If light-load efficiency is unimportant (in
desktop 5V-to-3V 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.
Current-Sense Resistor Value
The current-sense resistor value is calculated accord-
ing to the worst-case-low current-limit threshold voltage
(from the
Electrical Characteristics
table) and the peak
inductor current. The continuous-mode peak inductor-
current calculations that follow are also useful for sizing
the switches and specifying the inductor-current satu-
ration ratings. In order to simplify the calculation, I
LOAD
may be used in place of I
PEAK
if the inductor value has
been set for LIR = 0.3 or less (high inductor values)
and 300kHz operation is selected. Low-inductance
resistors, such as surface-mount metal-film resistors,
are preferred.
80mV
R
SENSE
= ————
I
PEAK
Input Capacitor Value
Place a small ceramic capacitor (0.1µF) between V+
and GND, close to the device. Also, connect a low-ESR
bulk capacitor directly to the drain of the high-side
MOSFET. Select the bulk input filter capacitor accord-
ing to input ripple-current requirements and voltage rat-
ing, rather than capacitor value. Electrolytic capacitors
that have low enough ESR to meet the ripple-current
requirement invariably have more than adequate
capacitance values. Aluminum-electrolytic capacitors
such as Sanyo OS-CON or Nichicon PL are preferred
over tantalum types, which could cause power-up
surge-current failure, especially when connecting to
robust AC adapters or low-impedance batteries. RMS
input ripple current is determined by the input voltage
and load current, with the worst possible case occur-
ring at V
IN
= 2 x V
OUT
:
————————
√
V
OUT
(V
IN
- V
OUT
)
I
RMS
= I
LOAD
x ——————————
V
IN
I
RMS
= I
LOAD
/ 2 when V
IN
is 2 x V
OUT
Output Filter Capacitor Value
The output filter capacitor values are generally deter-
mined by the ESR (effective series resistance) and volt-
age rating requirements rather than actual capacitance
requirements for loop stability. In other words, the low-
ESR electrolytic capacitor that meets the ESR require-
ment usually has more output capacitance than is
required for AC stability. Use only specialized 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:
V
REF
(1 + V
OUT
/ V
IN(MIN)
)
C
F
> ––––––––––––––––———–––
V
OUT
x R
SENSE
x f
R
SENSE
x V
OUT
R
ESR
< ————————
V
REF
(can be multiplied by 1.5, see note below)