Rainbow Electronics MAX1655 - Manual (Page 9)

MAX1649/MAX1651
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
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To maximize efficiency and reduce the size and cost
of external components, minimize the peak current.
However, since the available output current is a func-
tion of the peak current, the peak current must not be
too low.
To choose the proper current-sense resistor for a par-
ticular output voltage, determine the minimum input
voltage and the maximum load current. Next, refer-
ring to Figures 5a or 5b, using the minimum input volt-
age, find the curve with the largest sense resistor that
provides sufficient output current. It is not necessary
to perform worst-case calculations. These curves take
into account the sense-resistor (±5%) and inductor
(47µH ±10%) values, the diode drop (0.4), and the
IC’s current-sense trip level (85mV); an external MOS-
FET on-resistance of 0.07
Ω
is assumed for V GS = -5V.
Standard wire-wound and metal-film resistors have an
inductance high enough to degrade performance.
Surface-mount (chip) resistors have very little inductance
and are well suited for use as current-sense resistors.
A U-shaped wire resistor made by IRC works well in
through-hole applications. Because this resistor is a
band of metal shaped as a “U”, its inductance is less
than 10nH (an order of magnitude less than metal film
resistors). Resistance values between 5m
Ω
and 0.1
Ω
are available (see Table 1).
Inductor Selection
The MAX1649/MAX1651 operate with a wide range of
inductor values, although for most applications coils
between 10µH and 68µH take best advantage of the con-
trollers’ high switching frequency. With a high inductor
value, the MAX1649/MAX1651 will begin continuous-cur-
rent operation (see
Detailed Description) at a lower frac-
tion of full-load current. In general, smaller values pro-
duce higher ripple (see below) while larger values require
larger size for a given current rating.
In both the continuous and discontinuous modes, the
lower limit of the inductor is important. With a too-small
inductor value, the current rises faster and overshoots the
desired peak current limit because the current-limit com-
parator has a finite response time (300ns). This reduces
efficiency and, more importantly, could cause the current
rating of the external components to be exceeded.
Calculate the minimum inductor value as follows:
(V+(max) - V OUT ) x 0.3µs
L(min) = ——————————––——
∆ I x I LIM
where ∆ I is the inductor-current overshoot factor,
I LIM = V CS /R SENSE , and 0.3µs is the time it takes the com-
parator to switch. Set ∆ I = 0.1 for an overshoot of 10%.
For highest efficiency, use a coil with low DC resis-
tance; a value smaller than 0.1V/I
LIM works best. To
minimize radiated noise, use a toroid, pot core, or
shielded-bobbin inductor. Inductors with a ferrite core
or equivalent are recommended. Make sure the induc-
tor’s saturation-current rating is greater than I
LIM (max).
However, it is generally acceptable to bias the inductor
into saturation by about 20% (the point where the
inductance is 20% below its nominal value).
5.0 5.4 5.8 6.2 6.6 16.0
1649 Fig05a
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0
0.5
r s = 0.030
V OUT = 5V
r s = 0.040
r s = 0.050
r s = 0.060
r s = 0.080
r s = 0.100
Figure 5a. MAX1649 Current-Sense Resistor Graph
3.0 3.4 3.8 4.2 4.6 16.0
1651 Fig05b
INPUT VOLTAGE (V)
MAXIMUM OUTPUT CURRENT (A)
3.0
2.5
2.0
1.5
1.0
0
0.5
r s = 0.030
r s = 0.040
r s = 0.050
r s = 0.060
r s = 0.080
r s = 0.100
V OUT = 3.3V
Figure 5b. MAX1651 Current-Sense Resistor Graph