5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
MAX1649/MAX1651
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 0.07Ω 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 5mΩ and 0.1Ω
are available (see Table 1).
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).
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-
1649 Fig05a
V
OUT
= 5V
r
s
= 0.030
r
s
= 0.040
r
s
= 0.050
V
OUT
= 3.3V
r
s
= 0.030
r
s
= 0.040
r
s
= 0.050
MAXIMUM OUTPUT CURRENT (A)
2.5
2.0
1.5
1.0
0.5
0
5.0
5.4
5.8
MAXIMUM OUTPUT CURRENT (A)
2.5
2.0
1.5
1.0
0.5
0
r
s
= 0.060
r
s
= 0.080
r
s
= 0.100
r
s
= 0.060
r
s
= 0.080
r
s
= 0.100
6.2
6.6
16.0
3.0
3.4
3.8
4.2
4.6
16.0
INPUT VOLTAGE (V)
INPUT VOLTAGE (V)
Figure 5a. MAX1649 Current-Sense Resistor Graph
Figure 5b. MAX1651 Current-Sense Resistor Graph
9
_______________________________________________________________________________________
1651 Fig05b
3.0
3.0
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
