5 V/3 .3 V o r Ad ju s t a b le , Hig h -Effic ie n c y,
Lo w -Dro p o u t , S t e p -Do w n DC-DC Co n t ro lle rs
49/MAX651
To maximize efficiency and reduce the size and cost
of e xte rna l c omp one nts, minimize the pe a k c urre nt.
However, since the available output current is a func -
tion of the peak current, the peak current must not be
too low.
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.
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 a c c ount the s e ns e -re s is tor (± 5%) a nd ind uc tor
(47µH ± 10%) va lue s , the d iod e d rop (0.4), a nd the
IC’s current-sense trip level (85mV); an external MOS-
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
) x 0.3µs
OUT
FET on-resistance of 0.07Ω is assumed for V = -5V.
GS
L(min) = ——————————––——
∆I x I
LIM
Standard wire-wound and metal-film resistors have an
ind uc ta nc e hig h e noug h to d e g ra d e p e rforma nc e .
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).
whe re ∆I is the ind uc tor-c urre nt ove rs hoot fa c tor,
= V /R , and 0.3µs is the time it takes the com-
I
LIM
CS SENSE
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
works best. To
LIM
minimize ra d ia te d nois e , us e a toroid , p ot c ore , 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 (max).
LIM
However, it is generally acceptable to bias the inductor
into s a tura tion b y a b out 20% (the p oint whe re the
inductance is 20% below its nominal value).
In d u c t o r S e le c t io n
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-
3.0
3.0
V
OUT
= 5V
V
OUT
= 3.3V
r
r
r
= 0.030
= 0.040
= 0.050
r
r
r
= 0.030
= 0.040
= 0.050
s
s
s
s
s
s
2.5
2.0
1.5
1.0
0.5
0
2.5
2.0
1.5
1.0
0.5
0
r
r
= 0.060
= 0.080
r
r
= 0.060
= 0.080
s
s
s
s
r
s
= 0.100
r
s
= 0.100
5.0
5.4
5.8
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
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
