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
V
IN
C4
C1
0.1µF 100µF
5
V+
R1
0.05Ω
MAX1649
MAX1651
6
CS
1.5A
1A
P1
Si9430
L1
47µH
3
4
OUTPUT
@ 1.5A
7
SHDN
EXT
1
2
0A
REF
OUT
FB
R2
GND
8
C2
330µF
C3
0.1µF
2µs/div
CIRCUIT OF FIGURE 1, R1 = 75mΩ
D1
1N5820
V+ = 10V, I
= 1.3A
R3
150k
LOAD
V
OUT
49/MAX651
R2 = R3
– 1
(
)
V
REF
V
REF
= 1.5V
Figure 3. MAX1649 Continuous-Conduction Mode, Heavy
Load-Current Waveform (500mA/div)
Figure 4. Adjustable-Output Operation
Mo d e s o f Op e ra t io n
When delivering high output currents, the MAX1649/
MAX1651 operate in continuous-conduction mode. In
this mode, current always flows in the inductor, and
the control circuit adjusts the switch duty cycle to main-
tain regulation without exceeding the switch current
capability (Figure 3). This provides excellent load-tran-
sient response and high efficiency.
__________________De s ig n P ro c e d u re
S e t t in g t h e Ou t p u t Vo lt a g e
The MAX1649/MAX1651 are preset for 5V and 3.3V out-
put voltages, respectively; tie FB to GND for fixed-output
operation. They may also be adjusted from 1.5V (the
reference voltage) to the input voltage, using external
resistors R2 and R3 configured as shown in Figure 4. For
adjustable-output operation, 150kΩ is recommended for
resistor R3—high enough to avoid wasting energy, yet
low enough to avoid RC delays caused by parasitic
capacitance at FB. R2 is given by:
In discontinuous-conduction mode, current through the
ind uc tor s ta rts a t ze ro, ris e s to a p e a k va lue , the n
ramps down to zero. Although efficiency is still excel-
lent, the output ripple increases slightly, and the switch
waveform exhibits ringing (at the inductor's self-reso-
nant frequency). This ringing is to be expected and
poses no operational problems.
V
OUT
——— -1
R2 = R3 x
(
)
V
REF
where V
= 1.5V.
REF
Dro p o u t
The MAX1649/MAX1651 are in dropout when the input
voltage (V+) is low enough that the output drops below
the minimum outp ut volta g e s p e c ific a tion (s e e
Electrical Characteristics). The dropout voltage is the
difference between the input and output voltage when
d rop out oc c urs . Se e the Typ ic a l Op e ra ting
Cha ra c te ris tic s for the Drop out Volta g e vs . Loa d
Current and Dropout Voltage vs. Temperature graphs.
When using external resistors, it does no harm to con-
nect OUT and the output together, or to leave OUT
unconnected.
Cu rre n t -S e n s e Re s is t o r S e le c t io n
The current-sense resistor limits the peak switch cur-
rent to 110mV/R
, where R
is the value of
SENSE
SENSE
the current-sense resistor, and 110mV is the current-
limit trip level (see Electrical Characteristics).
8
_______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
