MAX1649 Maxim, MAX1649 Datasheet - Page 8

• Manufacturer Part Number: MAX1649
• Description: 5V/3.3V or Adjustable / High-Efficiency / Low-Dropout / Step-Down DC-DC Controllers
• Manufacturer: Maxim

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.
In discontinuous-conduction mode, current through the
inductor starts at zero, rises to a peak value, then
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.
The MAX1649/MAX1651 are in dropout when the input
voltage (V+) is low enough that the output drops below
the minimum output voltage specification (see
Electrical Characteristics ). The dropout voltage is the
difference between the input and output voltage when
dropout
Characteristics for the Dropout Voltage vs. Load
Current and Dropout Voltage vs. Temperature graphs.
5V/3.3V or Adjustable, High-Efficiency,
Low-Dropout, Step-Down DC-DC Controllers
Figure 3. MAX1649 Continuous-Conduction Mode, Heavy
Load-Current Waveform (500mA/div)
8
_______________________________________________________________________________________
V+ = 10V, I
CIRCUIT OF FIGURE 1, R1 = 75m
occurs.
LOAD
= 1.3A
See
2 s/div
the
Modes of Operation
Typical
Operating
Dropout
0A
1.5A
1A
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:
where V
When using external resistors, it does no harm to con-
nect OUT and the output together, or to leave OUT
unconnected.
The current-sense resistor limits the peak switch cur-
rent to 110mV/R
the current-sense resistor, and 110mV is the current-
limit trip level (see Electrical Characteristics ).
Figure 4. Adjustable-Output Operation
__________________Design Procedure
V
V
IN
REF
R2 = R3
C3
0.1 F
3
4
REF
= 1.5V
SHDN
REF
(
= 1.5V.
Current-Sense Resistor Selection
V
V
MAX1649
MAX1651
OUT
REF
GND
– 1
SENSE
R2 = R3 x
8
)
Setting the Output Voltage
OUT
EXT
CS
V+
FB
, where R
2
5
6
7
1
1N5820
0.05
(
——— -1
V
V
R1
D1
OUT
REF
SENSE
P1
Si9430
)
0.1 F
C4
47 H
is the value of
L1
330 F
R2
R3
150k
C2
100 F
C1
OUTPUT
@ 1.5A