MAX649/MAX651/MAX652
5V/3.3V/3V or Adjustable, High-Efficiency,
Low I
Q
, Step-Down DC-DC Controllers
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_______________Detailed Description
The MAX649/MAX651/MAX652 are BiCMOS, step-
down, switch-mode power-supply controllers that pro-
vide fixed outputs of 5V, 3.3V, and 3V, respectively.
Their unique control scheme combines the advantages
of pulse-frequency-modulation (low supply current)
and pulse-width-modulation (high efficiency at high
loads). An external P-channel power MOSFET allows
peak currents in excess of 3A, increasing the output
current capability over previous PFM devices. Figure 2
is the block diagram.
The MAX649/MAX651/MAX652 offer three main
improvements over prior solutions:
1) The converters operate with tiny (less than 9mm
diameter) surface-mount inductors, due to their
300kHz switching frequency.
2) The current-limited PFM control scheme allows
greater than 90% efficiencies over a wide range of
load currents (1.0mA to 1.5A).
3) The maximum supply current is only 100µA.
PFM Control Scheme
The MAX649/MAX651/MAX652 use a proprietary, cur-
rent-limited PFM control scheme. As with traditional
PFM converters, the external power MOSFET is turned
on when the voltage comparator senses that the output
is out of regulation. However, unlike traditional PFM
converters, switching is accomplished through the
combination of a peak current limit and a pair of one-
shots that set the maximum switch on-time (16µs) and
minimum switch off-time (2.3µs). Once off, the minimum
off-time one-shot holds the switch off for 2.3µs. After
this minimum time, the switch either 1) stays off if the
output is in regulation, or 2) turns on again if the output
is out of regulation.
The MAX649/MAX651/MAX652 also limit the peak induc-
tor current, which allows them to run in continuous-con-
duction mode and maintain high efficiency with heavy loads
(Figure 3a). This current-limiting feature is a key compo-
nent of the control circuitry. Once turned on, the switch
stays on until either 1) the maximum on-time one-shot turns
it off (16µs later), or 2) the current limit is reached.
To increase light-load efficiency, the current limit for
the first two pulses is set to half the peak current limit.
If those pulses bring the output voltage into regulation,
the voltage comparator holds the MOSFET off and the
current limit remains at half its peak. If the output vol-
tage is still out of regulation after two pulses, the
current limit for the next pulse is raised to its peak (Figure
3b). Calculate the peak current limit by dividing the
Current-Limit Trip Level (see
Electrical Characteristics
)
by the value of the current-sense resistor.
Shutdown Mode
When SHDN is high, the MAX649/MAX651/MAX652 enter
shutdown mode. In this mode, the internal biasing circuit-
ry is turned off (including the reference) and the supply
current drops to less than 5µA. EXT goes high, turning off
the external MOSFET. SHDN is a TTL/CMOS logic-level
input. Connect SHDN to GND for normal operation.
Quiescent Current
In normal operation, the quiescent current is less than
100µA. However, this current is measured by forcing
the external transistor switch off. In an actual applica-
tion, even with no load, additional current is drawn to
supply external feedback resistors (if used) and the
diode and capacitor leakage currents. In the circuit of
Figure 1, with V+ at 5V and V
OUT
at 3.3V, the typical
quiescent current is 90µA.
EXT Drive Voltage Range
EXT swings from V+ to GND and provides the drive out-
put for an external P-channel power MOSFET.
Modes of Operation
When delivering high output currents, the MAX649/
MAX651/MAX652 operate in continuous-conduction
mode (CCM). In this mode, current always flows in the
