Low-Noise, 5.5V-Input,
PWM Step-Down Regulator
MAX1692
amounts of reverse current to flow from the output dur-
ing light loads. This allows regulation with a constant-
switching frequency and eliminates minimum load
requirements. The NEGLIM comparator threshold is
50mA if V
FB
< 1.25V, and decreases as V
FB
exceeds
1.25V to prevent the output from rising. The NEGLIM
threshold in PFM mode is fixed at 50mA. (See
Forced
PWM and PWM/PFM Operation
section.)
SYNC Input and Frequency Control
The MAX1692’s internal oscillator is set for a fixed-
switching frequency of 750kHz or can be synchronized
to an external clock. Connect SYNC to IN for forced-
PWM operation. Do not leave SYNC/PWM unconnect-
ed. Connecting SYNC/PWM to GND enables PWM/PFM
operation to reduce supply current at light loads.
SYNC/PWM is a negative-edge triggered input that
allows synchronization to an external frequency ranging
between 500kHz and 1000kHz. When SYNC/PWM is
clocked by an external signal, the converter operates in
forced PWM mode. If SYNC is low or high for more than
100µs, the oscillator defaults to 750kHz.
Forced PWM and PWM/PFM Operation
Connect SYNC/PWM to IN for normal forced PWM
operation. Forced PWM operation is desirable in sensi-
tive RF and data-acquisition applications, to ensure that
switching-noise harmonics do not interfere with sensi-
tive IF and data-sampling frequencies. A minimum load
is not required during forced PWM operation, since the
synchronous rectifier passes reverse-inductor current
as needed to allow constant-frequency operation with
no load. Forced PWM operation uses higher supply
current with no load (2mA typ).
Connecting SYNC/PWM to GND enables PWM/PFM
operation. This proprietary control scheme overrides
PWM mode and places the MAX1692 in PFM mode at
light loads to improve efficiency and reduce quiescent
current to 85µA. With PWM/PFM enabled, the MAX1692
initiates pulse-skipping PFM operation when the peak
inductor current drops below 120mA. During PFM oper-
ation, the MAX1692 switches only as needed to service
the load, reducing the switching frequency and associ-
ated losses in the internal switch, the synchronous rec-
tifier, and the external inductor.
During PFM mode, a switching cycle initiates when the
PFM comparator senses that the output voltage has
dropped too low. The P-channel MOSFET switch turns
on and conducts current to the output-filter capacitor
and load until the inductor current reaches the PFM
peak current limit (120mA). Then the switch turns off
and the magnetic field in the inductor collapses, forcing
current through the synchronous rectifier to the output
filter capacitor and load. Then the MAX1692 waits until
the PFM comparator senses a low output voltage again.
The PFM current comparator controls both entry into
PWM mode and the peak switching current during PFM
mode. Consequently, some jitter is normal during tran-
sition from PFM to PWM modes with loads around
100mA, and it has no adverse impact on regulation.
Output ripple is higher during PFM operation. A larger
output-filter capacitor can be used to minimize ripple.
Shutdown Mode
Connecting
SHDN
to GND places the MAX1692 in
shutdown mode. In shutdown, the reference, control
circuitry, internal switching MOSFET, and the synchro-
nous rectifier turn off and the output falls to 0V. Connect
SHDN
to IN for normal operation.
Current-Sense Comparators
The MAX1692 uses several internal current-sense com-
parators. In PWM operation, the PWM comparator sets
the cycle-by-cycle current limit (Figure 1) and provides
improved load and line response, allowing tighter spec-
ification of the inductor-saturation current limit to
reduce inductor cost. A second 120mA current-sense
comparator used across the P-channel switch controls
entry into PFM mode. A third current-sense comparator
monitors current through the internal N-channel MOSFET
to set the NEGLIM threshold and determine when to turn
off the synchronous rectifier. A fourth comparator (LIM)
used at the P-channel MOSFET switch detects overcur-
rent. This protects the system, external components, and
internal MOSFETs under overload conditions.
Applications Information
Output Voltage Selection
Select an output voltage between 1.25V and V
IN
by
connecting FB to a resistor-divider between the output
and GND (Figure 2). Select feedback resistor R2 in the
5kΩ to 500kΩ range. R1 is then given by:
R1 = R2 [(V
OUT
/ V
FB
) - 1]
where V
FB
= 1.232V (See Note 2 of the
Electrical
Characteristics).
Add a small ceramic capacitor (C5)
around 47pF to 100pF in parallel with R1 to compensate
for stray capacitance at the FB pin and output capacitor
equivalent series resistance (ESR).
8
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