Rainbow Electronics MAX887 - Manual (Page 9)

MAX887
100% Duty Cycle, Low-Noise,
Step-Down, PWM DC-DC Converter
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backward through the inductor, from the output filter
capacitor to GND, or through the switch and synchro-
nous rectifier to GND.
During PWM operation, the NEGLIM threshold adjusts
to permit small amounts of reverse current to flow from
the output during light loads. This allows regulation with
a constant switching frequency and eliminates mini-
mum load requirements. The NEGLIM comparator
threshold is 0mA if VFB < 1.25V, and decreases as VFB
exceeds 1.25V to prevent the output from rising. The
NEGLIM threshold in PFM mode is 0mA. (See
Forced
PWM and Idle Mode operation. )
Forced PWM and Idle Mode Operation
Connect SYNC to VL for normal forced PWM operation.
Forced PWM operation is desirable in sensitive RF and
data-acquisition applications, to ensure that switching-
noise harmonics do not interfere with sensitive IF and
data-sampling frequencies. A minimum load is not
required during forced PWM operation, since the syn-
chronous rectifier passes reverse inductor current as
needed to allow constant-frequency operation with no
load.
Connecting SYNC to GND enables Idle Mode opera-
tion. This proprietary control scheme places the
MAX887 in PFM mode at light loads to improve efficien-
cy and reduce quiescent current to 200µA typ. With
Idle Mode enabled, the MAX887 initiates PFM operation
when the output current drops below 100mA. During
PFM operation, the MAX887 switches only as needed
to service the load, reducing the switching frequency
and associated losses in the internal switch and
synchronous rectifier, Schottky diode, and external
inductor.
During PFM mode, a switching cycle is initiated 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 (100mA). Then the switch turns off
and the magnetic field in the inductor collapses, forcing
current through the output diode to the output filter
capacitor and load. The output filter capacitor stores
charge when the inductor current is high and releases
charge when it is low, smoothing the voltage across the
load. Then the MAX887 waits until the PFM comparator
senses a low output voltage again. During PFM mode,
the synchronous rectifier is disabled and the external
Schottky diode is used as an output rectifier.
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 has no adverse impact on regulation.
Output ripple is higher during PFM operation, and the
output filter capacitor should be selected on this basis
when PFM mode is used. Output ripple and noise are
higher during PFM operation.
SYNC Input and Frequency Control
The MAX887H comes with an internal oscillator set for a
fixed switching frequency of 300kHz. Connect SYNC to
VL for normal forced-PWM operation. Do not leave
SYNC floating. Connecting SYNC to GND enables Idle
Mode operation to reduce supply current at light loads.
SYNC is a logic-level input useful for operating-mode
selection and frequency control. It is a negative edge
triggered input that allows synchronization to an exter-
nal frequency between 25kHz and 440kHz. When
SYNC is clocked by an external signal, the converter
operates in PWM mode. If SYNC is low or high for more
than 100µs, the oscillator defaults to 300kHz. Operating
at a lower switching frequency reduces quiescent cur-
rent, but reduces maximum load current as well
(Table 1). For example, at 330kHz, maximum output
current is 600mA, while at 30kHz, maximum output cur-
rent is only 30mA. Note that 100% duty cycle will only
occur for f
SYNC > f OSC /4.
VL Regulator
The MAX887 uses an internal 3.3V linear regulator for
logic power in the IC. This logic supply is brought out
using the VL pin for bypassing and compensation with
an external 2.2µF capacitor to GND. Connect this
capacitor close to the MAX887, within 0.2in (5mm).
Shutdown
Connecting SHDN to GND places the MAX887 in a low-
current shutdown mode (I Q = 2.5µA typ at V+ = 7V). In
shutdown, the reference, VL regulator, control circuitry,
internal switching MOSFET, and the synchronous recti-
fier turn off and the output falls to 0V. Connect
SHDN to
V+ for normal operation.
Current-Sense Comparators
Several internal current-sense comparators are used
inside the MAX887. In PWM operation, the PWM com-
parator is used for current-mode control. Current-mode
control imparts cycle-by-cycle current limiting and pro-
vides improved load and line response, allowing tighter
specification of the inductor saturation current limit to
reduce inductor cost. A second 100mA current-sense
comparator is used across the P-channel switch to con-
trol entry into PFM mode. A third current-sense com-
parator monitors current through the internal N-channel
MOSFET to set the NEGLIM threshold and determine