MAX1684 Datasheet, PDF (10/16 Pages) Maxim Integrated Products – Low-Noise, 14V Input, 1A, PWM Step-Down Converters

Low-Noise, 14V Input, 1A, PWM
Step-Down Converters
inductor peak current during the first half of each cycle,
based on the output error voltage. The MAX1684/
MAX1685’s loop gain is relatively low to enable the use
of a small, low-valued output filter capacitor. The 1.4%
transient load regulation from 0 to 1A is compensated
by an integrator circuit that lowers DC load regulation
to 0.01% typical. Slope compensation accounts for the
inductor-current waveform’s down slope during the
second half of each cycle, and eliminates the inductor-
current staircasing characteristic of current-mode con-
trollers at high duty cycles.
PFM Control
In low-power mode, the MAX1684/MAX1685 devices
switch only as needed to service the load. This reduces
the switching frequency and associated losses in the
P-channel switch, the synchronous rectifier, and the
external inductor. During this PFM operation, a switch-
ing cycle initiates when the PFM comparator senses
that the output voltage has dropped too low. The
P-channel MOSFET switch turns on and conducts cur-
rent to the output-filter capacitor and load. The
MAX1684/MAX1685 then wait until the PFM comparator
senses a low output voltage again.
In normal mode at light load (<150mA), the device also
operates in PFM. The PFM current comparator controls
both entry into PWM mode and the peak switch current
during PFM operation. Consequently, some jitter is nor-
mal during transition from PFM to PWM with loads
around 150mA, and it has no adverse impact on regu-
lation.
100% Duty-Cycle Operation
As the input voltage drops, the duty cycle increases
until the P-channel MOSFET turns on continuously,
achieving 100% duty cycle. Dropout voltage in 100%
duty cycle is the output current multiplied by the on-
resistance of the internal switch and inductor, approxi-
mately 0.35V (I_OUT = 1A).
Very Low Duty-Cycle Operation
Because of the P-channel minimum on-time and dead-
time (duration when both switches are off), the
MAX1684/MAX1685’s switching frequency must
decrease in PWM or normal mode to maintain regulation
at a very low duty cycle. The total P-channel on-time and
dead-time is 290ns typical. As a result, the MAX1684/
MAX1685 will maintain fixed-frequency regulation at no
load for V_IN up to 10V_OUT and 5V_OUT, respectively (see
PWM Fixed-Frequency Operation Area graph in the
Typical Operating Characteristics). For higher V_IN at no
load, the frequency decreases based on the following
equation:
f = V_OUT / (V_IN · 290ns)
At medium- to full-load current (>100mA), V_IN can
increase slightly higher before the frequency decreases.
Synchronous Rectification
Although the primary rectifier is an external Schottky
diode, a small internal N-channel synchronous rectifier
allows PWM operation at light loads. During the second
half of each cycle, when the inductor current ramps
below the zero-crossing threshold or when the oscilla-
tor period ends, the synchronous rectifier turns off. This
keeps excess current from flowing backward through
the inductor. Choose an appropriate inductor to limit
the PWM ripple current through the N-channel FET to
400mAp-p.
Current Limit and Soft-Start
The voltage at ILIM/SS sets the PWM current limit
(I_LIM = 1.75A) and the low-power current limit (I_LIMLP =
380mA). The PWM current limit applies when the
device is in PWM mode, in synchronized PWM mode,
or delivering a heavy load in normal mode (Table 1).
The I_LIMLP limit applies when the device is in low-power
mode. An internal 4µA current source pulls ILIM/SS up
to CVL. To use the maximum current-limit thresholds,
leave ILIM/SS unconnected or connect it to a soft-start
capacitor. Connect an external resistor from ILIM/SS to
AGND to adjust the current-limit thresholds.
The PWM current-limit threshold is (I_LIM · R_ILIM/SS ·
4µA) / V_REF and is adjustable from 0.5A to 1.75A.
The low-power current-limit threshold is equal to (I_LIMLP ·
R_ILIM/SS · 4µA) / V_REF and is adjustable from 110mA to
380mA.
For example, when R_ILIM/SS is 156kΩ, the PWM current
limit threshold is 0.88A and the low-power current limit
threshold is 0.19A.
Connect a low-value capacitor from ILIM/SS to AGND
to achieve soft-start, limiting inrush current. ILIM/SS
internally shorts to AGND in shutdown to discharge the
soft-start capacitor. Do not connect ILIM/SS to REF or
CVL. Determine the soft-start duration by:
t_SOFT-START = C_ILIM/SS(1.25V / 4µA)
where t_SOFT-START is the time from SHDN going high to
the regulator being able to supply full load current. For
example, a 0.1µF capacitor yields 31ms of soft-start.
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