Lo w -No is e , 1 4 V In p u t , 1 A, P WM
S t e p -Do w n Co n ve rt e rs
inductor peak current during the first half of each cycle,
b a s e d on the outp ut e rror volta g e . 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.
load, the frequency decreases based on the following
equation:
f = V
/ (V · 290ns)
IN
OUT
At me d ium- to full-loa d c urre nt (>100mA), V c a n
IN
increase slightly higher before the frequency decreases.
S yn c h ro n o u s Re c t ific a t io n
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.
P FM Co n t ro l
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
tha t the outp ut volta g e ha s d rop p e d too low. The
P-channel MOSFET switch turns on and conducts cur-
re nt to the outp ut-filte r c a p a c itor a nd loa d . The
MAX1684/MAX1685 then wait until the PFM comparator
senses a low output voltage again.
Cu rre n t Lim it a n d S o ft -S t a rt
The volta g e a t ILIM/SS s e ts the PWM c urre nt limit
4/MAX1685
(I
LIM
= 1.75A) and the low-power current limit (I
=
LIMLP
380mA). The PWM c urre nt limit a p p lie s whe n the
device is in PWM mode, in synchronized PWM mode,
or delivering a heavy load in normal mode (Table 1).
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-
ma l d uring tra ns ition from PFM to PWM with loa d s
around 150mA, and it has no adverse impact on regu-
lation.
The I
limit applies when the device is in low-power
LIMLP
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
· R
·
LIM
ILIM/SS
4µA) / V
and is adjustable from 0.5A to 1.75A.
REF
1 0 0 % Du t y-Cyc le Op e ra t io n
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-
The low-power current-limit threshold is equal to (I
·
LIMLP
R
· 4µA) / V
and is adjustable from 110mA to
ILIM/SS
REF
380mA.
For example, when R
limit threshold is 0.88A and the low-power current limit
threshold is 0.19A.
is 156kΩ, the PWM current
ILIM/SS
mately 0.35V (I
= 1A).
OUT
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:
Ve ry Lo w Du t y-Cyc le Op e ra t io n
Because of the P-channel minimum on-time and dead-
time (d ura tion whe n b oth s witc he s a re off), the
MAX1684/MAX1685’s s witc hing fre q ue nc y mus t
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
t
= C
(1.25V / 4µA)
ILIM/SS
SOFT-START
where t
is the time from SHDN going high to
SOFT-START
the regulator being able to supply full load current. For
example, a 0.1µF capacitor yields 31ms of soft-start.
load for V up to 10V
and 5V , respectively (see
OUT
IN
OUT
PWM Fixed-Frequency Operation Area graph in the
Typical Operating Characteristics). For higher V at no
IN
10 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
