Hig h -S p e e d , Dig it a lly Ad ju s t e d
S t e p -Do w n Co n t ro lle rs fo r No t e b o o k CP Us
-I
PEAK
∆i
∆t
V
-V
BATT OUT
=
L
-I
PEAK
I
LOAD
I
= I
/2
LOAD PEAK
I
LIMIT
LX-PGND I
THRESHOLD = 100mV (NOMINAL, DEFAULT)
LIMIT
VOLTAGE DROP ACROSS Q2
0
TIME
0
ON-TIME
TIME
Figure 4. ‘‘Valley’’ Current-Limit Threshold Point
Figure 3. Pulse-Skipping/Discontinuous Crossover Point
about 1/4 full load. The crossover point occurs at an
even lower value if a swinging (soft-saturation) inductor
is used.
the c urre nt-s e ns e s ig na l is a b ove the c urre nt-limit
threshold, the PWM is not allowed to initiate a new cycle
(Figure 4). The actual peak current is greater than the
current-limit threshold by an amount equal to the induc-
tor ripple current. Therefore the exact current-limit char-
acteristic and maximum load capability are a function of
the MOSFET on-resistance, inductor value, and battery
voltage. The reward for this uncertainty is robust, loss-
less overcurrent sensing. When combined with the UVP
protection circuit, this current-limit method is effective in
almost every circumstance.
0/MAX71
The switching waveforms may appear noisy and asyn-
chronous when light loading causes pulse-skipping
operation, but this is a normal operating condition that
results in high light-load efficiency. Trade-offs in PFM
noise vs. light-load efficiency can be made by varying
the inductor value. Generally, low inductor values pro-
duce a broader efficiency vs. load curve, while higher
values result in higher full-load efficiency (assuming that
the coil resistance remains fixed) and less output voltage
ripple. Penalties for using higher inductor values include
la rg e r p hys ic a l s ize a nd d e g ra d e d loa d -tra ns ie nt
response (especially at low input voltage levels).
There is also a negative current limit that prevents exces-
sive reverse inductor currents when V
is sinking cur-
OUT
re nt. The ne g a tive c urre nt-limit thre s hold is s e t to
approximately 120% of the positive current limit, and
therefore tracks the positive current limit when ILIM is
adjusted.
SKIP
Fo rc e d -P WM Mo d e (
= Hig h )
The low-noise, forced-PWM mode (SKIP driven high) dis-
ables the zero-crossing comparator, which controls the
low-side switch on-time. This causes the low-side gate-
drive waveform to become the complement of the high-
s id e g a te -d rive wa ve form. This in turn c a us e s the
inductor current to reverse at light loads, as the PWM
The current-limit threshold can be adjusted with an exter-
nal resistor (R ) at ILIM. A precision 5µA pull-up cur-
LIM
rent source at ILIM sets a voltage drop on this resistor,
a d jus ting the c urre nt-limit thre s hold from 50mV to
200mV. In the adjustable mode, the current-limit thresh-
old voltage is precisely 1/10th the voltage seen at ILIM.
loop strives to maintain a duty ratio of V /V . The
OUT IN
Therefore, choose R
equal to 2kΩ/mV of the current-
LIM
benefit of forced-PWM mode is to keep the switching fre-
quency fairly constant, but it comes at a cost: the no-
load battery current can be as high as 40mA or more.
limit threshold. The threshold defaults to 100mV when
ILIM is tied to V . The logic threshold for switchover to
CC
the 100mV default value is approximately V - 1V.
CC
Forced-PWM mode is most useful for reducing audio-fre-
quency noise, improving load-transient response, pro-
viding sink-current capability for dynamic output voltage
adjustment, and improving the cross-regulation of multi-
ple-output applications that use a flyback transformer or
coupled inductor.
The a d jus ta b le c urre nt limit c a n a c c ommod a te
MOSFETs with atypical on-resistance characteristics
(see Design Procedure).
A capacitor in parallel with R
soft-start function.
can provide a variable
LIM
Cu rre n t -Lim it Circ u it (ILIM)
The current-limit circuit employs a unique “valley” cur-
rent-sensing algorithm that uses the on-state resistance
of the low-side MOSFET as a current-sensing element. If
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors don’t corrupt the cur-
rent-sense signals seen by LX and PGND. The IC must
be mounted close to the low-side MOSFET with short,
14 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
