MAX1711 Maxim, MAX1711 Datasheet - Page 14

• Manufacturer Part Number: MAX1711
• Description: High-Speed / Digitally Adjusted Step-Down Controllers for Notebook CPUs
• Manufacturer: Maxim

High-Speed, Digitally Adjusted
Step-Down Controllers for Notebook CPUs
about 1/4 full load. The crossover point occurs at an
even lower value if a swinging (soft-saturation) inductor
is used.
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
larger physical size and degraded load-transient
response (especially at low input voltage levels).
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-
side gate-drive waveform. This in turn causes the
inductor current to reverse at light loads, as the PWM
loop strives to maintain a duty ratio of V
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.
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 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
14
Figure 3. Pulse-Skipping/Discontinuous Crossover Point
______________________________________________________________________________________
0
ON-TIME
i
t
Forced-PWM Mode (
=
V
BATT
Current-Limit Circuit (ILIM)
-V
L
OUT
TIME
-I
PEAK
S S K K I I P P = High)
I
LOAD
OUT
= I
/V
PEAK
IN
/2
. The
Figure 4. ‘‘Valley’’ Current-Limit Threshold Point
the current-sense signal is above the current-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.
There is also a negative current limit that prevents exces-
sive reverse inductor currents when V
rent. The negative current-limit threshold is set to
approximately 120% of the positive current limit, and
therefore tracks the positive current limit when ILIM is
adjusted.
The current-limit threshold can be adjusted with an exter-
nal resistor (R
rent source at ILIM sets a voltage drop on this resistor,
adjusting the current-limit threshold from 50mV to
200mV. In the adjustable mode, the current-limit thresh-
old voltage is precisely 1/10th the voltage seen at ILIM.
Therefore, choose R
limit threshold. The threshold defaults to 100mV when
ILIM is tied to V
the 100mV default value is approximately V
The adjustable current limit can accommodate
MOSFETs with atypical on-resistance characteristics
(see Design Procedure ).
A capacitor in parallel with R
soft-start function.
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,
0
LIM
LX-PGND I
VOLTAGE DROP ACROSS Q2
CC
) at ILIM. A precision 5µA pull-up cur-
. The logic threshold for switchover to
LIMIT
LIM
THRESHOLD = 100mV (NOMINAL, DEFAULT)
TIME
equal to 2kΩ/mV of the current-
LIM
can provide a variable
OUT
-I
PEAK
I
I
LOAD
LIMIT
is sinking cur-
CC
- 1V.