MAX1710/MAX1711/MAX1712
High-Speed, Digitally Adjusted
Step-Down Controllers for Notebook CPUs
minimize the energy transferred from inductor to capaci-
tor during load-step recovery. Even so, the amount of
overshoot is high enough (80mV) that for the MAX1710,
it’s wise to disable OVP or use the MAX1711/MAX1712
with their fixed 2.25V overvoltage protection threshold to
avoid tripping the fault latch (see the overshoot equation
in the Output Capacitor Selection section). The efficiency
penalty for operating at 550kHz is about 2% to 3%,
depending on the input voltage.
Two optional 1kΩ resistors are placed in series with FB
and FBS. These resistors prevent the negative output
voltage spike (that results from tripping OVP) from
pulling SHDN low via its internal ESD diode, which tends
to clear the fault latch, causing “hiccup” restarts.
Setting V
OUT
with a Resistor-Divider
The output voltage can be adjusted with a resistor-
divider rather than the DAC if desired (Figure 8). The
drawback of this practice is that the on-time doesn’t
automatically receive correct compensation for changing
output voltage levels. This can result in variable switch-
ing frequency as the resistor ratio is changed and/or
excessive switching frequency. The equation for adjust-
ing the output voltage is:
where V
FB
is the currently selected DAC value. When
using external resistors, FBS remote sensing is not rec-
ommended, but GNDS remote sensing is still possible.
Connect FBS to FB and GNDS to remote ground loca-
tion. In resistor-adjusted circuits, the DAC code should
be set as close as possible to the actual output voltage
so that the switching frequency doesn’t become exces-
sive. For highest accuracy, use the MAX1710 when
adjusting V
OUT
with external resistors. The MAX1710 FB
node has very high impedance, while the MAX1711/
MAX1712 have a 180kΩ ±35% FB impedance, which
degrades V
OUT
accuracy.
Adjusting V
OUT
Above 2V
The feed-forward circuit that makes the on-time depen-
dent on battery voltage maintains a nearly constant
switching frequency as V
IN
, I
LOAD
, and the DAC code
are changed. This works extremely well as long as FB is
connected directly to the output.
When the output is adjusted higher than 2V with a resis-
tor-divider, the switching frequency can be increased to
relatively unreasonable levels as the actual off-time
decreases and isn’t compensated for by a change in on-
time; 3.3V is about the maximum limit to the practical
adjustment range. Even at the slowest TON setting and
with the DAC set to 2V, the switching rate will exceed
600kHz.
The trip threshold for output overvoltage protection
scales with the nominal output voltage setting.
2-Stage (5V-Powered) Notebook CPU
Buck Regulator
The most efficient and overall cost-effective solution for
stepping down a high-voltage battery to very low output
voltage is to use a single-stage buck regulator that’s
powered directly from the battery. However, there may
be situations where the battery bus can’t be routed near
the CPU, or where space constraints dictate the smallest
possible local DC-DC converter. In such cases, the 5V-
powered circuit of Figure 9 may be appropriate. The
reduced input voltage allows a higher switching frequen-
cy and a much smaller inductor value.
Dynamic DAC Code Changes
(MAX1711/MAX1712)
Changing the output voltage dynamically by switching
DAC codes “on-the-fly” can be used to help make
power-savings/performance trade-offs in the host sys-
tem. Several important design issues arise from this
practice.
First, know that attempting to slew the output upward
quickly causes large current surges at the battery as the
IC goes into output current limiting during the transition.
Surge currents can be controlled either by counting the
DAC code slowly (50kHz or slower rate suggested), or
by modulating the I
LIM
current-limit threshold.
The DAC inputs must be driven quickly to the new value
so the device doesn’t wrongly interpret a disallowed
