Rainbow Electronics MAX1712 - Manual (Page 15)

MAX1710/MAX1711/MAX1712
High-Speed, Digitally Adjusted
Step-Down Controllers for Notebook CPUs
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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
OUT is sinking cur-
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
LIM ) at ILIM. A precision 5µA pullup current
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
LIM equal to 2k Ω/mV of the current-
limit threshold. The threshold defaults to 100mV when
ILIM is tied to V
CC . The logic threshold for switchover to
the 100mV default value is approximately V
CC
- 1V.
The adjustable current limit can accommodate
MOSFETs with atypical on-resistance characteristics
(see Design Procedure).
A capacitor in parallel with R LIM can provide a variable
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,
direct traces making a Kelvin-sense connection to the
source and drain terminals.
MOSFET Gate Drivers (DH, DL)
The DH and DL drivers are optimized for driving moder-
ate-size, high-side and larger, low-side power MOSFETs.
This is consistent with the low duty factor seen in the
notebook CPU environment, where a large V
BATT - V OUT
differential exists. An adaptive dead-time circuit monitors
the DL output and prevents the high-side FET from turn-
ing on until DL is fully off. There must be a low-resis-
tance, low-inductance path from the DL driver to the
MOSFET gate in order for the adaptive dead-time circuit
to work properly. Otherwise, the sense circuitry in the
MAX1710/MAX1711/MAX1712 will interpret the MOSFET
gate as “off” while there is actually still charge left on the
gate. Use very short, wide traces measuring 10 to 20
squares (50 to 100 mils wide if the MOSFET is 1 inch
from the MAX1710/MAX1711/MAX1712).
The dead time at the other edge (DH turning off) is deter-
mined by a fixed 35ns (typ) internal delay.
The internal pulldown transistor that drives DL low is
robust, with a 0.5
Ω (typ) on-resistance. This helps pre-
vent DL from being pulled up during the fast rise time of
the inductor node, due to capacitive coupling from the
drain to the gate of the massive low-side synchronous-
rectifier MOSFET. However, you might still encounter
some combinations of high- and low-side FETs that will
cause excessive gate-drain coupling, which can lead to
efficiency-killing, EMI-producing shoot-through currents.
This can often be remedied by adding a resistor in series
with BST, which increases the turn-on time of the high-
side FET without degrading the turn-off time (Figure 5).
DAC Converter (D0–D4)
The DAC programs the output voltage. It receives a digi-
tal code from pins on the CPU module that are either
hard-wired to GND or left open-circuit. The
MAX1710/MAX1711/MAX1712 contain weak internal
BST
+5V
V BATT
5 Ω
DH
LX MAX1710
MAX1711
MAX1712
Figure 5. Reducing the Switching-Node Rise Time
Figure 4.”Valley’’ Current-Limit Threshold Point
INDUCTOR CURRENT
I LIMIT
I LOAD
0 TIME
LX-PGND I LIMIT THRESHOLD = 100mV (NOMINAL, DEFAULT)
VOLTAGE DROP ACROSS Q2
-I PEAK