MAX1711 Maxim, MAX1711 Datasheet - Page 22

• 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
where V
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
node has very high impedance, while the MAX1711 has
a 180kΩ ±35% FB impedance, which degrades V
accuracy.
The feed-forward circuit that makes the on-time depen-
dent on battery voltage maintains a nearly constant
switching frequency as V
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
22
Figure 9. 5V-Powered, 7A CPU Buck Regulator
______________________________________________________________________________________
FB
OUT
is the currently selected DAC value. When
** MAX1711 ONLY
* MAX1710 ONLY
with external resistors. The MAX1710 FB
Adjusting V
IN
ON/OFF
INPUTS
, I
D/A
100k
LOAD
0.22 F
V
470pF
CC
, and the DAC code
1 F
OUT
SHDN
D0
D1
D2
D3
D4**
REF
CC
PGOOD
I
LIM
TON
Above 2V
V
CC
MAX1710
MAX1711
20
SKIP
OUT
V+
OVP*
PGND
GNDS
V
GND
BST
FBS
DD
DH
LX
DL
FB
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.
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.
1k
0.1 F
1 F
TO REMOTE
LOAD
2-Stage (5V-Powered) Notebook CPU
4.5V TO 5.5V
1k
V
IN
IRF7805
IRF7805
C1
4 x 10 F/25V
0.5 H
L1
C2
3 x 470 F
KEMET
T510
1.6V AT 7A
V
Buck Regulator
OUT