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
V
IN
4.5V TO 5.5V
1µF
20Ω
C1
4 x 10µF/25V
1µF
V+ V
I
V
DD
LIM CC
SHDN
D0
ON/OFF
BST
DH
IRF7805
D1
D/A
INPUTS
D2
V
OUT
0.1µF
L1
0.5µH
1.6V AT 7A
D3
C2
D4**
MAX1710
MAX1711
LX
3 x 470µF
KEMET
T510
0.22µF
470pF
REF
CC
DL
IRF7805
PGND
0/MAX71
FB
V
CC
1k
GND
1k
TO REMOTE
LOAD
100k
GNDS
FBS
PGOOD
TON SKIP OVP*
* MAX1710 ONLY
** MAX1711 ONLY
Figure 9. 5V-Powered, 7A CPU Buck Regulator
where V is the currently selected DAC value. When
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.
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
The trip threshold for output overvoltage protection
scales with the nominal output voltage setting.
adjusting V
with external resistors. The MAX1710 FB
2 -S t a g e (5 V-P o w e re d ) No t e b o o k CP U
Bu c k Re g u la t o r
OUT
node has very high impedance, while the MAX1711 has
a 180kΩ ±35% FB impedance, which degrades V
accuracy.
OUT
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.
Ad ju s t in g V
Ab o ve 2 V
OUT
The feed-forward circuit that makes the on-time depen-
dent on battery voltage maintains a nearly constant
switching frequency as V , I
are changed. This works extremely well as long as FB is
connected directly to the output.
, and the DAC code
IN LOAD
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 ______________________________________________________________________________________
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
