Owner's manual

MAX8760
Dual-Phase, Quick-PWM Controller for AMD
Mobile Turion 64 CPU Core Power Supplies
36 ______________________________________________________________________________________
slew the inductor current higher in response to
increased load, and must always be greater than 1. As
h approaches 1, the absolute minimum dropout point,
the inductor current cannot increase as much during
each switching cycle and V
SAG
greatly increases
unless additional output capacitance is used.
A reasonable minimum value for h is 1.5, but adjusting
this up or down allows tradeoffs between V
SAG
, output
capacitance, and minimum operating voltage. For a
given value of h, the minimum operating voltage can be
calculated as:
where η
OUTPH
is the total number of out-of-phase switch-
ing regulators, V
VPS
is the voltage-positioning droop,
V
DROP1
and V
DROP2
are the parasitic voltage drops in
the discharge and charge paths (see the On-Time One-
Shot (TON) section), t
OFF(MIN)
is from the Electrical
Characteristics table, and K is taken from Table 6. The
absolute minimum input voltage is calculated with h = 1.
If the calculated V
IN(MIN)
is greater than the required min-
imum input voltage, then reduce the operating frequency
or add output capacitance to obtain an acceptable V
SAG
.
If operation near dropout is anticipated, calculate V
SAG
to
be sure of adequate transient response.
Dropout design example:
V
FB
= 1.4V
K
MIN
= 3µs for f
SW
= 300kHz
t
OFF(MIN)
= 400ns
V
VPS
= 3mV/A × 30A = 90mV
V
DROP1
= V
DROP2
= 150mV (30A load)
h = 1.5 and η
OUTPH
= 2
Calculating again with h = 1 gives the absolute limit of
dropout:
Therefore, V
IN
must be greater than 4.1V, even with very
large output capacitance, and a practical input voltage
with reasonable output capacitance would be 5V.
Vx
VmV mV
xsx s
mV mV mV V
IN MIN()
.
(. . / .
.
=
+






++=
2
14 90 150
12 04 10 30
150 150 90 4 07
-
-
-
µµ
Vx
VmV mV
xsx s
mV mV mV V
IN MIN()
.
(. . / .
.
=
+






++=
2
14 90 150
12 04 15 30
150 150 90 4 96
-
-
-
µµ
V
VV V
hxt
K
VVV
IN MIN
OUTPH
FB VPS DROP
OUTPH
OFF MIN
DROP DROP VPS
()
()
=
+


















++
η
η
-
-
-
1
21
1
MAIN
PHASE
SECOND
PHASE
PC BOARD TRACE
RESISTANCE
ERROR
COMPARATOR
R
F
R
A
R
A
R
B
R
B
OAIN+
OAIN-
FB
PC BOARD TRACE
RESISTANCE
CPU SENSE
POINT
CMP
CMN
CSP
CSN
L1
R
SENSE
R
FBS
L2
R
SENSE
MAX8760
Figure 10. Voltage-Positioning Gain