Datasheet

when OVP and shutdown discharge are disabled
(OVP/UVP = REF or GND) or forcing DL high when OVP
and shutdown discharge are enabled (OVP/UVP =
AV
DD
or OPEN). See Table 3 for a detailed truth table
for OVP/UVP and shutdown settings. When AV
DD
rises
above 4.25V, the controller activates the buck regulator
and initializes the internal soft-start.
The buck regulator’s internal soft-start allows a gradual
increase of the current-limit level during startup to
reduce the input surge currents. The MAX8632 divides
the soft-start period into five phases. During the first
phase, the controller limits the current limit to only 20%
of the full current limit. If the output does not reach reg-
ulation within 425µs, soft-start enters the second phase,
and the current limit is increased by another 20%. This
process repeats until the maximum current limit is
reached, after 1.7ms, or when the output reaches the
nominal regulation voltage, whichever occurs first.
Adding a capacitor in parallel with the external ILIM
resistors creates a continuously adjustable analog soft-
start function for the buck regulator’s output.
Soft-start in the LDO section can be realized by con-
necting a capacitor between the SS pin and ground.
When VTT is turned off or placed in standby mode, or
during thermal shutdown of the LDOs, the SS capacitor
is discharged. When VTT is turned on or when the ther-
mal limit is removed, an internal 4µA (typ) current
charges the SS capacitor. The resulting ramp voltage
on SS linearly increases the current-limit comparator
threshold to the VTT output, until full current limit is
attained when SS reaches approximately 1.6V. This
lowering of the current limit during startup limits the ini-
tial inrush current peaks, particularly when driving
capacitors. Choose the value of the SS capacitor
appropriately to set the soft-start time window. Leave
SS floating to disable the soft-start feature.
Power-OK (POK1)
POK1 is an open-drain output for a window comparator
that continuously monitors V
OUT
. POK1 is actively held
MAX8632
Integrated DDR Power-Supply Solution for
Desktops, Notebooks, and Graphic Cards
______________________________________________________________________________________ 15
Figure 4. Valley Current-Limit Threshold
INDUCTOR CURRENT
I
LOAD
I
LIMIT
0
TIME
I
PEAK
I
LOAD(MAX)
I
LIM(VAL) =
1 - x I
LOAD
LIR
2
()
Figure 3. Adjustable Current-Limit Threshold
ILIM
TO PWM
CONTROLLER
(SEE FIGURE 1)
V
DD
- 1V
MAX8632
LX
1.0V
C
REF
C
ILIM
R
A
R
B
REF