Datasheet

MAX8632
remains fixed) and less output voltage ripple. Penalties
for using higher inductor values include larger physical
size and degraded load-transient response, especially
at low input-voltage levels.
DC output accuracy specifications refer to the threshold
of the error comparator. When the inductor is in continu-
ous conduction, the MAX8632 regulates the valley of the
output ripple, so the actual DC output voltage is higher
than the trip level by 50% of the output ripple voltage. In
discontinuous conduction (SKIP = GND and I
LOAD
<
I
LOAD(SKIP)
), the output voltage has a DC regulation
level higher than the error-comparator threshold by
approximately 1.5% due to slope compensation.
Forced-PWM Mode ( SKIP = AV
DD
)
The low-noise forced-PWM mode (SKIP = AV
DD
) dis-
ables the zero-crossing comparator, which controls the
low-side switch on-time. This forces the low-side gate-
drive waveform to constantly be the complement of the
high-side gate-drive waveform, so the inductor current
reverses at light loads while DH maintains a duty factor
of V
OUT
/ V
IN
. Forced-PWM mode keeps the switching
frequency fairly constant. However, forced-PWM opera-
tion comes at a cost where the no-load V
DD
bias cur-
rent remains between 2mA and 20mA due to the
external MOSFET’s gate charge and switching frequen-
cy. Forced-PWM mode is most useful for reducing
audio frequency noise, improving load-transient
response, and providing sink-current capability for
dynamic output-voltage adjustment.
Current-Limit Buck Regulator (ILIM)
Valley Current Limit
The current-limit circuit for the buck regulator portion of
the MAX8632 employs a unique “valley” current-sensing
algorithm that senses the voltage drop across LX and
PGND1 and uses the on-resistance of the rectifying
MOSFET (Q2 in Figure 8) as the current-sensing ele-
ment. If the magnitude of the current-sense signal is
above the valley current-limit threshold, the PWM con-
troller is not allowed to initiate a new cycle (Figure 4).
With valley current-limit sensing, the actual peak current
is greater than the valley current-limit threshold by an
amount equal to the inductor current ripple. Therefore,
the exact current-limit characteristic and maximum load
capability are a function of the current-sense resistance,
inductor value, and input voltage. When combined with
the undervoltage-protection circuit, this current-limit
method is effective in almost every circumstance.
In forced-PWM mode, the MAX8632 also implements a
negative current limit to prevent excessive reverse induc-
tor currents when the buck regulator output is sinking
current. The negative current-limit threshold is set to
approximately 120% of the positive current limit and
tracks the positive current limit when V
ILIM
is adjusted.
The current-limit threshold is adjusted with an external
resistor-divider at ILIM. A 2µA to 20µA divider current is
recommended for accuracy and noise immunity.
The current-limit threshold adjustment range is from
25mV to 200mV. In the adjustable mode, the current-
limit threshold voltage (from PGND1 to LX) is precisely
1/10th the voltage seen at ILIM. The threshold defaults
to 50mV when ILIM is connected to AV
DD
. The logic
threshold for switchover to the 50mV default value is
approximately AV
DD
- 1V.
Carefully observe the PC board layout guidelines to
ensure that noise and DC errors do not corrupt the differ-
ential current-sense signals seen between LX and GND.
POR, UVLO, and Soft-Start
Internal power-on reset (POR) occurs when AV
DD
rises
above approximately 2V, resetting the fault latch and
the soft-start counter, powering up the reference, and
preparing the buck regulator for operation. Until AV
DD
reaches 4.25V (typ), AV
DD
undervoltage-lockout
(UVLO) circuitry inhibits switching. The controller
inhibits switching by pulling DH low and holding DL low
Integrated DDR Power-Supply Solution for
Desktops, Notebooks, and Graphic Cards
14 ______________________________________________________________________________________
Figure 2. Pulse-Skipping/Discontinuous Crossover Point
INDUCTOR CURRENT
ON-TIME0 TIME
I
PEAK
I
LOAD
= I
PEAK
/ 2
∆I
∆t
V
IN
- V
OUT
L
=