Datasheet

The overshoot during a full-load to no-load transient
due to stored inductor energy can be calculated as:
Applications Information
Dropout Performance (Buck)
The output-voltage adjustable range for continuous-
conduction operation is restricted by the nonadjustable
minimum off-time one-shot. For best dropout perfor-
mance, use the slower (200kHz) on-time setting. When
working with low input voltages, the duty-factor limit
must be calculated using worst-case values for on- and
off-times. Manufacturing tolerances and internal propa-
gation delays introduce an error to the TON K-factor.
This error is greater at higher frequencies (see Table
1). Also, keep in mind that transient-response perfor-
mance of buck regulators operated too close to
dropout is poor, and bulk output capacitance must
often be added (see the V
SAG
equation in the Design
Procedure section).
The absolute point of dropout is when the inductor cur-
rent ramps down during the minimum off-time (∆I
DOWN
)
as much as it ramps up during the on-time (∆I
UP
). The
ratio h = ∆I
UP
/ ∆I
DOWN
indicates the controller’s ability
to 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 trade-offs between V
SAG
, output
capacitance, and minimum operating voltage. For a
given value of h, the minimum operating voltage can be
calculated as:
where 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, and K is taken from Table 1.
The absolute minimum input voltage is calculated with
h = 1.
If the calculated V
IN(MIN)
is greater than the required
minimum input voltage, then the operating frequency
must be reduced or output capacitance added to
obtain an acceptable V
SAG
. If operation near dropout is
anticipated, calculate V
SAG
to be sure of adequate
transient response.
A dropout design example follows:
V
OUT
= 2.5V
f
SW
= 600kHz
K = 1.7µs
t
OFF(MIN)
= 450ns
V
DROP1
= V
DROP2
= 100mV
h = 1.5
Voltage Positioning (Buck)
In applications where fast-load transients occur, the
output voltage changes instantly by R
ESR
× C
OUT
×
∆I
LOAD
. Voltage positioning allows the use of fewer out-
put capacitors for such applications, and maximizes
the output-voltage AC and DC tolerance window in
tight-tolerance applications.
Figure 9 shows the connection of OUT and FB in a volt-
age-positioned circuit. In nonvoltage-positioned cir-
cuits, the MAX8632 regulates at the output capacitor. In
voltage-positioned circuits, the MAX8632 regulates on
the inductor side of the voltage-positioning resistor.
V
OUT
is reduced to:
PC Board Layout Guidelines
Careful PC board layout is critical to achieve low
switching losses and clean, stable operation. The
switching power stage requires particular attention. If
possible, mount all the power components on the top
side of the board, with their ground terminals flush
against one another. Follow these guidelines for good
PC board layout:
• Keep the high-current paths short, especially at the
ground terminals. This practice is essential for sta-
ble, jitter-free operation.
• Keep the power traces and load connections short.
This practice is essential for high efficiency. Using
VV RI
OUT VPS OUT NO LOAD POS LOAD() (_ )
=× -
V
VV
ns
s
VV V
IN MIN()
. .
.
.
. . .=
+
×




















+=
25 01
1
15 450
17
01 01 43
-
-
µ
V
VV
ht
K
VV
IN MIN
OUT DROP
OFF MIN
DROP DROP()
()
=
+
×




















+
1
21
1-
-
V
IL
CV
SOAR
LOAD MAX
OUT OUT
=
×
××
()
∆
2
2
MAX8632
Integrated DDR Power-Supply Solution for
Desktops, Notebooks, and Graphic Cards
______________________________________________________________________________________ 25