IC PWR SUPPLY DDR 28-TQFN

MAX8632ETI+T

Manufacturer Part NumberMAX8632ETI+T
DescriptionIC PWR SUPPLY DDR 28-TQFN
ManufacturerMaxim Integrated Products
MAX8632ETI+T datasheet
 


Specifications of MAX8632ETI+T

ApplicationsController, DDRVoltage - Input2 ~ 28 V
Number Of Outputs1Voltage - Output1.8V, 2.5V, 0.7 ~ 5.5 V
Operating Temperature-40°C ~ 85°CMounting TypeSurface Mount
Package / Case28-TQFN Exposed PadOutput Voltage1.8 V, 2.5 V, 0.7 V to 5.5 V
Output Current15 AInput Voltage2 V to 28 V
Mounting StyleSMD/SMTMaximum Operating Temperature+ 85 C
Minimum Operating Temperature- 40 CLead Free Status / RoHS StatusLead free / RoHS Compliant
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Integrated DDR Power-Supply Solution for
Desktops, Notebooks, and Graphic Cards
The on-time one-shot has good accuracy at the operat-
ing points specified in the Electrical Characteristics
table (approximately ±12.5% at 600kHz and 450kHz,
and ±10% at 200kHz and 300kHz). On-times at operat-
ing points far removed from the conditions specified in
the Electrical Characteristics table can vary over a
wider range. For example, the 600kHz setting typically
runs approximately 10% slower with inputs much
greater than 5V due to the very short on-times required.
The constant on-time translates only roughly to a con-
stant switching frequency. The on-times guaranteed in
the Electrical Characteristics table are influenced by
resistive losses and by switching delays in the high-
side MOSFET. Resistive losses, which include the
inductor, both MOSFETs, the output capacitor’s ESR,
and any PC board copper losses in the output and
ground, tend to raise the switching frequency as the
load increases. The dead-time effect increases the
effective on-time, reducing the switching frequency as
one or both dead times are added to the effective on-
time. The dead time occurs only in PWM mode (SKIP =
V
) and during dynamic output-voltage transitions
DD
when the inductor current reverses at light or negative
load currents. With reversed inductor current, the induc-
tor’s EMF causes LX to go high earlier than normal,
extending the on-time by a period equal to the DH-rising
dead time. For loads above the critical conduction point,
where the dead-time effect is no longer a factor, the
actual switching frequency is:
+
V
V
OUT
DROP
=
f
(
SW
+
t
V
V
ON IN
where V
is the sum of the parasitic voltage drops
DROP1
in the inductor discharge path, including the synchro-
nous rectifier, the inductor, and any PC board resis-
tances; V
is the sum of the resistances in the
DROP2
charging path, including the high-side switch (Q1 in
Figure 8), the inductor, and any PC board resistances,
and t
is the one-shot on-time (see the On-Time One-
ON
Shot (TON) section.
Automatic Pulse-Skipping Mode
In skip mode (SKIP = GND), an inherent automatic
switchover to PFM takes place at light loads (Figure 2).
This switchover is affected by a comparator that trun-
cates the low-side switch on-time at the inductor cur-
rent’s zero crossing. The zero-crossing comparator
differentially senses the inductor current across the
synchronous-rectifier MOSFET (Q2 in Figure 8). Once
V
- V
drops below 5% of the current-limit thresh-
PGND
LX
old (2.5mV for the default 50mV current-limit threshold),
______________________________________________________________________________________
the comparator forces DL low (Figure 1). This mecha-
nism causes the threshold between pulse-skipping
PFM and nonskipping PWM operation to coincide with
the boundary between continuous and discontinuous
inductor-current operation (also known as the critical
conduction point). The load-current level at which
PFM/PWM crossover occurs, I
half the peak-to-peak ripple current, which is a function
of the inductor value (Figure 2). This threshold is rela-
tively constant, with only a minor dependence on the
input voltage (V
I
where K is the on-time scale factor (see Table 1). For
example, in Figure 8 (K = 1.7µs, V
12V, and L = 1µH), the pulse-skipping switchover
occurs at:
The crossover point occurs at an even lower value if a
swinging (soft-saturation) inductor is used. The switching
waveforms can appear noisy and asynchronous when
light loading causes pulse-skipping operation, but this is
a normal operating condition that results in high light-
load efficiency. Trade-offs in PFM noise vs. light-load
efficiency are made by varying the inductor value.
1
Generally, low inductor values produce a broader effi-
)
ciency vs. load curve, while higher values result in higher
DROP
2
full-load efficiency (assuming that the coil resistance
Table 1. Approximate K-Factor Errors
TON SETTING
SKIP = GND)
(
200
(TON = AV
300
(TON = open)
450
(TON = REF)
600
(TON = GND)
LOAD(SKIP)
):
IN
×
V
K
V
-
OUT
IN
=
LOAD SKIP
(
)
2
L
V
OUT
×
µ
2 5
.
V
1 7
.
s
12
V
- .5V
2
 =
1 68
×
µ
2
1
H
12
V
MINIMUM V
TYPICAL
K-FACTOR
(h = 1.5; SEE THE
K-FACTOR
ERROR
(µs)
(%)
PERFORMANCE
(BUCK) SECTION)
±10
5.0
)
DD
±10
3.3
±12.5
2.2
±12.5
1.7
, is equal to
V
OUT
IN
= 2.5V, V
=
IN
.
A
AT
IN
V
= 2.5V
OUT
DROPOUT
3.15
3.47
4.13
5.61
13