MAX8632ETI+T Maxim Integrated Products, MAX8632ETI+T Datasheet - Page 13

• Manufacturer Part Number: MAX8632ETI+T
• Description: IC PWR SUPPLY DDR 28-TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8632ETI+T

• Applications: Controller, DDR
• Voltage - Input: 2 ~ 28 V
• Number Of Outputs: 1
• Voltage - Output: 1.8V, 2.5V, 0.7 ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 28-TQFN Exposed Pad
• Output Voltage: 1.8 V, 2.5 V, 0.7 V to 5.5 V
• Output Current: 15 A
• Input Voltage: 2 V to 28 V
• Mounting Style: SMD/SMT
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
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:
where V
in the inductor discharge path, including the synchro-
nous rectifier, the inductor, and any PC board resis-
tances; V
charging path, including the high-side switch (Q1 in
Figure 8), the inductor, and any PC board resistances,
and t
Shot (TON) section.
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
old (2.5mV for the default 50mV current-limit threshold),
DD
PGND
) and during dynamic output-voltage transitions
ON
- V
DROP1
is the one-shot on-time (see the On-Time One-
DROP2
LX
drops below 5% of the current-limit thresh-
f
SW
Integrated DDR Power-Supply Solution for
Automatic Pulse-Skipping Mode
is the sum of the parasitic voltage drops
Desktops, Notebooks, and Graphic Cards
is the sum of the resistances in the
______________________________________________________________________________________
=
t
ON IN
V
OUT
(
V
+
+
V
V
DROP
DROP
1
(
2
SKIP = GND)
)
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
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.
Generally, low inductor values produce a broader effi-
ciency vs. load curve, while higher values result in higher
full-load efficiency (assuming that the coil resistance
Table 1. Approximate K-Factor Errors
TON SETTING
(TON = AV
(TON = open)
(TON = GND)
(TON = REF)
200
300
450
600
I
LOAD SKIP



DD
2 5
.
(
)
2
V
K-FACTOR
×
IN
×
TYPICAL
1
)
):
µ
1 7
(µs)
5.0
3.3
2.2
1.7
H
=
.
µ



s
V



OUT



12
K-FACTOR
2
ERROR
L
V
±12.5
±12.5
×
12
±10
±10
(%)
- .5V
LOAD(SKIP)
K
V
2






V
OUT
IN

 =

(BUCK) SECTION)
(h = 1.5; SEE THE
MINIMUM V
PERFORMANCE
V
-
1 68
V
= 2.5V, V
IN
.
DROPOUT
V
OUT
, is equal to
OUT
3.15
3.47
4.13
5.61
A
= 2.5V



IN
IN
AT
13
=