MAX8550ETI+ Maxim Integrated Products, MAX8550ETI+ Datasheet - Page 18

• Manufacturer Part Number: MAX8550ETI+
• Description: IC PWR SUP DDR INTEG 28TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8550ETI+

• Applications: Controller, DDR
• Voltage - Input: 2 ~ 28 V
• Number Of Outputs: 2
• 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: 0.7 V to 5.5 V, 1.8 V, 2.5 V
• Output Current: 20 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

shut off, go high impedance, and restart after the die
portion of the IC cools by 15°C. Both thermal faults are
independent. For example, if the VTT output is over-
loaded to the point that it triggers its thermal fault, the
buck regulator continues to function.
Firmly establish the input voltage range (V
mum load current (I
choosing a switching frequency and inductor operating
point (ripple current ratio or LIR). The primary design
trade-off lies in choosing a good switching frequency
and inductor operating point, and the following four fac-
tors dictate the rest of the design:
• Input Voltage Range. The maximum value (V
• Maximum Load Current. There are two values to con-
• Switching Frequency. This choice determines the
• Inductor Operating Point. This choice provides trade-
Integrated DDR Power-Supply Solutions for
Desktops, Notebooks, and Graphic Cards
18
must accommodate the worst-case voltage. The mini-
mum value (V
voltage after drops due to connectors and fuses. If
there is a choice, lower input voltages result in better
efficiency.
sider. The peak load current (I
instantaneous component stresses and filtering
requirements and thus drives output capacitor selec-
tion, inductor saturation rating, and the design of the
current-limit circuit. The continuous load current
(I
drives the selection of input capacitors, MOSFETs,
and other critical heat-contributing components.
basic trade-off between size and efficiency. The opti-
mal frequency is largely a function of maximum input
voltage, due to MOSFET switching losses proportion-
al to frequency and V
also a moving target, due to rapid improvements in
MOSFET technology that are making higher frequen-
cies more practical.
offs: size vs. efficiency and transient response vs. out-
put ripple. Low inductor values provide better
transient response and smaller physical size but also
result in lower efficiency and higher output ripple due
to increased ripple currents. The minimum practical
inductor value is one that causes the circuit to operate
at the edge of critical conduction (where the inductor
current just touches zero with every cycle at maximum
load). Inductor values lower than this grant no further
size-reduction benefit. The optimum operating point is
usually found between 20% and 50% ripple current.
When pulse skipping (SKIP = low at light loads), the
inductor value also determines the load-current value
at which PFM/PWM switchover occurs.
LOAD
______________________________________________________________________________________
) determines the thermal stresses and thus
IN(MIN)
LOAD
) must account for the lowest
IN
) in the buck regulator before
2
Design Procedure
. The optimum frequency is
PEAK
) determines the
IN
) and maxi-
IN(MAX)
)
The MAX8550/MAX8551s’ Dual-Mode operation allows
the selection of common voltages without requiring
external components (Figure 5). Connect FB to GND for
a fixed 2.5V output, to AV
connect FB directly to OUT for a fixed 0.7V output.
The buck-regulator output voltage can be adjusted from
0.7V to 5.5V using a resistive voltage-divider (Figure 6).
The MAX8550/MAX8551 regulate FB to a fixed refer-
ence voltage (0.7V). The adjusted output voltage is:
where V
and V
The termination power-supply output (VTT) can be set by
two different methods. First, the VTT output can be con-
nected directly to the VTTS input to force VTT to regulate
to V
higher than V
Figure 5. Dual-Mode Feedback Decoder
REF (2.0V)
Setting the VTT and VTTR Voltages (LDO)
REFIN
0.1V
Setting the Buck Regulator Output (V
FB
RIPPLE
FB
V
V
RIPPLE
/ 2. Secondly, VTT can be forced to regulate
OUT
Setting the Output Voltage (Buck)
is 0.7V, R
is:
REFIN
AMPLIFIER
ERROR
=
=
TO
/ 2 by connecting a resistive
V
LIR
FB
C
Resistive Voltage-Divider at FB
and R
⎛
⎜
⎝
×
1
DD
I
+
LOAD MAX
R
R
for a fixed 1.8V output, or
D
D
Preset Output Voltages
C
are shown in Figure 6,
(
⎞
⎟ +
⎠
MAX8550/
MAX8551
V
)
RIPPLE
×
2
R
ESR
OUT
) with a
(FIXED)
(FIXED)
1.8V
2.5V
OUT