MAX8744ETJ+ Maxim Integrated Products, MAX8744ETJ+ Datasheet - Page 26

• Manufacturer Part Number: MAX8744ETJ+
• Description: IC CNTRLR PWR SUP QUAD 32TQFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX8744ETJ+

• Applications: Controller, Notebook Computers
• Voltage - Input: 6 ~ 26 V
• Number Of Outputs: 4
• Voltage - Output: 3.3V, 5V, 1 ~ 26 V
• Operating Temperature: 0°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 32-TQFN Exposed Pad
• Duty Cycle (max): 99 %
• Output Voltage: 3.315 V, 5.015 V, 2 V to 5.5 V
• Mounting Style: SMD/SMT
• Switching Frequency: 200 KHz, 300 KHz, 500 KHz
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Synchronous Pin: No
• Topology: Boost, Flyback, Forward
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

High-Efficiency, Quad-Output, Main Power-
Supply Controllers for Notebook Computers
The switching frequency and inductor operating point
determine the inductor value as follows:
For example: I
f
Find a low-loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. Most induc-
tor manufacturers provide inductors in standard values,
such as 1.0µH, 1.5µH, 2.2µH, 3.3µH, etc. Also look for
nonstandard values, which can provide a better compro-
mise in LIR across the input voltage range. If using a
swinging inductor (where the no-load inductance
decreases linearly with increasing current), evaluate the
LIR with properly scaled inductance values. For the
selected inductance value, the actual peak-to-peak
inductor ripple current (ΔI
Ferrite cores are often the best choice, although pow-
dered iron is inexpensive and can work well at 200kHz.
The core must be large enough not to saturate at the
peak inductor current (I
A coupled inductor or transformer can be substituted
for the inductor in the 5V SMPS to create an auxiliary
output (Figure 1). The MAX8744/MAX8745 is particular-
ly well suited for such applications because the sec-
ondary feedback threshold automatically triggers DL5
even if the 5V output is lightly loaded.
The power requirements of the auxiliary supply must be
considered in the design of the main output. The trans-
former must be designed to deliver the required current
in both the primary and the secondary outputs with the
proper turns ratio and inductance. The power ratings of
the synchronous-rectifier MOSFETs and the current limit
in the MAX8744/MAX8745 must also be adjusted
26
OSC
______________________________________________________________________________________
= 300kHz, 30% ripple current or LIR = 0.3:
MAX8744/MAX8745 Auxiliary Output)
L
=
I
ΔI
PEAK
12
INDUCTOR
L
V x
LOAD(MAX)
=
=
5
V f
V x
300
I
LOAD MAX
IN OSC LOAD MAX
V
OUT IN
(
kHz x A x
12
=
PEAK
(
V
INDUCTOR
Transformer Design (for
(
I
V
V
= 5A, V
−
OUT IN
5
5
):
)
V
−
V f
+
)
(
(
IN OSC
V
V
Inductor Selection
Δ
OUT
0 3
I
IN
INDUCTOR
.
) is defined by:
−
)
LIR
= 12V, V
=
)
V
L
2
OUT
6 50
.
)
μ
H
OUT
= 5V,
accordingly. Extremes of low input-output differentials,
widely different output loading levels, and high turns
ratios can further complicate the design due to parasitic
transformer parameters such as interwinding capaci-
tance, secondary resistance, and leakage inductance.
Power from the main and secondary outputs is com-
bined to get an equivalent current referred to the main
output. Use this total current to determine the current
limit (see the Setting the Current Limit section):
where I
to the main output, and P
power from both the main output and the secondary
output:
where N is the transformer turns ratio, V
mum required rectified secondary voltage, V
forward drop across the secondary rectifier,
V
voltage, and V
the synchronous-rectifier MOSFET. The transformer
secondary return is often connected to the main output
voltage instead of ground in order to reduce the neces-
sary turns ratio. In this case, subtract V
secondary voltage (V
turns-ratio equation above. The secondary diode in
coupled-inductor applications must withstand flyback
voltages greater than 60V. Common silicon rectifiers,
such as the 1N4001, are also prohibited because they
are too slow. Fast silicon rectifiers such as the MURS120
are the only choice. The flyback voltage across the recti-
fier is related to the V
the transformer turns ratio:
where N is the transformer turns ratio (secondary wind-
ings/primary windings), and V
ondary DC output voltage. If the secondary winding is
returned to V
from V
reverse breakdown voltage rating must also accommo-
date any ringing due to leakage inductance. The
diode’s current rating should be at least twice the DC
load current on the secondary output.
The inductor ripple current also impacts transient-
response performance, especially at low V
ferentials. Low inductor values allow the inductor
OUT5(MIN)
FLYBACK
TOTAL
V
FLYBACK
N
is the minimum value of the main output
OUT5
=
is the equivalent output current referred
RECT
I
V
TOTAL
in the equation above. The diode’s
OUT
= V
instead of ground, subtract V
is the on-state voltage drop across
IN
5
SEC
V
SEC
= P
- V
+
SEC
TOTAL
V
OUT5
RECT
TOTAL
- V
+ (V
+
Transient Response
V
OUT5
SEC
FWD
IN
difference, according to
is the sum of the output
+
/V
V
– V
OUT5
is the maximum sec-
SENSE
) in the transformer
OUT5
SEC
OUT5
) x N
IN
is the mini-
- V
FWD
from the
OUT
is the
OUT5
dif-