MAX8734AEEI Maxim Integrated Products, MAX8734AEEI Datasheet - Page 29

• Manufacturer Part Number: MAX8734AEEI
• Description: Current & Power Monitors & Regulators Quad-Out Main Power Supply Controller
• Manufacturer: Maxim Integrated Products

Specifications of MAX8734AEEI

• Product: Power Monitors
• Supply Voltage (max): 4.5 V
• Supply Voltage (min): 24 V
• Operating Temperature Range: - 40 C to + 85 C
• Mounting Style: SMD/SMT
• Package / Case: QSOP-28
• Supply Current (max): 50 uA
• Lead Free Status / Rohs Status: Lead free / RoHS Compliant

the minimum V+ is:
Calculating with h = 1 yields:
Therefore, V+ must be greater than 6.65V. A practical
input voltage with reasonable output capacitance
would be 7.5V.
A coupled inductor or transformer can be substituted for
the inductor in the 5V or 3.3V SMPS to create an auxiliary
output (Figure 12). The MAX8732A/MAX8733A/
MAX8734A are particularly well suited for such applica-
tions because they can be configured in ultrasonic or
forced-PWM mode to ensure good load regulation when
the main supplies are lightly loaded. An additional
postregulation circuit can be used to improve load regula-
tion and limit output current.
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 MAX8732A/MAX8733A/MAX8734A must also be
adjusted accordingly. Extremes of low input-output dif-
ferentials, widely different output loading levels, and high
turns ratios can further complicate the design due to par-
asitic transformer parameters such as interwinding
capacitance, secondary resistance, and leakage induc-
tance. Power from the main and secondary outputs is
combined to get an equivalent current referred to the
main output. Use this total current to determine the cur-
rent limit (see the Determining the Current Limit section):
where I
to the main output and P
power from both the main output and the secondary
output:
V
V
+
+
(
MIN
(
MIN
TOTAL
)
Use of Coupled Inductors to Create
Supply Controllers for Notebook Computers
)
=
=
High-Efficiency, Quad-Output, Main Power-
1
1
is the equivalent output current referred
I
−
TOTAL
−



(



(
5
5
0 35
0 35
______________________________________________________________________________________
V
.
V
.
2 25
+
2 25
+
.
.
µ ×
0 1
µ ×
=
0 1
s
.
s
µ
.
TOTAL
µ
P
V
s
V
s
TOTAL
)
1 5
1
)
.






+
is the sum of the output
0 1
+
Auxiliary Outputs
/
.
0 1
V
V
OUT
.
V
−
−
0 1
.
0 1
V
.
V
=
=
6 04
.
6 65
.
V
V
where L
transformer turns ratio, V
rectified secondary voltage, V
across the secondary rectifier, V
value of the main output voltage, and V
state voltage drop across the synchronous rectifier
MOSFET. The transformer secondary return is often con-
nected to the main output voltage instead of ground to
reduce the necessary turns ratio. In this case, subtract
V
transformer turns-ratio equation above.
The secondary diode in coupled-inductor applications
must withstand flyback voltages greater than 60V, which
usually rules out most Schottky rectifiers. Common sili-
con rectifiers, such as the 1N4001, are also prohibited
because they are too slow. This often makes fast silicon
rectifiers such as the MURS120 the only choice. The fly-
back voltage across the rectifier is related to the V
V
where N is the transformer turns ratio (secondary wind-
ings/primary windings), V
DC output voltage, and V
put voltage. If the secondary winding is returned to V
instead of ground, subtract V
equation above. The diode’s reverse breakdown voltage
rating must also accommodate any ringing due to leak-
age inductance. The diode’s current rating should be at
least twice the DC load current on the secondary output.
The optional linear postregulator must be selected to
deliver the required load current from the transformer’s
rectified DC output. The linear regulator should be con-
figured to run close to dropout to minimize power dissi-
pation and should have good output accuracy under
those conditions. Input and output capacitors are cho-
sen to meet line regulation, stability, and transient
requirements. There is a wide variety of linear regulators
appropriate for this application; consult the specific lin-
ear-regulator data sheet for details.
Widely different output loads affect load regulation. In
particular, when the secondary output is left unloaded
while the main output is fully loaded, the secondary out-
put capacitor may become overcharged by the leakage
inductance, reaching voltages much higher than intend-
ed. In this case, a minimum load or overvoltage protec-
OUT
OUT
difference, according to the transformer turns ratio:
from the secondary voltage (V
L
PRIMARY
PRIMARY
V
FLYBACK
N
=
is the primary inductance, N is the
=
= V
V
V
OUT MIN
IN MAX
V
V
(
SEC
OUT IN MAX
SEC
SEC
(
OUT
SEC
+ (V
(
)
+
V
is the maximum secondary
OUT
)
× ƒ ×
is the primary (main) out-
V
is the minimum-required
FWD
+
OUT(MIN)
FWD
(
IN
V
RECT
from V
I
- V
TOTAL
is the forward drop
)
SEC
OUT
−
V
RECT
OUT
FLYBACK
is the minimum
)
- V
×
✕
LIR
N
OUT
is the on-
) in the
in the
OUT
IN
29
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