MAX8734 Maxim Integrated Products, MAX8734 Datasheet - Page 28

• Manufacturer Part Number: MAX8734
• Description: (MAX8732 - MAX8734) Main Power-Supply Controllers
• Manufacturer: Maxim Integrated Products

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High-Efficiency, Quad-Output, Main Power-
Supply Controllers for Notebook Computers
1N5819 type for loads up to 3A, or a 1N5822 type for
loads up to 10A. The rectifier’s rated reverse breakdown
voltage must be at least equal to the maximum input volt-
age, preferably with a 20% derating factor.
A signal diode, such as a 1N4148, works well in most
applications. Use a small (20mA) Schottky diode for
slightly improved efficiency and dropout characteris-
tics, if the input voltage can go below 6V. Do not use
large power diodes, such as 1N5817 or 1N4001, since
high-junction capacitance can force LDO5 to excessive
voltages.
The output voltage-adjust range for continuous-conduc-
tion operation is restricted by the nonadjustable 350ns
(max) minimum off-time one-shot. Use the slower 5V
SMPS for the higher of the two output voltages for best
dropout performance in adjustable feedback mode. The
duty-factor limit must be calculated using worst-case val-
ues for on- and off-times, when working with low input
voltages. Manufacturing tolerances and internal propaga-
tion delays introduce an error to the t
keep in mind that transient-response performance of
buck regulators operated close to dropout is poor, and
bulk output capacitance must often be added (see the
V
The absolute point of dropout occurs when the inductor
current ramps down during the minimum off-time
(∆I
(∆I
slew the inductor current higher in response to
increased load, and must always be greater than 1. As
h approaches 1, the absolute minimum dropout point,
the inductor current is less able to increase during each
switching cycle and V
additional output capacitance is used.
A reasonable minimum value for h is 1.5, but this can
be adjusted up or down to allow tradeoffs between
V
voltage. For a given value of h, the minimum operating
voltage can be calculated as:
where V
drops in the discharge and charge paths (see the On-
28
SAG
SAG
DOWN
UP
V
+
______________________________________________________________________________________
). The ratio h = ∆I
, output capacitance, and minimum operating
equation in the Output Capacitor Selection section).
(
MIN
DROP1
) as much as it ramps up during the on-time
)
=
1
(
−
V
Applications Information
and V
OUT




t
OFF MIN
_
UP
DROP2
+
(
SAG
K
V
/∆I
DROP
)
DOWN
Dropout Performance
×
greatly increases unless
are the parasitic voltage
Boost Supply Diode
1
h
)




indicates the ability to
+
V
DROP
ON
K-factor. Also,
2
−
V
DROP
1
Time One-Shot section), t
and K is taken from Table 2. The absolute minimum
input voltage is calculated with h = 1.
Operating frequency must be reduced or h must be
increased and output capacitance added to obtain an
acceptable V
the required minimum input voltage. Calculate V
be sure of adequate transient response if operation
near dropout is anticipated.
MAX8733: With V
t
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.
Figure 12. Transformer-Coupled Secondary Output
OFF(MIN)
V
V
+
+
T1 = TRANSPOWER TECHNOLOGIES TTI-5870
(
MIN
(
MIN
MAX8732
MAX8733
MAX8734
= 350ns, V
)
)
=
=
1
SAG
1
−
−
DH_
DL_
(






5
OUT5
(
5
0 35
0 35
V
if calculated V+
DROP1
.
V
.
2 25
+
2 25
.
+
.
µ ×
0 1
µ ×
= 5V, fsw = 400kHz, K = 2.25µs,
0 1
.
V+
s
s
µ
.
V
µ
= V
s
OFF(MIN)
V
s
)
1
1 5
)



T1
10µH
1:2.2
DROP2
.
Dropout Design Example
+



0 1
+
.
0 1
(MIN)
is from the EC table,
MAX1658/
MAX1659
= 100mV, and h = 1.5,
V
.
LDO
−
V
0 1
−
.
is greater than
0 1
V
.
V
=
12V
POSITIVE
SECONDARY
OUTPUT
5V
MAIN
OUTPUT
=
6 04
.
6 65
SAG
.
V
V
to