MAX8734AEEI+G104 Maxim Integrated, MAX8734AEEI+G104 Datasheet - Page 28

• Manufacturer Part Number: MAX8734AEEI+G104
• Description: Current & Power Monitors & Regulators High-Efficiency Quad-Output Main Power-Supply Controllers for Notebook Computers
• Manufacturer: Maxim Integrated

Specifications of MAX8734AEEI+G104

• Product: Power Monitors
• Supply Voltage - Max: 24 V
• Supply Voltage - Min: 4.5 V
• Operating Temperature Range: - 40 C to + 85 C
• Accuracy: 1.50%
• Input Voltage Range: 4.5 V to 24 V
• Supply Current: 50 uA

High-Efficiency, Quad-Output, Main Power-
Supply Controllers for Notebook Computers
Current circulates from ground to the junction of both
MOSFETs and the inductor when the high-side switch is
off. As a consequence, the polarity of the switching
node is negative with respect to ground. This voltage is
approximately -0.7V (a diode drop) at both transition
edges while both switches are off (dead time). The drop
is I
The rectifier is a clamp across the synchronous rectifier
that catches the negative inductor swing during the dead
time between turning the high-side MOSFET off and the
synchronous rectifier on. The MOSFETs incorporate a
high-speed silicon body diode as an adequate clamp
diode if efficiency is not of primary importance. Place a
Schottky diode in parallel with the body diode to reduce
the forward-voltage drop and prevent the N2/N4 MOSFET
body diodes from turning on during the dead time.
Typically, the external diode improves the efficiency by
1% to 2%. Use a Schottky diode with a DC current rating
equal to 1/3 of the load current. For example, use an
MBR0530 (500mA-rated) type for loads up to 1.5A, a
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
28
SAG
DOWN
L
x R
______________________________________________________________________________________
equation in the Output-Capacitor Selection section).
DS(ON)
) as much as it ramps up during the on-time
when the low-side switch conducts.
Applications Information
Dropout Performance
Boost Supply Diode
Rectifier Selection
ON
K-factor. Also,
(∆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-
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.
MAX8733A: With V
t
Figure 12. Transformer-Coupled Secondary Output
OFF(MIN)
SAG
UP
V
+
T1 = TRANSPOWER TECHNOLOGIES TTI-5870
). The ratio h = ∆I
, output capacitance, and minimum operating
(
MIN
MAX8732A
MAX8733A
MAX8734A
DROP1
= 350ns, V
)
=
SAG
1
(
−
V
and V
DH_
DL_
OUT




OUT5
t
if calculated V+
DROP1
OFF MIN
_
UP
DROP2
+
(
= 5V, fsw = 400kHz, K = 2.25µs,
SAG
V+
V
K
/∆I
= V
DROP
OFF(MIN)
)
DOWN
×
DROP2
greatly increases unless
Dropout Design Example
T1
10µH
1:2.2
are the parasitic voltage
1
h
)




indicates the ability to
+
(MIN)
= 100mV, and h = 1.5,
is from the EC table,
MAX1658/
MAX1659
V
DROP
LDO
is greater than
2
−
12V
POSITIVE
SECONDARY
OUTPUT
5V
MAIN
OUTPUT
V
DROP
SAG
1
to