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| 型号： | MAX8734AEEI+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 28/33页 |
| 文件大小： | 0K |
| 描述： | IC PWR SUPPLY CONTROLLER 28QSOP |
| 产品培训模块： | Lead (SnPb) Finish for COTS Obsolescence Mitigation Program |
| 标准包装： | 2,500 |
| 应用： | 电源控制器 |
| 输入电压： | 4.5 V ~ 24 V |
| 电流 - 电源： | 25µA |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-QSOP |
| 供应商设备封装： | 28-QSOP |
| 包装： | 带卷 (TR) |
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High-Efficiency, Quad-Output, Main Power-
Supply Controllers for Notebook Computers
OFF ( MIN ) × h
?
?
?
?
Rectifier Selection
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 L x R DS(ON) when the low-side switch conducts.
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.
Boost Supply Diode
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.
Applications Information
Dropout Performance
( ? I UP ). The ratio h = ? I UP / ? I DOWN indicates the ability to
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 SAG greatly increases unless
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 SAG , output capacitance, and minimum operating
voltage. For a given value of h, the minimum operating
voltage can be calculated as:
V + ( MIN ) = ( V OUT _ + V DROP 1 ) + V DROP 2 ? V DROP 1
? t ?
1 ? ? ?
K
where V DROP1 and V DROP2 are the parasitic voltage
drops in the discharge and charge paths (see the On-
Time, One-Shot section), t OFF(MIN) is from the EC table,
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 SAG if calculated V+ (MIN) is greater than
the required minimum input voltage. Calculate V SAG to
be sure of adequate transient response if operation
near dropout is anticipated.
Dropout Design Example
MAX8733A: With V OUT5 = 5V, fsw = 400kHz, K = 2.25μs,
t OFF(MIN) = 350ns, V DROP1 = V DROP2 = 100mV, and h = 1.5,
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
DH_
V+
MAX1658/
MAX1659
LDO
12V
POSITIVE
SECONDARY
OUTPUT
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 ON K-factor. Also,
keep in mind that transient-response performance of
buck regulators operated close to dropout is poor, and
MAX8732A
MAX8733A
MAX8734A
DL_
T1
10 μ H
1:2.2
5V
MAIN
OUTPUT
bulk output capacitance must often be added (see the
V SAG equation in the Output-Capacitor Selection section).
The absolute point of dropout occurs when the inductor
current ramps down during the minimum off-time
( ? I DOWN ) as much as it ramps up during the on-time
T1 = TRANSPOWER TECHNOLOGIES TTI-5870
Figure 12. Transformer-Coupled Secondary Output
28
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