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| 型号： | MAX8734AEEI+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 24/33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
Power-Up Sequencing and
On/Off Controls (ON3, ON5)
ON3 and ON5 control SMPS power-up sequencing.
ON3 or ON5 rising above 2.4V enables the respective
outputs. ON3 or ON5 falling below 1.6V disables the
respective outputs.
Connecting ON3 or ON5 to REF forces the respective
outputs off while the other output is below regulation and
starts after that output regulates. The second SMPS
remains on until the first SMPS turns off, the device shuts
down, a fault occurs, or LDO5 goes into undervoltage
lockout. Both supplies begin their power-down sequence
immediately when the first supply turns off. Driving ON_
below 0.8V clears the overvoltage, undervoltage, and
thermal fault latches.
Adjustable-Output Feedback
(Dual-Mode FB)
Connect FB_ to GND to enable the fixed, preset SMPS
output voltages (3.3V and 5V). Connect a resistive volt-
age-divider at FB_ between OUT_ and GND to adjust
the respective output voltage between 2V and 5.5V
(Figure 11). Choose R2 to be approximately 10k ?, and
solve for R1 using the equation:
When using the adjustable-output mode, set the 3.3V
SMPS lower than the 5V SMPS. LDO5 connects to OUT5
through an internal switch only when OUT5 is above the
LDO5 bootstrap-switch threshold (4.56V). LDO3 con-
nects to OUT3 through an internal switch only when
OUT3 is above the LDO3 bootstrap switch threshold
(2.91V). Bootstrapping is most effective when the fixed
output voltages are used. Once LDO_ is bootstrapped
from OUT_, the internal linear regulator turns off. This
reduces internal power dissipation and improves effi-
ciency when LDO_ is powered with a high input voltage.
Design Procedure
Establish the input voltage range and maximum load
current before choosing an inductor and its associated
ripple-current ratio (LIR). The following four factors dic-
tate the rest of the design:
1) Input Voltage Range. The maximum value (V+ (MAX) )
must accommodate the maximum AC adapter volt-
age. The minimum value (V+ (MIN) ) must account for
the lowest input voltage after drops due to connec-
tors, fuses, and battery selector switches. Lower input
voltages result in better efficiency.
2) Maximum Load Current. The peak load current
? V
? 1 ?
?
? V OUT _
R 1 = R 2 × ?
? FB
where V FB = 2V nominal.
V+
DH_
?
?
(I LOAD(MAX) ) determines the instantaneous compo-
nent stress and filtering requirements, and thus dri-
ves output capacitor selection, inductor saturation
rating, and the design of the current-limit circuit.
The continuous load current (I LOAD ) determines the
thermal stress and drives the selection of input
capacitors, MOSFETs, and other critical heat-con-
tributing components.
3) Switching Frequency. This choice determines the
basic trade-off between size and efficiency. The
optimal frequency is largely a function of maximum
input voltage and MOSFET switching losses. The
MAX8732A has a nominal switching frequency of
MAX8732A
MAX8733A
MAX8734A
DL_
GND
OUT_
FB_
R1
R2
V OUT_
200kHz for the 5V SMPS and 300kHz for the 3.3V
SMPS. The MAX8733A has a nominal switching fre-
quency of 400kHz for the 5V SMPS and 500kHz for
the 3.3V SMPS. The MAX8734A has a pin-selec-
table switching frequency.
4) Inductor Ripple Current Ratio (LIR). LIR is the
ratio of the peak-to-peak ripple current to the aver-
age inductor current. Size and efficiency trade-offs
must be considered when setting the inductor rip-
ple current ratio. Low inductor values cause large
ripple currents, resulting in the smallest size, but
poor efficiency and high output noise. The minimum
practical inductor value is one that causes the cir-
cuit to operate at critical conduction (where the
Figure 11. Setting V OUT_ with a Resistor-Divider
inductor current just touches zero with every cycle
24
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