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参数资料
| 型号： | MAX8734AEEI+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 25/33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
V OUT_ ( V +? V OUT_ )
V + × f × LIR × I LOAD(MAX)
( )
at maximum load).Inductor values lower than this
grant no further size-reduction benefit.
The MAX8732A/MAX8733A/MAX8734As’ pulse-skip-
ping algorithm (SKIP = GND) initiates skip mode at the
critical conduction point, so the inductor’s operating
point also determines the load current at which
PWM/PFM switchover occurs. The optimum point is
usually found between 20% and 50% ripple current.
Inductor Selection
The switching frequency (on-time) and operating point
(% ripple or LIR) determine the inductor value as follows:
L =
Example: I LOAD(MAX) = 5A, V+ = 12V, V OUT5 = 5V, f =
200kHz, 35% ripple current or LIR = 0.35:
5 V 12 V ? 5 V
L = = 8 . 3 μ H
12 V × 200 kHz × 0 . 35 × 5 A
Find a low-loss inductor with the lowest possible DC
resistance that fits in the allotted dimensions. Ferrite cores
are often the best choice. The core must be large enough
not to saturate at the peak inductor current (I PEAK ):
I PEAK = I LOAD(MAX) + [(LIR / 2) x I LOAD(MAX) ]
The inductor ripple current also impacts transient-
response performance, especially at low V+ - V OUT_
differences. Low inductor values allow the inductor cur-
rent to slew faster, replenishing charge removed from
the output filter capacitors by a sudden load step. The
peak amplitude of the output transient (V SAG ) is also a
function of the maximum duty factor, which can be cal-
culated from the on-time and minimum off-time:
ley of the inductor current occurs at I LOAD(MAX) minus
half of the ripple current; therefore:
I LIMIT(LOW) > I LOAD(MAX) - [(LIR / 2) x I LOAD(MAX) ]
where I LIMIT(LOW) = minimum current-limit threshold
voltage divided by the R DS(ON) of N2/N4 (MAX8734A).
For the MAX8732A/MAX8733A/MAX8734A, the mini-
mum current-limit threshold voltage is 93mV (ILIM_ =
V CC ). Use the worst-case maximum value for R DS(ON)
from the MOSFET N2/N4 data sheet and add some
margin for the rise in R DS(ON) with temperature. A good
general rule is to allow 0.5% additional resistance for
each °C of temperature rise.
Examining the 5A circuit example with a maximum
R DS(ON) = 12m ? at high temperature reveals the following:
I LIMIT(LOW) = 93mV / 12m ? > 5A - (0.35 / 2) 5A
7.75A > 4.125A
7.75A is greater than the valley current of 4.125A, so
the circuit can easily deliver the full-rated 5A using the
fixed 100mV nominal current-limit threshold voltage.
Connect the source of the synchronous rectifier to a
current-sense resistor to GND (MAX8732A/MAX8733A),
and connect CS_ to that junction to set the current limit
for the device. The MAX8732A/MAX8733A/MAX8734A
limit the current with the sense resistor instead of the
R DS(ON) of N2/N4. The maximum value of the sense
resistor can be calculated with the equation:
I LIM_ = 93mV / R SENSE
Output-Capacitor Selection
The output filter capacitor must have low enough equiv-
alent series resistance (ESR) to meet output ripple and
load-transient requirements, yet have high enough ESR
to satisfy stability requirements. The output capaci-
tance must also be high enough to absorb the inductor
energy while transitioning from full-load to no-load con-
( ? I LOAD ( MAX ) )
? ?
V OUT _
× L ? K + t OFF ( MIN ) ?
V +
?
?
? ? V +? V
?? ? t OFF ( MIN ) ?
2 × C OUT × V OUT _ ? K ?
? ? ?
?
V SAG =
2
? ?
? ?
OUT _
? ?
V + ?
ditions without tripping the overvoltage fault latch. In
applications where the output is subject to large load
transients, the output capacitor’s size depends on how
much ESR is needed to prevent the output from dip-
ping too low under a load transient. Ignoring the sag
due to finite capacitance:
where minimum off-time = 0.350μs (max) and K is from
Table 2.
R ESR ≤
V DIP
I LOAD ( MAX )
Determining the Current Limit
The minimum current-limit threshold must be great
enough to support the maximum load current when the
current limit is at the minimum tolerance value. The val-
where V DIP is the maximum-tolerable transient voltage
drop. In non-CPU applications, the output capacitor’s
size depends on how much ESR is needed to maintain
an acceptable level of output voltage ripple:
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