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| 型号： | MAX8717ETI+T |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 22/30页 |
| 文件大小： | 0K |
| 描述： | IC CNTRLR PWR SUP 28-TQFN |
| 标准包装： | 2,500 |
| 应用： | 控制器，笔记本电脑电源系统 |
| 输入电压： | 4 V ~ 26 V |
| 输出数： | 2 |
| 输出电压： | 3.3V，5V，1 V ~ 5.5 V |
| 工作温度： | 0°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 28-WFQFN 裸露焊盘 |
| 供应商设备封装： | 28-TQFN-EP（5x5） |
| 包装： | 带卷 (TR) |
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Interleaved High-Efficiency, Dual Power-Supply
Controllers for Notebook Computers
I LIMIT LOAD ( MAX ) + ?
? Δ I INDUCTOR ?
?
?
look for nonstandard values, which can provide a better
compromise in LIR across the input voltage range. If
using a swinging inductor (where the no-load induc-
tance decreases linearly with increasing current), evalu-
ate the LIR with properly scaled inductance values. For
the selected inductance value, the actual peak-to-peak
inductor ripple current ( Δ I INDUCTOR ) is defined by:
> I ?
2
where I LIMIT_ equals the minimum current-limit thresh-
old voltage divided by the current-sense resistance
(R SENSE ). For the 50mV default setting, the minimum
current-limit threshold is 50mV.
Δ I INDUCTOR = OUT IN OUT
V ( V ? V
V IN ? OSC L
)
Connect ILIM_ to V CC for a default 50mV current-limit
threshold. In adjustable mode, the current-limit thresh-
old is precisely 1/10 the voltage seen at ILIM_. For an
Ferrite cores are often the best choice, although pow-
dered iron is inexpensive and can work well at 200kHz.
The core must be large enough not to saturate at the
peak inductor current (I PEAK ):
adjustable threshold, connect a resistive divider from
REF to analog ground (GND) with ILIM_ connected to
the center tap. The external 500mV to 2V adjustment
range corresponds to a 50mV to 200mV current-limit
threshold. When adjusting the current limit, use 1% tol-
I PEAK = I LOAD ( MAX ) +
Δ I INDUCTOR
2
erance resistors and a divider current of approximately
10μA to prevent significant inaccuracy in the current-
limit tolerance.
Transient Response
The inductor ripple current also impacts transient-
response performance, especially at low V IN - V OUT dif-
ferentials. Low inductor values allow the inductor
current to slew faster, replenishing charge removed
from the output filter capacitors by a sudden load step.
The total output voltage sag is the sum of the voltage
sag while the inductor is ramping up and the voltage
sag before the next pulse can occur:
The current-sense method (Figure 8) and magnitude
determines the achievable current-limit accuracy and
power loss. Typically, higher current-sense limits pro-
vide tighter accuracy, but also dissipate more power.
Most applications employ a current-limit threshold
(V LIM ) of 50mV to 100mV, so the sense resistor can be
determined by:
R SENSE_ = V LIM_ / I LIM_
For the best current-sense accuracy and overcurrent
V SAG =
L ( Δ I LOAD ( MAX ) ) 2
2 C OUT ( V IN × D MAX ? V OUT )
+
Δ I LOAD ( MAX ) ( T ? Δ T )
C OUT
protection, use a 1% tolerance current-sense resistor
between the inductor and output as shown in Figure 8a.
This configuration constantly monitors the inductor cur-
rent, allowing accurate current-limit protection.
where D MAX is maximum duty factor (see the Electrical
Characteristics), T is the switching period (1 / f OSC ), and
Δ T equals V OUT / V IN x T when in PWM mode, or L x 0.2
x I MAX / (V IN - V OUT ) when in skip mode. The amount of
overshoot during a full-load to no-load transient due to
stored inductor energy can be calculated as:
Alternatively, high-power applications that do not
require highly accurate current-limit protection may
reduce the overall power dissipation by connecting a
series RC circuit across the inductor (Figure 8b) with an
equivalent time constant:
V SOAR ≈
( Δ I LOAD(MAX ) ) 2 L
2 C OUT V OUT
L
R L
= C EQ × R EQ
where R L is the inductor’s series DC resistance. In this
Setting 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
peak inductor current occurs at I LOAD(MAX) plus half
the ripple current; therefore:
configuration, the current-sense resistance equals the
inductor’s DC resistance (R SENSE = R L ). Use the worst-
case inductance and R L values provided by the induc-
tor manufacturer, adding some margin for the
inductance drop over temperature and load.
22
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