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| 型號(hào)： | LM2698 |
| 文件頁(yè)數(shù)： | 9/17頁(yè) |
| 文件大?。?/td> | 503K |
| 代理商： | LM2698 |
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Operation
(Continued)
If V
is constant, di
/ dt must be constant, thus the current
in the inductor changes at a constant rate. This is the case in
DC/DC converters since the voltages at the input and output
can be approximated as a constant. The current through the
inductor of the LM2698 boost converter is shown in Figure
2(a). The important quantities in determining a proper induc-
tance value are I
(the average inductor current) and
i
L
(the inductor current ripple). If
i
is larger than I
, the
inductor current will drop to zero for a portion of the cycle and
the converter will operate in discontinuous conduction mode.
If
i
is smaller than I
, the inductor current will stay
above zero and the converter will operate in continuous
conduction mode (CCM). All the analysis in this datasheet
assumes operation in continuous conduction mode. To op-
erate in CCM:
I
L(AVG)
>
i
L
Choose the minimum I
OUT
to determine the minimum L for
CCM operation. A common choice is to set
i
L
to 30% of
I
L(AVG)
.
The inductance value will also affect the stability of the
converter. Because the LM2698 utilizes current mode con-
trol, the inductor value must be carefully chosen. See the
COMPENSATION section for recommended inductance val-
ues.
Choosing an appropriate core size for the inductor involves
calculating the average and peak currents expected through
the inductor. In a boost converter,
and
I
L(Peak)
= I
L(AVG)
+
i
L
,
where
A core size with ratings higher than these values should be
chosen. If the core is not properly rated, saturation will
dramatically reduce overall efficiency.
Current Limit
The current limit in the LM2698 is referenced to the peak
switch current. The peak currents in the switch of a boost
converter will always be higher than the average current
supplied to the load. To determine the maximum average
output current that the LM2698 can supply, use:
I
OUT(MAX)
= (I
CL
i
L
)
*
(1D) = (I
CL
i
L
)
*
V
IN
/V
OUT
Where I
is the switch current limit (see Electrical Chara-
teristics table and Typical Performance Curves). Hence, as
V
increases, the maximum current that can be supplied to
the load increases, as shown in Figure 3
Diode
The diode in a boost converter such as the LM2698 acts as
a switch to the output. During the first cycle, when the
transistor is closed, the diode is reverse biased and current
is blocked; the load current is supplied by the output capaci-
tor. In the second cycle, the transistor is open and the diode
is forward biased; the load current is supplied by the induc-
tor.
Observation of the boost converter circuit shows that the
average current through the diode is the average load cur-
rent, and the peak current through the diode is the peak
current through the inductor. The diode should be rated to
handle more than its peak current. To improve efficiency, a
low forward drop Schottky diode is recommended.
Input Capacitor
Due to the presence of an inductor at the input of a boost
converter, the input current waveform is continuous and
triangular. The inductor ensures that the input capacitor sees
fairly low ripple currents. However, as the inductor gets
smaller, the input ripple increases. The rms current in the
input capacitor is given by:
The input capacitor should be capable of handling the rms
current.Although the input capacitor is not so critical in boost
applications, a 10 μF or higher value, good quality capacitor
prevents any impedance interactions with the input supply.
A 0.1μF or 1μF ceramic bypass capacitor is also recom-
mended on the V
IN
pin (pin 6) of the IC. This capacitor must
20012673
FIGURE 3. Maximum Output Current vs Input Voltage
L
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| LM2698EVAL | 功能描述:電源管理IC開發(fā)工具 EVAL BOARDFOR LM2698 RoHS:否 制造商:Maxim Integrated 產(chǎn)品:Evaluation Kits 類型:Battery Management 工具用于評(píng)估:MAX17710GB 輸入電壓: 輸出電壓:1.8 V |
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| LM2698MM-ADJ/NOPB | 制造商:Texas Instruments 功能描述:DC/DC Converter (DC-DC) / Switching Regu |
| LM2698MMX-ADJ | 功能描述:直流/直流開關(guān)轉(zhuǎn)換器 RoHS:否 制造商:STMicroelectronics 最大輸入電壓:4.5 V 開關(guān)頻率:1.5 MHz 輸出電壓:4.6 V 輸出電流:250 mA 輸出端數(shù)量:2 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT |
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