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参数资料
| 型号： | LM2734XMKX/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 稳压器 |
| 英文描述： | 2.5 A SWITCHING REGULATOR, 1900 kHz SWITCHING FREQ-MAX, PDSO6 |
| 封装： | TSOT-23, 6 PIN |
| 文件页数： | 23/24页 |
| 文件大小： | 466K |
| 代理商： | LM2734XMKX/NOPB |
for best efficiency. V
BOOST – VSW should not exceed the max-
imum operating limit of 5.5V.
5.5V > V
BOOST – VSW > 2.5V for best performance.
20102308
FIGURE 3. V
OUT Charges CBOOST
When the LM2734 starts up, internal circuitry from the
BOOST pin supplies a maximum of 20mA to C
BOOST. This
current charges C
BOOST to a voltage sufficient to turn the
switch on. The BOOST pin will continue to source current to
C
BOOST until the voltage at the feedback pin is greater than
0.76V.
There are various methods to derive V
BOOST:
1.
From the input voltage (V
IN)
2.
From the output voltage (V
OUT)
3.
From an external distributed voltage rail (V
EXT)
4.
From a shunt or series zener diode
In the Simplifed Block Diagram of Figure 1, capacitor
C
BOOST and diode D2 supply the gate-drive current for the
NMOS switch. Capacitor C
BOOST is charged via diode D2 by
V
IN. During a normal switching cycle, when the internal NMOS
control switch is off (T
V
IN minus the forward voltage of D2 (VFD2), during which the
current in the inductor (L) forward biases the Schottky diode
D1 (V
FD1). Therefore the voltage stored across CBOOST is
V
BOOST - VSW = VIN - VFD2 + VFD1
When the NMOS switch turns on (T
ON), the switch pin rises
to
V
SW = VIN – (RDSON x IL),
forcing V
BOOST to rise thus reverse biasing D2. The voltage at
V
BOOST is then
V
BOOST = 2VIN – (RDSON x IL) – VFD2 + VFD1
which is approximately
2V
IN - 0.4V
for many applications. Thus the gate-drive voltage of the
NMOS switch is approximately
V
IN - 0.2V
An alternate method for charging C
BOOST is to connect D2 to
the output as shown in Figure 3. The output voltage should
be between 2.5V and 5.5V, so that proper gate voltage will be
applied to the internal switch. In this circuit, C
BOOST provides
a gate drive voltage that is slightly less than V
OUT.
In applications where both V
IN and VOUT are greater than
5.5V, or less than 3V, C
BOOST cannot be charged directly from
these voltages. If V
IN and VOUT are greater than 5.5V,
C
BOOST can be charged from VIN or VOUT minus a zener volt-
age by placing a zener diode D3 in series with D2, as shown
in Figure 4. When using a series zener diode from the input,
ensure that the regulation of the input supply doesn’t create
a voltage that falls outside the recommended V
BOOST voltage.
(V
INMAX – VD3) < 5.5V
(V
INMIN – VD3) > 1.6V
20102309
FIGURE 4. Zener Reduces Boost Voltage from V
IN
An alternative method is to place the zener diode D3 in a
shunt configuration as shown in Figure 5. A small 350mW to
500mW 5.1V zener in a SOT-23 or SOD package can be used
for this purpose. A small ceramic capacitor such as a 6.3V,
0.1F capacitor (C4) should be placed in parallel with the
zener diode. When the internal NMOS switch turns on, a pulse
of current is drawn to charge the internal NMOS gate capac-
itance. The 0.1 F parallel shunt capacitor ensures that the
V
BOOST voltage is maintained during this time.
Resistor R3 should be chosen to provide enough RMS current
to the zener diode (D3) and to the BOOST pin. A recom-
mended choice for the zener current (I
ZENER) is 1 mA. The
current I
BOOST into the BOOST pin supplies the gate current
of the NMOS control switch and varies typically according to
the following formula for the X version:
I
BOOST = 0.56 x (D + 0.54) x (VZENER – VD2) mA
I
BOOST can be calculated for the Y version using the following:
I
BOOST = 0.22 x (D + 0.54) x (VZENER - VD2) A
where D is the duty cycle, V
ZENER and VD2 are in volts, and
I
BOOST is in milliamps. VZENER is the voltage applied to the
anode of the boost diode (D2), and V
D2 is the average forward
voltage across D2. Note that this formula for I
BOOST gives typ-
ical current. For the worst case I
BOOST, increase the current
by 40%. In that case, the worst case boost current will be
I
BOOST-MAX = 1.4 x IBOOST
R3 will then be given by
R3 = (V
IN - VZENER) / (1.4 x IBOOST + IZENER)
For example, using the X-version let V
IN = 10V, VZENER = 5V,
V
D2 = 0.7V, IZENER = 1mA, and duty cycle D = 50%. Then
I
BOOST = 0.56 x (0.5 + 0.54) x (5 - 0.7) mA = 2.5mA
R3 = (10V - 5V) / (1.4 x 2.5mA + 1mA) = 1.11k
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LM2734
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相关代理商/技术参数 |
参数描述 |
|---|---|
| LM2734XQMK | 制造商:Texas Instruments 功能描述:DC DC REG BUCK 1A 1.6MHZ 6TSOT 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT, Primary Input Voltage:20V, No. of Outputs:1, 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT, Primary Input Voltage:20V, No. of Outputs:1, Output Voltage:18V, Output Current:1A, Voltage Regulator Case Style:SOT-23, No. of Pins:6, Operating Temperature Min:-40C, Operating Temperature , RoHS Compliant: Yes |
| LM2734XQMK/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| LM2734XQMKE | 制造商:Texas Instruments 功能描述:DC DC REG, BUCK, 1A, 1.6MHZ, 6TSOT |
| LM2734XQMKE/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
| LM2734XQMKX/NOPB | 功能描述:直流/直流开关调节器 RoHS:否 制造商:International Rectifier 最大输入电压:21 V 开关频率:1.5 MHz 输出电压:0.5 V to 0.86 V 输出电流:4 A 输出端数量: 最大工作温度: 安装风格:SMD/SMT 封装 / 箱体:PQFN 4 x 5 |
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