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| 型号： | LX1732CDUT |
| 厂商： | MICROSEMI CORP-ANALOG MIXED SIGNAL GROUP |
| 元件分类： | 稳压器 |
| 英文描述： | 1 A SWITCHING REGULATOR, 135 kHz SWITCHING FREQ-MAX, PDSO8 |
| 封装： | PLASTIC, MSOP-8 |
| 文件页数： | 9/12页 |
| 文件大小： | 450K |
| 代理商： | LX1732CDUT |
Microsemi
Integrated Products
11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570
Page 6
Copyright
2000
Rev. 1.1, 2003-07-29
WWW
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LX1732
High Current PFM Boost Converter
PRODUCTION DATA SHEET
I N T E GRA T ED
PR ODUC T S
APPLICATION INFORMATION
INDUCTOR SELECTION
A smaller value inductor tends to have a smaller package
size. Also using a smaller value inductor can reduce output
voltage ripple. The inductor value must be large enough to
maintain a reasonable level of inductor current ripple (during a
burst period) since this will increase the DC output power
capability of the converter. The ripple current can be
estimated as:
+
×
≈
FWD
OUT
IN
SW
L
IN
V
f
I
V
L
1
THERMAL CONSIDERATIONS
Calculating maximum power dissipation for a given
operating condition is achieved using the following
relationship:
PD(max) = [TJ(max) – TA(max)]/ΘJA
The maximum device junction temperature is specified
at 150oC and the 8 pin MSOP package thermal resistance is
206oC/W.
The LX1732 operates within specified
parameters up to a maximum ambient temperature of 70oC.
The maximum power dissipation achievable under these
constraints is (150oC - 70oC)/ 206oC/W = 0.38W and
increases to 0.58W at a device ambient temperature of 30oC.
Designers should pay close attention to PCB design, device
thermal coupling, proximity to other active components, and
access to airflow in applications that require the device to
operate close to the maximum junction temperature.
CIRCUIT DESIGN EXAMPLE
Example 1
VIN = 3.0; VOUT = 5.0V+5%; IOUT = 150mA (max);
Efficiency > 80%; VOUT(ripple) < 100mV.
Step 1: Program the output voltage. This value was
already determined in the example on page 5 as 316k
.
Step 2: Determine an appropriate inductor value.
Determine the inductor that will result in a ripple current of
200mA. Assuming a diode forward voltage drop of 350mV
and a nominal switching frequency of 130KHz, based on the
equation above : L = 51uF; use 47uF standard value.
Step 3: Determine output capacitance.
The value of the
output capacitor effects output voltage ripple and transient
performance. The ripple voltage on the output (ignoring
ESR) is the summation of the comparator overdrive voltage,
the voltage undershoot (which usually occurs during the first
switch “on” time) and overshoot that occurs at the end of the
burst when the stored energy in the inductor is delivered to
the output capacitor. An approximation of the output ripple
voltage is given by this relationship:
VOUTpp = {(IOUT*tON) / COUT } + { 0.5*(L /
COUT)*(IPEAK - IOUT) / (VOUT – VIN) } + IPEAK*
ESRC + 10mV.
Based upon this equation, making the output capacitor large,
the inductor value small, and the peak current small will
help reduce ripple. Figure 14 shows IPEAK equal to 400mA
for a 150mA load condition..
Two 100
F tantalum
capacitors were placed in-parallel at the output. The total
ESRC is approximately 0.10. The estimated ripple voltage
based upon these values is calculated to be 59mV. The
actual ripple measured in Figure 14 is less than 40mV.
Variation in tON, actual ESRC and COUT contribute to the
error associated between the measured and calculated value.
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