LX1732 データシートの表示（PDF） - Microsemi Corporation

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LX1732

High Current PFM Boost Converter

Microsemi Corporation 

INTEGRATED PRODUCTS

LX1732

High Current PFM Boost Converter

PRODUCTION DATA SHEET

APPLICATION INFORMATION

INDUCTOR SELECTION

A smaller value inductor tends to have a smaller package

size. Also using a smaller value inductor can reduce output

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

voltage ripple. The inductor value must be large enough to ESR) is the summation of the comparator overdrive voltage,

maintain a reasonable level of inductor current ripple (during a the voltage undershoot (which usually occurs during the first

burst period) since this will increase the DC output power

switch “on” time) and overshoot that occurs at the end of the

capability of the converter. The ripple current can be

estimated as:

burst when the stored energy in the inductor is delivered to

the output capacitor. An approximation of the output ripple

L

≈

VIN

∆I L × f SW

×

⎜⎜⎝⎛1

−

VIN

VOUT + VFWD

⎟⎟⎠⎞

THERMAL CONSIDERATIONS

Calculating maximum power dissipation for a given

operating condition is achieved using the following

voltage is given by this relationship:

• ∆VOUTpp = {(IOUT*tON) / COUT } + { 0.5*(L /

COUT)*(IPEAK - IOUT)² / (VOUT – VIN) } + IPEAK*

ESRC + 10mV.

Based upon this equation, making the output capacitor large,

relationship:

the inductor value small, and the peak current small will

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

help reduce ripple. Figure 14 shows IPEAK equal to 400mA

for a 150mA load condition.. Two 100µF tantalum

capacitors were placed in-parallel at the output. The total

ESRC is approximately 0.10Ω. 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.

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 316kΩ.

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.

Copyright © 2000

Rev. 1.1c, 2005-03-03

Microsemi

Integrated Products

11861 Western Avenue, Garden Grove, CA. 92841, 714-898-8121, Fax: 714-893-2570

Page 6

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