LT1302CS8 datasheet(9/16 Pages) LINER

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LT1302/LT1302-5

APPLICATIONS INFORMATION

behavior in the 4th graticule is the result of the LT1302’s

Burst Mode comparator turning off all switching as output

voltage rises above its threshold.

In Figure 7c, the 0.1

µF capacitor has been replaced by a

0.01

µF unit. Undershoot is less but the response is still

underdamped. Figure 7d shows the results of the 0.1

µF

capacitor and a 10k resistor in series. Now some amount

of damping is observed, and behavior is more controlled.

Figure 7e details response with a 0.01

µF/10k series net-

work. Undershoot is down to around 100mV, or 2%. A

slight underdamping is still noticeable.

Finally, a 0.01

µF/24k series network results in the re-

sponse shown in Figure 7f. This has optimal damping,

undershoot less than 100mV and settles in less than 1ms.

The VC pin is sensitive to high frequency noise. Some

layouts may inject enough noise to modulate peak switch

current at 1/2 the switching frequency. A small capacitor

connected from VC to ground will eliminate this. Do not

exceed 1/10 of the compensation capacitor value.

Frequency Compensation

Obtaining proper RC values for the frequency compensa-

tion network is largely an empirical procedure, since

variations in input and output voltage, topology, capacitor

ESR and inductance make a simple formula elusive. As an

example, consider the case of a 2.5V to 5V boost converter

supplying 500mA. To determine optimum compensation,

the circuit is built and a transient load is applied to the

circuit. Figure 7 shows the setup.

In Figure 7a, the VC pin is simply left floating. Although

output voltage is maintained and transient response is

good, switch current rises instantaneously to the internal

current limit upon application of load. This is an undesir-

able situation as it places maximum stress on the switch

and the other power components. Additionally, efficiency

is well down from its optimal value. Next, a 0.1

µFcapacitor

is connected with no resistor. Figure 7b details response.

Although the circuit eventually stabilizes, the loop is quite

underdamped. Initial output “sag” exceeds 5%. Aberrant

Figure 7a. VC Pin Left Unconnected. Output Shows

Low Frequency Components Under Load

Figure 7b. 0.1

µF from VC to Ground.

Better, but More Improvement Needed

2ms/DIV

1302 F07b

VOUT

100mV/DIV

AC COUPLED

ILOAD

510mA

10mA

VOUT

100mV/DIV

AC COUPLED

2ms/DIV

1302 F07a

ILOAD

510mA

10mA

Figure 7. Boost Converter with Simulated Load

VIN

SHDN

SENSE

µH

MTP3055EL

PULSE

GENERATOR

PGND

0.1

µF

LT1302-5

GND

Ω

500

Ω

1302 F07

VIN

2.5V

Ω

220

µF

220

µF

C1, C2, C3 = AVX TPS SERIES

D1 = MOTOROLA MBRS130LT3

L1 = COILCRAFT DO3316-103K

330

µF

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