LT1302 データシートの表示（PDF） - Linear Technology

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LT1302

Micropower High Output Current Step-Up Adjustable and Fixed 5V DC/DC Converters

Linear Technology 

LT1302/LT1302-5

APPLICATIONS INFORMATION

ally reaches audio frequencies, but at a much lighter load

than without the IT feature. At some input voltage/load

current combinations, some residual bursting may occur

at frequencies out of the audio band.

VOUT

100mV/DIV

AC COUPLED

INDUCTOR

CURRENT

1A/DIV

ILOAD

525mA

25mA

1ms/DIV

1302 F08b

Figure 8b. 3.3k Resistor from IT Pin to Ground Forces

LT1302 into Current Mode Regardless of Load. Audio

Frequency Component Eliminated

90

IT FLOATING

80

70

3.3k IT TO GND

60

50

40

30

1

10

100

OUTPUT CURRENT (mA)

1000

1302 F08c

Figure 8c. 3.3k Resistor for IT to Ground Increases

Efficiency at Moderate Load, Decreases at Light Load

The IT pin cannot be used as a soft-start. Large capacitors

connected to the pin will cause erratic operation. If oper-

ating the device in Burst Mode, let the pin float. Keep high

dV/dt signals away from the pin.

Figure 8c details efficiency with and without the addition of

R1. Burst Mode operation keeps efficiency high at light

load with IT floating. Efficiency falls off at light load with

R1 added because the LT1302 cannot transition into Burst

Mode.

Layout

The high speed, high current switching associated with

the LT1302 mandates careful attention to layout. Follow

the suggested component placement in Figure 9 for proper

operation. High current functions are separated by the

package from sensitive control functions. Feedback resis-

tors R1 and R2 should be close to the feedback pin (pin4).

Noise can easily be coupled into this pin if care is not taken.

A small capacitor (100pF to 200pF) from FB to ground

provides a high frequency bypass. If the LT1302 is oper-

ated off a three-cell or higher input, R3 (2Ω to 10Ω) in

series with VIN is recommended. This isolates the device

from noise spikes on the input supply. Do not put in R3 if

the device must operate from a 2V input, as input current

will cause the voltage at the LT1302’s VIN pin to go below

2V. The 0.1µF ceramic bypass capacitor C3 (use X7R, not

Z5U) should be mounted as close as possible to the

package. When R3 is used, C3 should be a 1µF tantalum

unit. Grounding should be segregated as illustrated. C3’s

ground trace should not carry switch current. Run a

VIN

VOUT

L1

D1

C2

R3

2Ω

C1

C3 5

6

7

8

LT1302

4

3 R1

2

1

R2

200pF

RC

SHUTDOWN

CC

GND (BATTERY AND LOAD RETURN)

1302 F09

Figure 9. Suggested Component Placement for LT1302

11

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