LT3435 Просмотр технического описания (PDF) - Linear Technology

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LT3435

High Voltage 3A, 500kHz Step-Down Switching Regulator with 100µA Quiescent Current

Linear Technology 

LT3435

BLOCK DIAGRA

power from the VIN pin, but if the BIAS pin is connected to

an external voltage higher than 3V bias power will be

drawn from the external source (typically the regulated

output voltage). This improves efficiency.

High switch efficiency is attained by using the BOOST pin

to provide a voltage to the switch driver which is higher

than the input voltage, allowing switch to be saturated.

This boosted voltage is generated with an external capaci-

tor and diode.

To further optimize efficiency, the LT3435 automatically

switches to Burst Mode operation in light load situations.

In Burst Mode operation, all circuitry associated with

controlling the output switch is shut down reducing the

input supply current to 45µA.

The LT3435 contains a power good flag with a program-

mable threshold and delay time. A logic-level low on the

SHDN pin disables the IC and reduces input suppy current

to less than 1µA.

APPLICATIO S I FOR ATIO

FEEDBACK PIN FUNCTIONS

The feedback (FB) pin on the LT3435 is used to set output

voltage and provide several overload protection features.

The first part of this section deals with selecting resistors

to set output voltage and the remaining part talks about

frequency foldback and soft-start features. Please read

both parts before committing to a final design.

Referring to Figure 2, the output voltage is determined by

a voltage divider from VOUT to ground which generates

1.25V at the FB pin. Since the output divider is a load on the

output care must be taken when choosing the resistor

divider values. For light load applications the resistor

values should be as large as possible to achieve peak

efficiency in Burst Mode operation. Extremely large values

for resistor R1 will cause an output voltage error due to the

50nA FB pin input current. The suggested value for the

output divider resistor (see Figure 2) from FB to ground

(R2) is 100k or less. A formula for R1 is shown below. A

table of standard 1% values is shown in Table 1 for

common output voltages.

R1= R2 • VOUT – 1.25

1.25 + R2 • 50nA

More Than Just Voltage Feedback

The FB pin is used for more than just output voltage

sensing. It also reduces switching frequency and con-

trols the soft-start voltage ramp rate when output voltage

is below the regulated level (see the Frequency Foldback

Table 1

OUTPUT

VOLTAGE

(V)

2.5

3

3.3

5

6

8

10

12

LT3435

500kHz

OSCILLATOR

R2

(kΩ, 1%)

100

100

100

100

100

100

100

100

R1

NEAREST (1%)

(kΩ)

100

140

165

301

383

536

698

866

OUTPUT

ERROR

(%)

0

0

0.38

0.25

0.63

– 0.63

– 0.25

0.63

SOFT-START

SW 2

C1

CSS

9

VOUT

FOLDBACK

R1

DETECT

ERROR

AMP

FB

12

R2

1.25V

VC

11

3435 F02

Figure 2. Feedback Network

3435f

9

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