LTC3609IWKG-TRPBF(RevB) Просмотр технического описания (PDF) - Linear Technology

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LTC3609IWKG-TRPBF
(Rev.:RevB)

32V, 6A Monolithic Synchronous Step-Down DC/DC Converter

Linear Technology 

LTC3609

Applications Information

Because the voltage at the ION pin is about 0.7V, the cur-

rent into this pin is not exactly inversely proportional to

VIN, especially in applications with lower input voltages.

To correct for this error, an additional resistor RON2 con-

nected from the ION pin to the 5V INTVCC supply will further

stabilize the frequency.

RON2

=

5V

0.7V

RON

Changes in the load current magnitude will also cause

frequency shift. Parasitic resistance in the MOSFET

switches and inductor reduce the effective voltage across

the inductance, resulting in increased duty cycle as the

load current increases. By lengthening the on-time slightly

as current increases, constant frequency operation can be

maintained. This is accomplished with a resistive divider

from the ITH pin to the VON pin and VOUT. The values

required will depend on the parasitic resistances in the

specific application. A good starting point is to feed about

25% of the voltage change at the ITH pin to the VON pin

as shown in Figure 2a. Place capacitance on the VON pin

to filter out the ITH variations at the switching frequency.

The resistor load on ITH reduces the DC gain of the error

amp and degrades load regulation, which can be avoided

by using the PNP emitter follower of Figure 2b.

1000

VOUT = 1.5V

VOUT = 3.3V

VOUT = 2.5V

12

100

100

1000

RON (kΩ)

10000

3609 F01a

Figure 1a. Switching Frequency vs RON (VON = 0V)

1000

VOUT = 3.3V

VOUT = 12V

VOUT = 5V

100

100

1000

RON (kΩ)

10000

3609 F01b

Figure 1b. Switching Frequency vs RON (VON = INTVCC)

3609fb

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