LT1578 Просмотр технического описания (PDF) - Linear Technology

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LT1578

1.5A, 200kHz Step-Down Switching Regulator

Linear Technology 

LT1578/LT1578-2.5

APPLICATIONS INFORMATION

FEEDBACK PIN FUNCTIONS

The feedback (FB) pin on the LT1578 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

foldback frequency and current limiting created by the FB

pin. Please read both parts before committing to a final

design. The fixed 2.5V LT1578-2.5 has internal divider

resistors and the FB pin, renamed SENSE, is connected

directly to the 2.5V output.

The suggested value for the output divider resistor (see

Figure 2) from FB to ground (R2) is 5k or less, and a

formula for R1 is shown below. The output voltage error

caused by ignoring the input bias current on the FB pin is

less than 0.25% with R2 = 5k. Please read the following

if divider resistors are increased above the suggested

values.

( ) R1= R2 VOUT −1.21

1.21

More Than Just Voltage Feedback

The feedback pin is used for more than just output voltage

sensing. It also reduces switching frequency and current

limit when output voltage is very low (see the Frequency

Foldback graph in Typical Performance Characteristics).

This is done to control power dissipation in both the IC and

the external diode and inductor during short-circuit con-

ditions. A shorted output requires the switching regulator

to operate at very low duty cycles, and the average current

through the diode and inductor is equal to the short-circuit

current limit of the switch (typically 2A for the LT1578,

folding back to less than 0.77A). Minimum switch on time

limitations would prevent the switcher from attaining a

sufficiently low duty cycle if switching frequency were

maintained at 200kHz, so frequency is reduced by about

5:1 when the feedback pin voltage drops below 0.7V (see

Frequency Foldback graph). This does not affect operation

with normal load conditions; one simply sees a gear shift

in switching frequency during start-up as the output

voltage rises.

LT1578

TO FREQUENCY

SHIFTING

VSW

OUTPUT

5V

1.4V

Q1

ERROR

AMPLIFIER

1.21V

R3

R4

1k

1k

R1

FB

+

R5

5k

Q2

R2

5k

TO SYNC CIRCUIT

VC GND

1578 F02

Figure 2. Frequency and Current Limit Foldback

8

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