LT1074CK datasheet(9/16 Pages) LINER

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LT1074/LT1076

Figure 3. Frequency Shifting

Figure 4. Shutdown Pin Characteristics

current is well controlled even when switch duty cycle

must be extremely low. Theoretical switch “on” time for a

buck converter in continuous mode is:

OUT

•

VD = Catch diode forward voltage ( ≈ 0.5V)

f = Switching frequency

At f = 100kHz, tON must drop to 0.2µs when VIN = 25V

and the output is shorted (VOUT = 0V). In current limit,

the LT1074 can reduce tON to a minimum value of

≈0.6µs, much too long to control current correctly for

VOUT = 0. To correct this problem, switching frequency

is lowered from 100kHz to 20kHz as the FB pin drops

from 1.3V to 0.5V. This is accomplished by the circuitry

shown in Figure 3.

Q1 is off when the output is regulating (VFB = 2.21V). As

the output is pulled down by an overload, VFB will eventu-

ally reach 1.3V, turning on Q1. As the output continues to

drop, Q1 current increases proportionately and lowers the

frequency of the oscillator. Frequency shifting starts when

the output is

≈ 60% of normal value, and is down to its

minimum value of

≅20kHz when the output is ≅ 20% of

normal value. The rate at which frequency is shifted is

determined by both the internal 3k resistor R3 and the

external divider resistors. For this reason, R2 should not

be increased to more than 4k

Ω, if the LT1074 will be

subjected to the simultaneous conditions of high input

voltage and output short-circuit.

DESCRIPTIO S

SHUTDOWN PIN

The shutdown pin is used for undervoltage lockout, micro-

power shutdown, soft-start, delayed start, or as a general

purpose on/off control of the regulator output. It controls

switching action by pulling the ILIM pin low, which forces

the switch to a continuous “off” state. Full micropower

shutdown is initiated when the shutdown pin drops below

0.3V.

The V/I characteristics of the shutdown pin are shown in

Figure 4. For voltages between 2.5V and

≈VIN, a current of

µA flows out of the shutdown pin. This current in-

creases to

≈25µA as the shutdown pin moves through the

2.35V threshold. The current increases further to

≈30µAat

the 0.3V threshold, then drops to

≈15µA as the shutdown

voltage fall below 0.3V. The 10

µA current source is in-

cluded to pull the shutdown pin to its high or default state

when left open. It also provides a convenient pull-up for

delayed start applications with a capacitor on the shut-

down pin.

When activated, the typical collector current of Q1 in

Figure 5, is

≈2mA. A soft-start capacitor on the ILIM pinwill

delay regulator shutdown in response to C1, by

≈(5V)(CLIM)/2mA. Soft-start after full micropower shut-

down is ensured by coupling C2 to Q1.

–

2.21V

+2V

ERROR

AMPLIFIER

2.21k

VOUT

EXTERNAL

DIVIDER

OSCILLATOR

LT1074•PD03

VOLTAGE (V)

–40

1.0

2.0

LT1074•PC05

1.5

2.5

3.0

3.5

4.0

–35

–30

–25

–20

–15

–10

–5

SHUTDOWN

THRESHOLD

CURRENT FLOWS OUT

OF SHUTDOWN PIN

= 25

°C

0.5

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