LT3695 datasheet(19/28 Pages) LINER

19 / 28 page

LT3695

3695f

running properly. This minimum load will depend on input

and output voltages, and on the arrangement of the boost

circuit. The minimum load generally goes to zero once the

circuit has started. Figure 5 shows a plot of minimum load

to start and to run as a function of input voltage. In many

cases the discharged output capacitor will present a load

to the switcher, which will allow it to start. The plots show

the worst-case situation where VIN is ramping very slowly.

For lower start-up voltage, the boost diode can be tied to

VIN; however, this restricts the input range to one-half of

the absolute maximum rating of the BOOST pin. At light

loads, the inductor current becomes discontinuous and

the effective duty cycle can be very high. This reduces the

minimum input voltage to approximately 300mV above

VOUT. At higher load currents, the inductor current is

continuous and the duty cycle is limited by the maximum

duty cycle of the LT3695, requiring a higher input voltage

to maintain regulation.

APPLICATIONS INFORMATION

LT3695

VIN

VOUT

3695 F04a

BOOST

GND

PGND

(4a) For VOUT > 2.8V, VIN(MIN) = 4.3V if VOUT < 3V

LT3695

VIN

VOUT

3695 F04b

BOOST

GND

PGND

(4b) For 2.5V < VOUT < 2.8V, VIN(MIN) = 4.3V

LT3695

VIN

VOUT

3695 F04c

BOOST

GND

PGND

(4c) For VOUT < 2.5V, VIN(MAX) = 25V

Figure 4. Three Circuits for Generating

the Boost Voltage

by using a Zener diode between VOUT and BD to maintain

the BD pin voltage between 3V and 8V.

The minimum operating voltage of the LT3695 applica-

tion is limited by the minimum input voltage and by the

maximum duty cycle as outlined previously. For proper

start-up, the minimum input voltage is also limited by the

boost circuit. If the input voltage is ramped slowly, or the

LT3695 is turned on with its RUN/SS pin when the output

is already in regulation, then the boost capacitor may not

be fully charged. Because the boost capacitor is charged

with the energy stored in the inductor, the circuit will rely

on some minimum load current to get the boost circuit

LOAD CURRENT (mA)

4.5

5.0

5.5

1000

3.5

4.0

3.0

2.5

2.0

100

6.0

LOAD CURRENT(mA)

3695 F05

6.0

6.5

7.0

7.5

1000

4.5

4.0

5.5

5.0

3.5

2.5

3.0

2.0

8.0

TO RUN

TO START

(WORST CASE)

TO START

(WORST CASE)

VOUT = 3.3V

TA = 25˚C

L = 10μH

f = 800kHz

TO RUN

VOUT = 5V

TA = 25˚C

L = 10μH

f = 800kHz

Figure 5. The Minimum Input Voltage depends on

Output Voltage, Load Current and Boost Circuit

LT3695 Datasheet(PDF) 19 Page - Linear Technology

LT3695 Datasheet(HTML) 19 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL