LT3503 datasheet(13/20 Pages) LINER

13 / 20 page

LT3503

3503f

The minimum operating voltage of an LT3503 application

is limited by the undervoltage lockout (3.6V) and by the

maximum duty cycle as outlined above. For proper start-

up, the minimum input voltage is also limited by the boost

circuit. If the input voltage is ramped slowly, or the LT3503

is turned on with its SHDN 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 running

properly. This minimum load will depend on the 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 7 shows a plot of minimum load

APPLICATIO S I FOR ATIO

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 verly 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 discontinu-

ous and the effective duty cycle can be very high. This

reduces the minimum input voltage to approximately

400mV above VOUT. At higher load currents, the inductor

current is continuous and the duty cycle is limited by the

maximum duty cycle of the LT3503, requiring a higher

input voltage to maintain regulation.

(7a) Typical Minimum Input Voltage,

VOUT = 5V

(7b) Typical Minimum Input Voltage,

VOUT = 3.3V

Figure 7

Figure 6. Two Circuits for Generating the Boost Voltage

VIN

BOOST

GND

VIN

LT3503

(6a)

VOUT

VBOOST – VSW ≅ VOUT

MAX VBOOST ≅ VIN + VOUT

3503 F06a

VIN

BOOST

GND

VIN

LT3503

(6b)

3503 F06b

VOUT

VBOOST – VSW ≅ VIN

MAX VBOOST ≅ 2VIN

LOAD CURRENT (mA)

5.2

6.7

7.2

7.7

100

1000

3503 F07a

6.2

5.7

TO START

TO RUN

LOAD CURRENT (mA)

5.0

5.5

TO START

TO RUN

1OO

1000

3503 F07b

4.5

4.0

3.5

LT3503 Datasheet(PDF) 13 Page - Linear Technology

LT3503 Datasheet(HTML) 13 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

Datasheet Download

Link URL