LT3500 Ver la hoja de datos (PDF) - Linear Technology

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LT3500

Monolithic 2A Step-Down Regulator Plus Linear Regulator/Controller

Linear Technology 

TYPICAL PERFORMANCE CHARACTERISTICS

LDRV Dropout Voltage

vs Temperature

1.50

ILDRV = 5mA

1.45

1.40

1.35

1.30

1.25

1.20

1.15

1.10

1.05

1.00

–50 –25 0 25 50 75 100 125 150

TEMPERATURE (°C)

3500 G22

Switcher Dropout Operation

6

IVOUT1 = 1A

TA = 25°C

5

VVOUT1 = 5V

4

VVOUT1 = 3.3V

3

2

1

0

2.5 3.0 3.5 4.0 4.5 5.0 5.5

INPUT VOLTAGE (V)

3500 G23

LT3500

Inductor Value for 2A Maximum

Load Current (VOUT1 = 3.3V,

IRIPPLE = 250mA)

2500

2250

2000

1750

L = 1μH

L = 1.5μH

TA = 25°C

L = 2.2μH

1500

1250

1000

L = 3.3μH

L = 4.7μH

750

500

250

5

L = 6.8μH

L = 10μH

10 15 20 25 30 35 40

INPUT VOLTAGE (V)

3500 G24

PIN FUNCTIONS

VIN: The VIN pin powers the internal control circuitry and

is monitored by an undervoltage comparator. The VIN pin

is also connected to the collectors of the internal power

NPN switch and linear output NPN. The VIN pin has high

dI/dt edges and must be decoupled to ground close to

the pin of the device.

SHDN: The SHDN pin is used to shut down the LT3500

and reduce quiescent current to a typical value of

12μA. The accurate 0.76V threshold and input current

hysteresis can be used as an undervoltage lockout, prevent-

ing the regulator from operating until the input voltage has

reached a predetermined level. Force the SHDN pin above

its threshold or let it float for normal operation.

SS: The SS pin is used to control the slew rate of the

output of both the switching and linear regulators. A

single capacitor from the SS pin to ground determines

the regulators’ ramp rate. For soft-start details see the

Applications Information section.

PG: The power good pin is an open-collector output that

sinks current when the FB or LFB falls below 90% of its

nominal regulating voltage. For VIN above 2V, its output

state remains true, although during SHDN, VIN undervoltage

lockout, or thermal shutdown, its current sink capability

is reduced

VC: The VC pin is the output of the error amplifier and the

input to the peak switch current comparator. It is normally

used for frequency compensation, but can also be used

as a current clamp or control loop override. If the error

amplifier drives VC above the maximum switch current

level, a voltage clamp activates. This indicates that the

output is overloaded and current to be pulled from the

SS pin reducing the regulation point.

RT/SYNC: This RT/SYNC pin provides two modes of setting

the constant switch frequency.

Connecting a resistor from the RT/SYNC pin to ground

will set the RT/SYNC pin to a typical value of 1V. The

resultant switching frequency will be set by the resistor

value. The minimum value of 15kΩ and maximum value

of 200kΩ set the switching frequency to 2.5MHz and

250kHz respectively.

3500fc

7

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