LT1950 데이터 시트보기 (PDF) - Linear Technology

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LT1950

Single Switch PWM Controller with Auxiliary Boost Converter

Linear Technology 

LT1950

TYPICAL PERFOR A CE CHARACTERISTICS

BOOST Switch Off Time vs

Temperature

700

600

500

400

300

–50 –25

0 25 50 75

TEMPERATURE (°C)

100 125

1950 G16

Maximum Duty Cycle vs

VSEC Voltage

100

MAX DUTY CYCLE = (105/VSEC)%

90

1.25V < VSEC < 2.8V

TA = 25°C

80

70

60

50

40

30

0.8 1.2 1.6 2.0 2.4 2.8 3.2

VSEC VOLTAGE (V)

1950 G17

GATE Clamp Voltage vs

Temperature

16

15

14

13

12

11

10

–50 –25

0 25 50 75

TEMPERATURE (°C)

100 125

1950 G18

PI FU CTIO S

COMP (Pin 1): The COMP pin is the output of the error

amplifier. The error amplifier is a true op amp which allows

the use of an RC network to be connected between the

Comp and FB pins to compensate the feedback loop for

optimum transient response. The peak switch current in

the external MOSFET will be proportional to the voltage on

the COMP pin. Typical operating voltage range for this pin

is 1V to 2.5V.

FB (Pin 2): The FB pin is the inverting input to the error

amplifier. The output voltage is set with a resistor divider.

The error amplifier adjusts the peak switch current to

maintain the FB pin voltage at the value of the internal

reference voltage of 1.23V.

ROSC (Pin 3): A resistor from the ROSC pin to ground

programs the operating frequency of the LT1950. Operat-

ing frequency range is 100kHz to 500kHz. Nominal voltage

on the ROSC pin is 1V.

SYNC (Pin 4): The SYNC pin is used to synchronize the

internal oscillator to an external clock signal. The pin is

directly logic compatible and can be driven with any signal

with a duty cycle of 10% to 90%. If the SYNC function is

not used the pin can be left open circuit or connected to

ground.

SLOPE (Pin 5): The SLOPE pin is used to adjust the

amount of slope compensation. Leaving the pin open

circuit results in a default level of slope compensation. The

amount of slope compensation can be adjusted above this

default level by connecting a resistor from the SLOPE pin

to the VREF pin.

VREF (Pin 6): The VREF pin is the output of an internal 2.5V

reference. This pin is capable of sourcing up to 2.5mA for

external use. It is recommended that the VREF pin is

bypassed to ground with a 0.1µF ceramic capacitor.

SHDN (Pin 7): The SHDN pin is used to put the device into

a low power shutdown state. In shutdown the VIN supply

current drops to 5µA. The SHDN pin has an accurate

threshold of 1.32V which can be used to program an

undervoltage lockout threshold. Input current levels on

the SHDN pin can be used to program hysteresis into the

undervoltage lockout levels.

GND (Pin 8): The GND pin is the analog ground for the

internal circuitry of the LT1950. Sensitive circuitry such as

the feedback divider, frequency setting resistor, reference

bypass capacitor should be tied directly to this pin. See the

Applications Information section for recommendations

on ground connections.

1950fa

6

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