LT1242 データシートの表示（PDF） - Linear Technology

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LT1242

High Speed Current Mode Pulse Width Modulators

Linear Technology 

LT1241 Series

PI FU CTI S

COMP (Pin 1): Compensation Pin. This pin is the output of

the Error Amplifier and is made available for loop compen-

sation. It can also be used to adjust the maximum value of

the current sense clamp voltage to less than 1V. This pin

can source a minimum of 0.5mA (0.8mA typ) and sink a

minimum of 2mA (4mA typ)

FB (Pin 2) Voltage Feedback Pin. This pin is the inverting

input of the error amplifier. The output voltage is normally

fed back to this pin through a resistive divider. The non-

inverting input of the error amplifier is internally commit-

ted to a 2.5V reference point.

ISENSE (Pin 3): Current Sense Pin. This is the input to the

current sense comparator. The trip point of the compara-

tor is set by, and is proportional to, the output voltage of

the Error Amplifier.

RT/CT (Pin 4): The oscillator frequency and the deadtime

are set by connecting a resistor (RT) from VREF to RT/CT

and a capacitor (CT) from RT/CT to GND.

The rise time of the oscillator waveform is set by the RC

time constant of RT and CT. The fall time, which is equal to

the output deadtime, is set by a combination of the RC time

constant and the oscillator sink current (8.2mA typ).

GND (Pin 5): Ground.

OUTPUT (Pin 6): This pin is the output of a high current

totem pole output stage. It is capable of driving up to ±1A

of current into a capacitive load such as the gate of a

MOSFET.

VCC (Pin 7): This pin is the positive supply of the control

IC.

VREF (Pin 8): Reference. This is the reference output of the

IC. The reference output is used to supply charging current

to the external timing resistor RT. The reference provides

biasing to a large portion of the internal circuitry, and is

used to generate several internal reference levels includ-

ing the VFB level and the current sense clamp voltage.

APPLICATI S I FOR ATIO

START-UP

DEVICE

LT1241

LT1242

LT1243

LT1244

LT1245

MINIMUM

OPERATING

THRESHOLD

9.6V

16V

8.4V

16V

8.4V

MAXIMUM

VOLTAGE

7.6V

10V

7.6V

10V

7.6V

DUTY CYCLE REPLACES

50%

NONE

100%

UC1842

100%

UC1843

50%

UC1844

50%

UC1845

Oscillator

The LT1241 series devices are fixed frequency current

mode pulse width modulators. The oscillator frequency

and the oscillator discharge current are both trimmed and

tightly specified to minimize the variations in frequency

and deadtime. The oscillator frequency is set by choosing

a resistor and capacitor combination, RT and CT. This RC

combination will determine both the frequency and the

maximum duty cycle. The resistor RT is connected from

VREF (Pin 8) to the RT/CT pin (Pin 4). The capacitor CT is

connected from the RT/CT pin to ground. The charging

current for CT is determined by the value of RT. The

discharge current for CT is set by the difference between

the current supplied by RT and the discharge current of the

LT124X. The discharge current of the device is trimmed to

8.2mA. For large values of RT discharge time will be

determined by the discharge current of the device and the

value of CT. As the value of RT is reduced it will have more

effect on the discharge time of CT. During an oscillator

cycle capacitor CT is charged to approximately 2.8V and

discharged to approximately 1.1V. The output is enabled

during the charge time of CT and disabled, in an off state,

during the discharge time of CT. The deadtime of the circuit

is equal to the discharge time of CT. The maximum duty

cycle is limited by controlling the deadtime of the oscilla-

tor. There are many combinations of RT and CT that will

yield a given oscillator frequency, however there is only

one combination that will yield a specific deadtime at that

frequency. Curves of oscillator frequency and deadtime

7

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