LT1246CS8 datasheet(7/12 Pages) LINER

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LT1246/LT1247

GND (Pin 5): Ground.

OUTPUT (Pin 6): Current Output. 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): Supply Voltage. 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.

FU C

APPLICATI

I FOR ATIO

deadtime at that frequency. Curves of oscillator frequency

and deadtime for various values of RT and CT appear in the

Typical Performance Characteristics section. Frequency

and deadtime can also be calculated using the following

formulas:

Oscillator Rise Time: tr = 0.583 • RC

Oscillator Discharge Time:

Oscillator Period: tOSC = tr + td

Oscillator Frequency:

Maximum Duty Cycle:

The above formulas will give values that will be accurate

to approximately

±5%, at the oscillator, over the full

operating frequency range. This is due to the fact that the

oscillator trip levels are constant versus frequency and the

discharge current and initial oscillator frequency are

trimmed. Some fine adjustment may be required to achieve

more accurate results. Once the final RT/CT combination is

selected, the oscillator characteristics will be repeatable

from device to device. Note that there will be some slight

differences between maximum duty cycle at the oscillator

and maximum duty cycle at the output due to the finite rise

and fall times of the output.

Error Amplifier

The LT1246/LT1247 contain a fully compensated error

amplifier with a DC gain of 90dB and a unity-gain fre-

quency of 2MHz. Phase margin at unity-gain is 80

°. The

noninverting input is internally committed to a 2.5V refer-

ence point derived from the 5V reference of pin 8. The

Minimum

Start-Up

Operating

Maximum

Device

Threshold

Voltage

Duty Cycle

Replaces

LT1246

16V

10V

100%

UC1842

LT1247

8.4V

7.6V

100%

UC1843

Oscillator

The LT1246/LT1247 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

LT1246/LT1247. 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 oscil-

lator 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 oscillator. 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

d =

•

−

346

0 0164

11 73

OSC

MAX

OSC

−

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