LT1247CS8#TR Linear Technology, LT1247CS8#TR Datasheet - Page 7

• Manufacturer Part Number: LT1247CS8#TR
• Description: IC PWM CM 8SOIC
• Manufacturer: Linear Technology

Specifications of LT1247CS8#TR

• Pwm Type: Current Mode
• Number Of Outputs: 1
• Frequency - Max: 1MHz
• Duty Cycle: 100%
• Voltage - Supply: 8.2 V ~ 25 V
• Buck: Yes
• Boost: No
• Flyback: No
• Inverting: No
• Doubler: No
• Divider: No
• Cuk: No
• Isolated: Yes
• Operating Temperature: 0°C ~ 100°C
• Package / Case: 8-SOIC (3.9mm Width)
• Frequency-max: 1MHz
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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.
V
of the control IC.
Device
LT1246
LT1247
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, R
combination will determine both the frequency and the
maximum duty cycle. The resistor R
V
connected from the R
current for C
discharge current for C
the current supplied by R
LT1246/LT1247. The discharge current of the device is
trimmed to 8.2mA. For large values of R
will be determined by the discharge current of the device
and the value of C
more effect on the discharge time of C
lator cycle capacitor C
and discharged to approximately 1.1V. The output is
enabled during the charge time of C
off state, during the discharge time of C
the circuit is equal to the discharge time of C
maximum duty cycle is limited by controlling the deadtime
of the oscillator. There are many combinations of R
C
there is only one combination that will yield a specific
A
PI
CC
REF
T
PPLICATI
U
that will yield a given oscillator frequency, however
(Pin 7): Supply Voltage. This pin is the positive supply
(pin 8) to the R
FU
U
Threshold
Start-Up
T
C
8.4V
16V
TI
is determined by the value of R
O
T
. As the value of R
O
U
T
/C
U
T
S
T
T
/C
is charged to approximately 2.8V
T
S
T
Operating
Minimum
is set by the difference between
Voltage
pin (pin 4). The capacitor C
T
and the discharge current of the
I FOR ATIO
7.6V
10V
U
pin to ground. The charging
T
Duty Cycle
Maximum
T
is reduced it will have
W
T
100%
100%
and disabled, in an
is connected from
T
T
T
. During an oscil-
. The deadtime of
and C
T
discharge time
T
Replaces
. This RC
UC1842
UC1843
U
T
T
. The
. The
T
and
T
is
V
IC. The reference output is used to supply charging current
to the external timing resistor R
biasing to a large portion of the internal circuitry, and is
used to generate several internal reference levels includ-
ing the V
deadtime at that frequency. Curves of oscillator frequency
and deadtime for various values of R
Typical Performance Characteristics section. Frequency
and deadtime can also be calculated using the following
formulas:
The above formulas will give values that will be accurate
to approximately
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 R
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
REF
Oscillator Rise Time: t
Oscillator Discharge Time:
Oscillator Period: t
Oscillator Frequency:
Maximum Duty Cycle:
(Pin 8): Reference. This is the reference output of the
FB
level and the current sense clamp voltage.
5%, at the oscillator, over the full
OSC
f
r
OSC
= t
D
= 0.583 • RC
MAX
r
+ t
t
d
LT1246/LT1247
t
d
T
OSC
. The reference provides
1
t
0 0164
OSC
t
T
r
and C
T
3 46
/C
T
R
t
OSC
combination is
T
t
appear in the
OSC
RC
11 73
t
d
7